A new study raises a cautionary note on time-restricted eating (TRE), a type of intermittent fasting that is gaining popularity.
The observational analysis of over 20,000 US adults showed that those who limited their eating to a period of less than 8 hours per day had a higher risk for cardiovascular mortality compared with peers who ate across the typical 12-16 hours per day. This was the case in the overall sample and in those with cardiovascular disease (CVD) or cancer.
Lead author Victor Wenze Zhong, PhD, cautioned that the findings “require replication and we cannot demonstrate 8-hour TRE causes cardiovascular death in this observational study.”
“However, it’s important for patients, particularly those with existing heart conditions or cancer, to be aware of the positive association between an 8-hour eating window and cardiovascular death,” Zhong, professor and chair, Department of Epidemiology and Biostatistics, School of Public Health, Shanghai Jiao Tong University School of Medicine, Shanghai, China, told theheart.org | Medscape Cardiology.
The results (Abstract P192) were presented March 18 at the American Heart Association (AHA) Epidemiology and Prevention | Lifestyle and Cardiometabolic Health Scientific Sessions 2024.
‘Provocative’ Results
Short-term randomized controlled trials have suggested that 8-hour TRE may improve cardiometabolic risk profiles, but the potential long-term effects of this eating pattern are unknown.
The observation that TRE may have short-term benefits but long-term adverse effects is “interesting and provocative” and needs further study, Christopher D. Gardner, PhD, professor of medicine at Stanford University in California, who wasn’t involved in the study, said in a conference statement, and he agreed that much more research is needed.
The researchers analyzed data on dietary patterns for 20,078 adults (mean age, 48 years; 50% men; 73% non-Hispanic White) who participated in the 2003-2018 National Health and Nutrition Examination Surveys (NHANES). All of them completed two 24-hour dietary recall questionnaires within the first year of enrollment. Deaths through the end of 2019 were determined via the National Death Index.
During a median follow-up of 8 years, there were 2797 deaths due to any cause, including 840 CV deaths and 643 cancer deaths.
In the overall sample, compared with an eating duration of 12-16 hours, 8-hour TRE was significantly associated with an increased risk for CV mortality (hazard ratio [HR], 1.91; 95% CI, 1.20-3.03).
This association was also observed in adults with CVD (HR, 2.07; 95% CI, 1.14-3.78) and adults with cancer (HR, 3.04; 95% CI, 1.44-6.41).
Other eating durations were not associated with CV mortality, except for eating duration of 8 to less than 10 hours in people with CVD (HR, 1.66; 95% CI, 1.03-2.67).
No significant associations were found between eating duration and all-cause or cancer mortality in the overall sample and CVD/cancer subsamples, except that eating duration of more than 16 hours was associated with a lower risk for cancer mortality in people with cancer (HR, 0.47; 95% CI, 0.23-0.95).
Quality More Important Than Timing
Zhong noted that the study doesn’t address the underlying mechanisms driving the observed association between 8-hour TRE and CV death.
“However, we did observe that people who restricted eating to a period less than 8 hours per day had less lean muscle mass compared with those with typical eating duration of 12-16 hours. Loss of lean body mass has been linked to higher risk of cardiovascular mortality,” Zhong said.
“Based on the evidence as of now, focusing on what people eat appears to be more important than focusing on the time when they eat. There are certain dietary approaches with compelling health benefits to choose, such as DASH diet and Mediterranean diet,” Zhong told theheart.org | Medscape Cardiology.
Intermittent fasting is “certainly an interesting concept and one on which the potential mechanisms underlying the improvements in short outcome studies and preclinical studies in animals are strongly being pursued,” Sean Heffron, MD, cardiologist at the Center for the Prevention of Cardiovascular Disease at NYU Langone Heart, New York, NY, who wasn’t involved in the study, told theheart.org | Medscape Cardiology.
Heffron expressed skepticism about the study results calling them “far from complete” and noted that data on diet was based on only 2-day diet records without correction for confounding variables.
Heffron also noted that the restricted diet group has more smokers and more men. “I would “strongly anticipate that once appropriate corrections are made, the findings will no longer persist in statistical significance,” Heffron said.
He emphasized the need for more rigorous research before making clinical recommendations. When patients ask about intermittent fasting, Heffron said he tells them, “If it works for you, that’s fine,” but he doesn’t provide a recommendation for or against it.
Want better focus? Healthy cognition as you age? Or maybe just a better chess game? Then brain health is probably on your agenda.
Brainpower on your wishlist? Explore the connection between intermittent fasting and cognitive fitness. Your focus game is about to level up!
Aside from cool sci-fi scenarios with a magic pill or technological plug-in that makes you a genius, how else can you improve and/or preserve your brain health?
Some promising research suggests that intermittent fasting can help you be healthier and happier, not just physically but mentally capable, well into old age.
In this article, we’ll explore the science behind intermittent fasting for brain health.
Key takeaways
Generally, brain health requires overall body health. So, starting with basic healthy behaviors is a foundation for cognitive well-being.
Fasting stimulates physiological changes that may benefit brain health in particular.
Some people report better mood and focus when fasting, but folks vary in how they feel. Experiment and see what you notice for yourself.
While fasting can be part of an overall healthy lifestyle, if you have a known cognitive or neurological condition, always consult your healthcare provider before making any changes.
Ready for a brain health boost? Take the SIMPLE quiz to gain access to a personalized fasting routine, insightful nutrition scores, and a supportive community. Your brain and wellness journey starts here!
The metabolic switch of fasting
To understand whether fasting can support brain health, it helps to understand how fasting works in the body.
There are many types of intermittent fasting. For more on types of fasting, please check out our article.
However, they all generally share a process that scientists informally call “flipping the metabolic switch.”[1] This occurs somewhere around 8–12 hours after your last meal, depending on several factors.
With this switch, your body shifts from primarily using carbohydrate-based glycogen for energy to using stored fat from your fat tissues. This stored fat gets turned into substances called free fatty acids and then into ketones.
Thus, you might know this state as being in ketosis.
Want to see your metabolic switch flip in real time? You can see when your body switches to “fat-burning mode” by checking the Metabolic Status indicator in the SIMPLE app while fasting.
Your brain on ketosis: how fasting changes your brain
Unlock the power of your mind with intermittent fasting! Embrace the beach vibes, stretch it out, and let your brain revel in the benefits of fasting.
Why does the body do this metabolic switch into ketosis? Short answer: your brain.
Your brain isn’t that big — about 2% of your body size — but it needs a lot of energy to run relative to its size: about 20%–25% of your total daily calorie needs.[2,3] (Hey, being a genius takes fuel!)
Famines and food shortages were common for most of human history. Our brains needed to come up with a way to get fuel when food was scarce. Being smart, of course, it did: ketosis.
Ketosis occurs when our body makes ketones, or ketoacids, from fats or proteins. Ketones can provide energy when other fuels aren’t available.
During fasting, ketones made from stored body fat become the brain’s preferred energy source. Not only do they provide energy, but they may also help regulate important factors in the brain.
Boosting neuroplasticity with BDNF
Two key players are BHB (β-hydroxybutyrate) and AcAc (acetoacetate). These ketones are made in your liver during fasting and are sent to your brain. In animal studies, they seem to boost brain-derived neurotrophic factors (BDNF).[4]
BDNF helps create new brain cells, strengthens connections between them, and makes your brain more resilient to stress.
(By the way, BDNF is also stimulated by exercise. So perhaps a morning fasted walk might be a nice little brain boost!)
However, the research in humans isn’t as strong right now. The results vary, with some studies showing that fasting reduces BDNF.[5] Other factors like measurement methods, sex, and health status play a role.
So, we need more human research to draw some better conclusions. But animal studies look promising.
Fasting and autophagy
Intermittent fasting also helps your brain through a process called autophagy.
Autophagy is the process your body uses to break down and clear out damaged cells to make space for new ones. In the brain, this can mean tidying up damaged cells and cellular waste products.
Brief periods of fasting enhance the activity of genes linked to autophagy.[6] This suggests that intermittent fasting could potentially have a positive impact on autophagic activity. But since we don’t currently have a good way to measure autophagy in humans, we still can’t say for sure.
While good housekeeping is important in general, it’s likely especially important as we age. Just as some of us tend to accumulate clutter in our homes as we get older and acquire more “stuff,” our brains can do the same, building up cellular “gunk” that may diminish cognition or contribute to neurodegeneration.
So, fasting may also be part of healthy brain aging.
Fasting and neurodegeneration
Fasting is being explored as part of the treatment or prevention of many neurodegenerative diseases (i.e., diseases where brain or nervous system tissues break down or don’t work as well) due to its potential for enabling ketosis, among other physiological processes.[4,7]
For instance, evidence from research on Alzheimer’s disease suggests that fasting may help reduce the buildup of harmful brain plaques and slow down memory loss.
Although we’re not entirely sure why this works, some think it’s because intermittent fasting could activate pathways that make brain cells tougher and fight inflammation.[7] However, research in this area is still in its infancy, so it’s too soon to explain exactly how this works.
For Parkinson’s disease, intermittent fasting might offer some protection, too.
Some smaller studies have shown promising results with Parkinson’s patients on ketogenic diets, but we need much more research to say for sure.[8]
Fasting and insulin
High circulating insulin levels — which can lead to insulin resistance, a situation where cells can’t effectively respond to the signal of insulin — are increasingly linked to worse brain health.[9]
By lowering circulating insulin, fasting may help alleviate this and indirectly support brain health.[10] Exercise also helps support insulin sensitivity in the brain, so there’s one more vote for those nice little fasted strolls![11–13]
If you’re ready to give intermittent fasting a try, take our SIMPLE quiz and start your fasting journey!
Will fasting make you smarter?
While there’s interesting evidence (mainly from animal studies) that fasting may slow neurodegenerative processes, it’s not clear whether it reliably boosts cognitive function in humans, especially neurotypical people (i.e., people with no diagnosed cognitive conditions or neurological variations).[14]
However, many people report feeling “sharper” when fasting.
This is likely due to the release of catecholamine hormones such as adrenaline (epinephrine) and noradrenaline (norepinephrine), which are linked to alertness and focus.[15]
On the other hand, other people report feeling “brain fog” with less focus and concentration.
Likely, there are individual variations in how fasting affects people’s cognitive performance, focus, and concentration.
Take-home: Experiment and notice how you feel when you fast.
Do you feel more alert and focused? If so, then fasting may be a good fit for times when you need more mental horsepower.
Do you feel fuzzy and foggy? If so, then perhaps time your fast, so you aren’t doing anything important during that period.
Will fasting make you happier?
Cheers to feeling fantastic! Explore the happiness boost of intermittent fasting. Lift your glass, enjoy the journey, and let the positive vibes flow.
What’s good for your brain (and the rest of your body) is good for your emotions, and intermittent fasting may also play a positive role here.
Many people report feeling more energetic and positive while fasting.[15] In part, this may reflect the effects of dopamine, a neurotransmitter (brain chemical) involved in getting us motivated to seek rewards.
In our evolutionary past, hunger motivated us to leave the safety of our burrows and go find food. The dopamine system is part of helping us take risks and get moving.[16] For some people, this activation of dopamine and surge of activity may help alleviate symptoms of depression.[17,18]
There’s also emerging evidence that fasting may play a role in helping alleviate some types of chronic pain, likely by working through some similar mechanisms, but research is in its infancy, and we look forward to seeing what may come out of this.[19,20]
Take-home: Try fasting out for yourself and observe how you feel during your fasts. Do you feel better? If so, great!
SIMPLE’s expert opinion and final thoughts
Body health contributes to brain health. So, all the healthy behaviors that support a healthy body also support a healthy brain.
Notice how you feel and function while fasting. If you feel sharper, clearer, and more alert, that’s a great sign. If you feel fuzzier and more foolish, and it never improves, try adjusting your fasting window or exploring other approaches.
Try to cover all your brain health bases. Along with fasting, you can promote brain health with other healthy choices like:
good nutrition, especially omega-3s, antioxidants, B vitamins, and minerals, including magnesium and zinc from a range of whole grains, fruits, vegetables, and lean proteins[21]
regular exercise (at least 20 minutes a day of walking and some strength training is great)[22]
Make sure fasting is right for you. While fasting can be part of an overall healthy lifestyle, if you have a known cognitive or neurological condition, always consult your healthcare provider before making any changes.
We also don’t recommend fasting for people who:
have type 1 diabetes
are currently pregnant or breastfeeding
are younger than 18 or 80 and older
are on certain medications, including ones for blood pressure or blood sugar lowering
have an eating disorder or disordered eating tendencies (or have a history of one)
People with obesity and type 2 diabetes lost more weight using daily periods of fasting than by trying to restrict calories over a six-month period.
Blood sugar levels lowered in people in both groups, and no serious side effects were observed.
Time-restricted eating, also called intermittent fasting, may be an effective weight loss strategy. Katecat / Adobe Stock
Around 1 in 10 Americans live with type 2 diabetes, a disease in which levels of blood glucose, or blood sugar, are too high. Diabetes can lead to serious health issues such as heart disease, nerve damage, and eye problems.
Excess weight is a major risk factor for the development of type 2 diabetes, and weight loss is often recommended for those with excess weight and type 2 diabetes. Calorie restriction—reducing overall calorie intake—is a mainstay of most weight loss programs. But such regimens are very difficult to stick with over the long term.
Time-restricted eating, also called intermittent fasting, has emerged as an alternative weight loss paradigm. In this approach, the time of day during which food can be eaten is restricted, but the amount or types of food are not. Small studies have suggested that intermittent fasting is safe and promotes weight loss in people with type 2 diabetes. But these studies only tracked participants for a short period of time. They also didn’t compare the approach with traditional calorie restriction.
In a new clinical trial, an NIH-funded research team led by Dr. Krista Varady from the University of Illinois Chicago compared fasting and calorie restriction for weight loss and blood-sugar reduction. They recruited 75 people with obesity and type 2 diabetes. Of these, 70 were either Hispanic or non-Hispanic Black—two groups in the U.S. with an especially high prevalence of diabetes. The participants were randomly assigned to one of three diet groups for six months.
The fasting group could eat anything they wanted, but only between the hours of noon and 8 pm. The second group worked with a dietitian to reduce their calories by 25% of the amount needed to maintain their weight. A control group did not change their diet at all. All groups received education on healthy food choices and monitored their blood glucose closely during the study. The results were published on October 27, 2023, in JAMA Network Open.
After six months, participants in the fasting group lost an average of 3.6% percent of their body weight compared to those in the control group. In comparison, people in the calorie-restriction group did not lose a significant amount of weight compared to the control group.
Both groups had similarly healthy decreases in their average blood glucose levels. Both also had reductions in waist circumference. No serious side effects, including time outside of a safe blood glucose range, were seen in either treatment group. People in the fasting group reported that their diet was easier to adhere to than calorie restriction.
“Our study shows that time-restricted eating might be an effective alternative to traditional dieting for people who can’t do the traditional diet or are burned out on it,” Varady says. “For many people trying to lose weight, counting time is easier than counting calories.”
Some medications used to treat type 2 diabetes need adjustment for time-restricted eating. Therefore, people considering intermittent fasting should speak with a doctor before changing their eating pattern.
People with obesity and type 2 diabetes lost more weight using daily periods of fasting than by trying to restrict calories over a six-month period.
Blood sugar levels lowered in people in both groups, and no serious side effects were observed.
Time-restricted eating, also called intermittent fasting, may be an effective weight loss strategy.
Around 1 in 10 Americans live with type 2 diabetes, a disease in which levels of blood glucose, or blood sugar, are too high. Diabetes can lead to serious health issues such as heart disease, nerve damage, and eye problems.
Excess weight is a major risk factor for the development of type 2 diabetes, and weight loss is often recommended for those with excess weight and type 2 diabetes. Calorie restriction—reducing overall calorie intake—is a mainstay of most weight loss programs. But such regimens are very difficult to stick with over the long term.
Time-restricted eating, also called intermittent fasting, has emerged as an alternative weight loss paradigm. In this approach, the time of day during which food can be eaten is restricted, but the amount or types of food are not. Small studies have suggested that intermittent fasting is safe and promotes weight loss in people with type 2 diabetes. But these studies only tracked participants for a short period of time. They also didn’t compare the approach with traditional calorie restriction.
In a new clinical trial, an NIH-funded research team led by Dr. Krista Varady from the University of Illinois Chicago compared fasting and calorie restriction for weight loss and blood-sugar reduction. They recruited 75 people with obesity and type 2 diabetes. Of these, 70 were either Hispanic or non-Hispanic Black—two groups in the U.S. with an especially high prevalence of diabetes. The participants were randomly assigned to one of three diet groups for six months.
The fasting group could eat anything they wanted, but only between the hours of noon and 8 pm. The second group worked with a dietitian to reduce their calories by 25% of the amount needed to maintain their weight. A control group did not change their diet at all. All groups received education on healthy food choices and monitored their blood glucose closely during the study. The results were published on October 27, 2023, in JAMA Network Open.
After six months, participants in the fasting group lost an average of 3.6% percent of their body weight compared to those in the control group. In comparison, people in the calorie-restriction group did not lose a significant amount of weight compared to the control group.
Both groups had similarly healthy decreases in their average blood glucose levels. Both also had reductions in waist circumference. No serious side effects, including time outside of a safe blood glucose range, were seen in either treatment group. People in the fasting group reported that their diet was easier to adhere to than calorie restriction.
“Our study shows that time-restricted eating might be an effective alternative to traditional dieting for people who can’t do the traditional diet or are burned out on it,” Varady says. “For many people trying to lose weight, counting time is easier than counting calories.”
Some medications used to treat type 2 diabetes need adjustment for time-restricted eating. Therefore, people considering intermittent fasting should speak with a doctor before changing their eating pattern.
Some people undertaking intermittent fasting may experience headaches. This can be due to several factors, such as hypoglycemia, dehydration, and caffeine withdrawal.
Intermittent fasting is an eating method that a person may adopt for religious, dietary, or health reasons. It involves cycling between periods of eating and fasting.
Intermittent fasting may provide some health benefits, such as weight loss and decreased risk of certain diseases. However, fasting may not be suitable for all people, and some individuals may experience side effects, such as headaches.
Design by MNT; Photography by Blasius Erlinger/Getty Images & MirageC/Getty Images
People doing intermittent fasting may experience headaches. The results of a small 2023 study suggest that roughly 61%Trusted Source of individuals experience headaches while on an intermittent fast.
Secondary headaches refer to headaches that people can trace back to a specific cause. Common triggers of secondary headaches are changes to homeostasis. This term describes the internal system that helps regulate bodily functions.
As such, intermittent fasting can disturb homeostasis and result in headaches.
However, not everyone will experience headaches. Factors such as individual differences, fasting methods, and a person’s overall health may play a part in whether someone will have a headache during an intermittent fast.
Older research suggests that fasting headaches either occur in the frontal region, meaning a person feels the pain at the front of their head, or are diffuse, meaning the pain spreads. Usually, a fasting headache may also be non-pulsating and of mild or moderate intensity.
In addition to a headache, a person undertaking intermittent fasting may also experience feelings of dizziness and weakness.
There are several potential reasons why a person may experience a headache while fasting intermittently. These may include:
Hypoglycemia: Some people may experience hypoglycemia, or low blood sugar levels, duringTrusted Source periods of fasting. When blood sugar levels fall too low, a person may experience symptoms such as a headache.
Caffeine withdrawal: Many people rely on caffeine to stay alert or focused. During fasting, a person may reduce or eliminate their caffeine consumption. This can cause withdrawal symptomsTrusted Source, such as headaches.
Dehydration: Dehydration occurs when a person does not consume enough fluids. Headaches are a commonTrusted Source symptom of dehydration. When a person is undergoing an intermittent fast, the lack of regular meals may mean a person is not consuming enough fluids.
Stress and anxiety: Fasting can causeTrusted Source a person’s cortisol levels to increase. Cortisol is a hormone that can rise in response to feelings of stress and anxiety. Other research suggests a relationship between stress and the occurrence of headaches.
For people undertaking intermittent fasting, there are a few precautionary steps they can take to avoid having a headache. These may include:
Gradually transitioning: If a person is new to intermittent fasting, they may want to startTrusted Source with short periods and gradually increase their fasting duration. This may help the body adapt without triggering headaches.
Staying hydrated: It is important to be mindful of water consumption during the fasting period. A person can consume a variety of beverages, such as herbal teas and water with electrolyte supplements.
Monitoring blood sugar levels: Experts advise watching for symptoms of hypoglycemia. These can includeTrusted Source shakiness, dizziness, and sweating. Having a small snack or sugary beverage may help return glucose levels to a healthy range. However, it is best to consult a doctor about managing blood sugar levels.
Managing stress: Incorporating stress-reduction techniques, such as meditation, deep breathing exercises, and yoga, can help manage stress during fasting.
Moderating caffeine intake: If planning to eliminate caffeine during fasting, a person can try to gradually reduce their caffeine intake to minimize withdrawal symptoms, such as headaches.
If a person experiences severe or persistent headaches when fasting, it is best to consult a healthcare professional. They will be able to provide personalized guidance and ensure there are no underlying health issues causing the headaches.
Digestive changes: If a person’s body is not familiar with fasting, they may experience digestive discomfort, such as a stomachache or constipation. Additionally, individuals with preexisting digestive conditions may experience an exacerbationTrusted Source of symptoms.
Irritability and fatigue: Short-term fasting may causeTrusted Source a person to experience low mood, fatigue, and irritability.
Bad breath: During periods of fasting, a person may experience bad breath, which is also known as halitosis. Maintaining good oral hygiene and drinking plenty of water can help address this issue.
Sleep issues: Research is inconclusiveTrusted Source on whether intermittent fasting affects sleep. Anecdotal evidence suggests some people may experience disturbed sleep while fasting.
Malnutrition: Extended and unbalanced fasting can lead to malnutrition. It is important to ensure that during periods of eating, a person is consuming essential nutrients.
Additionally, it is important to consider that intermittent fasting may not suit all individuals. For example, experts advise not fasting if a person is ill or living with certain health conditions, such as diabetes.
Intermittent fasting is a dietary approach with various potential health benefits, but it can sometimes lead to headaches and other side effects.
To help prevent fasting headaches, it is essential to maintain proper hydration, manage blood sugar levels, and manage stress effectively. Gradually transitioning into fasting and consulting healthcare professionals as necessary can also make fasting more comfortable, particularly for individuals with preexisting health conditions.
While fasting headaches are common, many people may be able to fast intermittently with minimal side effects. However, a person should listen to their body and make adjustments as necessary to find the fasting routine that works best for them.
People with obesity and type 2 diabetes lost more weight using daily periods of fasting than by trying to restrict calories over a six-month period.
Blood sugar levels lowered in people in both groups, and no serious side effects were observed.
Time-restricted eating, also called intermittent fasting, may be an effective weight loss strategy. Katecat / Adobe Stock
Around 1 in 10 Americans live with type 2 diabetes, a disease in which levels of blood glucose, or blood sugar, are too high. Diabetes can lead to serious health issues such as heart disease, nerve damage, and eye problems.
Excess weight is a major risk factor for the development of type 2 diabetes, and weight loss is often recommended for those with excess weight and type 2 diabetes. Calorie restriction—reducing overall calorie intake—is a mainstay of most weight loss programs. But such regimens are very difficult to stick with over the long term.
Time-restricted eating, also called intermittent fasting, has emerged as an alternative weight loss paradigm. In this approach, the time of day during which food can be eaten is restricted, but the amount or types of food are not. Small studies have suggested that intermittent fasting is safe and promotes weight loss in people with type 2 diabetes. But these studies only tracked participants for a short period of time. They also didn’t compare the approach with traditional calorie restriction.
In a new clinical trial, an NIH-funded research team led by Dr. Krista Varady from the University of Illinois Chicago compared fasting and calorie restriction for weight loss and blood-sugar reduction. They recruited 75 people with obesity and type 2 diabetes. Of these, 70 were either Hispanic or non-Hispanic Black—two groups in the U.S. with an especially high prevalence of diabetes. The participants were randomly assigned to one of three diet groups for six months.
The fasting group could eat anything they wanted, but only between the hours of noon and 8 pm. The second group worked with a dietitian to reduce their calories by 25% of the amount needed to maintain their weight. A control group did not change their diet at all. All groups received education on healthy food choices and monitored their blood glucose closely during the study. The results were published on October 27, 2023, in JAMA Network Open.
After six months, participants in the fasting group lost an average of 3.6% percent of their body weight compared to those in the control group. In comparison, people in the calorie-restriction group did not lose a significant amount of weight compared to the control group.
Both groups had similarly healthy decreases in their average blood glucose levels. Both also had reductions in waist circumference. No serious side effects, including time outside of a safe blood glucose range, were seen in either treatment group. People in the fasting group reported that their diet was easier to adhere to than calorie restriction.
“Our study shows that time-restricted eating might be an effective alternative to traditional dieting for people who can’t do the traditional diet or are burned out on it,” Varady says. “For many people trying to lose weight, counting time is easier than counting calories.”
Some medications used to treat type 2 diabetes need adjustment for time-restricted eating. Therefore, people considering intermittent fasting should speak with a doctor before changing their eating pattern.
Intermittent fasting is a dietary pattern that has seen an increase in popularity in recent years, especially for weight management. But what actually is it, and is it right for you? This article aims to answer your questions, so you can feel ready for a conversation about weight management.
It may often seem like there is always a ‘new kid on the block’ when it comes to weight management methods, and intermittent fasting is one that has been slowly increasing in popularity in recent years, in particular the catchy 5:2 method. For those who have tried and tested several weight management approaches, it may be difficult to navigate the truth, and to try and understand what works best for you. The FAQs below are based on the science (albeit limited science so far!), so you can get a better understanding of the benefits and limitations of intermittent fasting.
It’s important to remember that people living with obesity are more likely to have micronutrient deficiencies, including an iron or vitamin D deficiency. Therefore, please speak to your healthcare provider before you start to restrict your diet, as they can perform the necessary assessments to support your health.
FAQs:
1. What is intermittent fasting, and how do you do it?
Intermittent fasting is a sweeping term that can be used when describing a program to restrict calorie-intake at certain times of the day, or week. This leads to periods of time known as extended fasting, where intake is typically around 25% of your energy needs.
One of the most popular and well known intermittent fasting methods is the 5:2 approach, where the extended fasting occurs on 2 non-consecutive days in the week. Another common approach is to follow ‘time-restricted eating’ where consumption is restricted to an 8-hour period within the day, and therefore the extended fast period is 16 hours daily.
It is important to understand that many people around the world also fast for religious reasons, and this should not be confused with intermittent fasting. For example, during the holy month of Ramadan, many complete a 12 hour fast each day, from sun rise to sunset.
2. How does intermittent fasting compare to popular diets?
It is important to remember that the scientific research for intermittent fasting is still evolving. Most recently, researchers who studied around 600 participants with overweight or obesity, have found that when following an intermittent fasting approach, people lost on average between 4-8% of their body weight over a six-month period. When compared to other diets that require a continuous restriction on energy intake, similar levels of weight loss were achieved.
When comparing weight management approaches, it is important to not just look at how much weight is possible to lose. The popular and well researched Mediterranean diet, where food intake can be characterised by a high level of plant-oriented food, and low levels of processed food, generally sees a similar level of weight loss, but has greater impact on your body’s cardiovascular system. A recent Spanish study found that an energy-unrestricted Mediterranean diet can reduce the risk of a major cardiovascular event by up to 30%. This study focussed specifically on people who lived in Mediterranean countries with an already high cardiovascular risk, so it’s important to understand your own body when comparing weight management approaches. You can find out more about the benefits of weight loss to your cardiovascular system here.
It is important to remember that when it comes to weight loss, changes to your diet are just one part of long-term management. Your healthcare provider will be able to recommend a weight management plan that is best for your lifestyle, as there is no ‘one size fits all’ method when it comes to managing your weight.
3. How does intermittent fasting affect your BMI?
Your Body Mass Index, often referred to as BMI, is a measurement of height compared with weight. It is the most common way to assess for overweight and obesity. For example, a BMI of 25 kg/m2 or ≥ 30 kg/m2, indicates overweight and obesity respectively.
For some people, intermittent fasting can lead to a reduction in BMI, although despite its widespread use in the medical community, BMI alone as an indicator for obesity has limitations that you should be aware of when using the indicator as a measurement of health.
4. Should you restrict your food intake if you are prescribed a weight management medication?
When talking to your healthcare provider about weight, they may prescribe you a weight management medication. There are several types of weight management medication, and they work with the chemistry in your brain that causes weight gain and prevents weight loss. Weight management medications are prescribed by healthcare professionals as a long-term approach to weight loss and can limit weight regain if continuously used.
Medications should always be taken alongside changes in nutrition and physical activity, rather than as an alternative.
As previously mentioned, for some, fasting is a religious practice rather than a tool for weight management. Studies into whether weight management medication is still effective during periods of fasting have so far shown that there is no change in how they work. However, some weight management medications can impact a person’s iron, vitamin D and other nutrient levels, so it is really important to consult your healthcare professional before you embark on periods of fasting when prescribed a weight management medication.
5. Do the outcomes of intermittent fasting differ between males and females?
As a common theme, the research into biological sex differences for intermittent fasting is limited, however there is no significant difference in how women respond to intermittent fasting versus men.
6. Should intermittent fasting be seen as a long-term approach to weight management?
It is important for your doctor to tailor a nutrition plan that meets your personal preferences, and treatment goals. This can take in a variety of factors that ensure safety, nutritionally sufficient, affordable and suitable changes for your lifestyle.For some, intermittent fasting may be appropriate, but there is limited evidence to show it can lead to weight reduction in the long-term.
Continuous calorie restriction may increase the chances of weight gain in the long-run, as this type of restriction can change the way your body signals hunger, satiety and body weight regulation.
In cases where behavioural interventions (diet and exercise) don’t work to manage your weight, psychological therapy, pharmacological therapy, or bariatric surgery can help you lose weight and prevent weight regain. For long-term weight management strategies, it is best to speak to your healthcare provider, who can assess your lifestyle to work out the best approach for you.
This approach can benefit individuals dealing with chronic insomnia and anxiety, too.
Intermittent fasting has become a go-to strategy for health and weight management for more and more people. Recent research suggests that avoiding eating for a minimum of 14 hours each night and restricting eating to a 10-hour window during the day (14:10) can enhance sleep, mood, and energy levels while reducing feelings of hunger.
This approach can benefit individuals dealing with chronic insomnia and anxiety, too. It can also be helpful for those facing issues such as fatigue and increased hunger resulting from dieting for weight loss.
The Different Forms of Fasting
There are various forms of intermittent fasting. In a 24-hour cycle, the most rigorous form is perhaps one-meal-a-day (23:1) fasting, while the more popular ones include 18:6 (essentially two meals a day, or eating only within a six-hour window) and 16:8 fasting. Intermittent fasting can also be structured on a weekly basis, such as the 5:2 (two days of light fasting each week whereby you eat less than 500 calories) and alternate-day fasting, whereby you eat less than 500 calories every other day.
Compared to these other intermittent fasting methods, the 14:10 fasting approach has a shorter fasting period and a longer eating window. It doesn’t significantly disrupt the usual three meals a day, making it relatively more flexible and easier to sustain. For example, if you start eating at 8 a.m., your last meal must be finished before 6 p.m., with no eating afterward.
What the Science Says
A research team from King’s College London conducted a study with 37,545 participants using the Zoe Health app. Results of the study were presented at the European Nutrition Conference on Nov. 14, 2023, in Belgrade, Serbia. In the first week of the study, participants were instructed to maintain their usual dietary habits, followed by two weeks during which they restricted their meals to a 10-hour window each day, referred to as the “eating window.” More than 36,231 participants chose to continue for additional weeks, with 27,371 considered “highly engaged.” Among this highly engaged group, 78 percent were female, with an average age of 60 and a body mass index (BMI) of 25.6 (normal range is between 18.5 and 24.9).
The study found that those who consistently ate within a fixed eating window had greater benefits than those with varying eating windows. Moreover, individuals who had the longest duration between their first and last bite each day before the intervention experienced greater improvements in their overall health.
Sarah Berry, associate professor at King’s College London and chief scientist at Zoe, said in a statement: “What’s really exciting is that the findings show that you don’t have to be very restrictive to see positive results.
“A ten-hour eating window, which was manageable for most people, improved mood, energy levels and hunger.”
However, she also emphasized the importance of consistency.
“We found for the first time that those who practiced time-restricted eating, but were not consistent day to day, did not have the same positive health effects as those who were dedicated every day,” she said.
Kate Bermingham from King’s College London and Zoe said that the study further demonstrates the importance of how we eat.
“The health impact of food is not just what you eat but the time at which you choose to consume your meals,” she said. “Findings show that we don’t need to be eating all the time. Many people will feel satiated and even lose weight if they restrict their food to a ten-hour window.”
A study published in Nature Medicine in April 2023 involving more than 200 participants and spanning 18 months found intermittent fasting more effective in reducing the risk of Type 2 diabetes than a calorie-restricted diet. In this experiment, intermittent fasting involved selecting three nonconsecutive days each week as fasting days, during which participants consumed two meals between 8 a.m. and 12 p.m., totaling only 30 percent of their daily energy requirements.
They refrained from eating anything for the remaining 20 hours and were free to eat ad libitum on the other four days of the week. After six months, participants practicing intermittent fasting showed a significant reduction in postprandial blood sugar, indicating enhanced glucose metabolism. This improvement was more pronounced than that of the group following a calorie-restricted diet.
A small-scale clinical trial published in Cell Metabolism in 2020 focusing on patients with metabolic syndrome found that following a 14:10 intermittent fasting plan for 12 weeks not only aided in weight loss but also resulted in reductions in waist circumference, body fat percentage, visceral fat, and improvements in blood pressure, atherogenic lipids, glycated hemoglobin (a blood sugar indicator), and sleep quality.
Exploring the Health Benefits of Intermittent Fasting
A review study published in The New England Journal of Medicine (NEJM) in 2020 summarized three intermittent fasting methods: alternate-day fasting, 5:2 fasting, and one-meal-a-day fasting, highlighting their numerous health benefits. These include improved glucose regulation, enhanced stress resistance, and suppression of inflammation. Intermittent fasting has demonstrated potent alleviating effects on various chronic diseases in animal models, such as effects on obesity, diabetes, cardiovascular diseases, cancer, and neurodegenerative brain disorders such as Alzheimer’s and Parkinson’s diseases.
Mark Mattson, a Johns Hopkins School of Medicine neuroscientist and one of the authors of the study, said in a statement that after several hours without eating, the body depletes its sugar stores and begins to burn fat. The purpose of intermittent fasting is to prolong the body’s fat-burning period.
The Key to Success in Intermittent Fasting
Regardless of the chosen intermittent fasting method, achieving optimal results relies on long-term consistency, as indicated by multiple studies. The body may take two to four weeks to adapt to intermittent fasting, during which you might experience hunger or irritability. After overcoming this initial phase, sustaining the practice in the long run becomes more manageable.
Therefore, it’s advisable to start intermittent fasting with the easiest method, such as 14:10. After adapting for a certain period, you can then progress to more rigorous fasting methods such as 16:8 or 18:6.
It’s worth noting that regular eating outside the fasting window doesn’t imply unrestrained indulgence, even though intermittent fasting doesn’t impose specific dietary restrictions. A study published in the Journal of the American Heart Association (JAHA) in January 2023 found that over a six-year follow-up period, the time interval between participants’ first and last meal each day wasn’t directly correlated with changes in body weight. Instead, reducing the frequency of large meals could be associated with weight control.
Additionally, longer fasting periods aren’t necessarily better. Extended durations of fasting, such as 24, 36, or 48 hours, pose risks and may not yield additional benefits. This is because prolonged periods without food might prompt your body to store more fat in response to hunger.
Is Intermittent Fasting Safe?
Intermittent fasting may lead to unpleasant side effects, including hunger, fatigue, insomnia, nausea, headaches, and more, but these symptoms usually subside within a month.
Intermittent fasting is generally safe for most people but may not suit everyone. Pregnant or breastfeeding women, children and adolescents under 18 years old, those with Type 1 diabetes taking insulin, and individuals with a history of eating disorders should avoid intermittent fasting. It’s advisable to consult a doctor before attempting an intermittent fasting regimen.
Combining aerobic exercise such as swimming with fasting may be beneficial for people with liver disease, a study suggests. George Pachantouris/Getty Images
Nonalcoholic fatty liver disease involves the buildup of fat in the liver.
Fat buildup in the liver is not initially dangerous, but it can put people at risk for other health problems.
Data from a recent study suggests that combining aerobic exercise, or cardio, with intermittent fasting may improve nonalcoholic fatty liver disease.
The liver is a crucial organ in the body that can influence many areas of health. The buildup of fat in the liver, referred to as nonalcoholic fatty liver disease (NAFLD),Trusted Source is becoming increasingly more common in certain countries.
Researchers are still working to understand the full impact of NAFLD and the best lifestyle choices to reduce the buildup of fat in the liver.
A recent studyTrusted Source examined the effectiveness of intermittent fasting and aerobic exercise in reducing fat buildup in the liver. The researchers found that combining these two methods effectively reduced fat levels in the liver.
NAFLDTrusted Source is when fat builds up in the liver. Under this broad term are two subtypes: nonalcoholic fatty liver (NAFL) and nonalcoholic steatohepatitis (NASH). Nonalcoholic fatty liver is when there is just fat buildup but no damage to the liver. Nonalcoholic steatohepatitis is when there is fat buildup, liver inflammation, and liver damage.
NAFLDTrusted Source is growing more common, including a high prevalence among adults who are obese or who have diabetes.
Dr. Na Li, gastroenterologist and hepatologist, who was not involved in the study, explained to Medical News Today:
“Nonalcoholic fatty liver disease is a medical condition when excessive fat accumulates in the liver. It may cause liver inflammation, scarring, and eventually cirrhosis. This condition has become the most common chronic liver disease and a leading cause for liver transplantation in the United States.”
“People with obesity and type 2 diabetes are particularly at high risk to develop nonalcoholic fatty liver disease.” — Dr. Na Li
Nonalcoholic fatty liver on its own doesn’t necessarily lead to more severe liver problems, but it can increase someone’s risk for cardiovascular disease and metabolic syndrome.
People with NAFLD can make helpful lifestyle modifications like reaching and maintaining a healthy body weight. However, specialists are still researching the best health option action steps for people with NAFLD.
This particular study was a randomized controlled trial. The study authors conducted the trial over three months and included 80 participants in their analysis. They wanted to look at the effectiveness of different lifestyle interventions in improving fat content in the liver. All participants had obesity and NAFLD.
Study author and intermittent fasting researcher Dr. Krista Varady, explained the basis of the team’s research to MNT:
“We noticed that the main lifestyle therapy for nonalcoholic fatty liver disease (NAFLD) was combining daily calorie restriction with aerobic exercise. We were curious if intermittent fasting combined with aerobic exercise would produce the same reductions in liver fat.”
Intermittent fasting involves only eating during certain time intervals or having specific days with a high amount of calorie restriction. Researchers divided participants into one of four groups to measure improvement in fatty liver.
The first group participated in regular moderate-intensity aerobic exercise. The second group participated in alternate-day fasting, where they only consumed about 600 calories on fast days, and alternated with feast days, where their diet wasn’t restricted.
The third group participated in both an exercise program and intermittent fasting. Finally, the last group was a control group with no interventions.
The combination group demonstrated a variety of improvements, some of which were superior to the other intervention groups.
“We found that liver fat was reduced by 5.5% in the group that participated in both fasting and exercise. This combination group also reduced body weight by 5%, fat mass, waist circumference, and liver enzyme (ALT) levels. We also saw increases in insulin sensitivity in the combination group, indicating better blood sugar control.” — Dr. Krista Varady
Researchers found that the reduction of liver fat and body weight was similar between the combination group and the group that only participated in intermittent fasting.
Researchers also found that all three intervention groups saw similar improvement in insulin resistance. Thus, the combination intervention could be an option for people with NAFLD but is not necessarily a far-superior method.
Dr. Li said the study “adds to current literature regarding the benefits of weight loss on fatty liver. It is in line with many other studies that calorie restriction is a key component for weight control.”
“Based on this study, alternate day fasting would be adopted as a beneficial strategy by clinicians and patients for weight control and fatty liver treatment,” she told MNT.
The study did have certain limitations that are important to consider.
First, the study included a limited number of participants and ran only for a short time. Over 80% of the participants were female. This and the ethnicity of participants indicate the need for further research that is more diverse.
Second, the combination intervention showed improvements in the liver but not back into a healthy range, which could indicate the need for people with NAFLD to pursue additional interventions. It is also unclear if the interventions would be effective in people with more severe NAFLD.
Because of baseline numerical differences between groups, there was a chance to observe larger average absolute differences in the combination intervention group.
Finally, because of when the study was conducted, the COVID-19 pandemic influenced the exercise interventions and may have led to more variation among participants.
Dr. Varady noted that one of the first steps in continued research would be studies conducted over a longer time frame.
“The study was only 3 months long. I think the next step would be to run longer-term trials in this area (6-12 months long). This will help us determine if these improvements can be sustained over longer periods of time,” she said.
Is intermittent fasting right for you? (Shutterstock)
Intermittent fasting as a means of weight loss and improving overall health has become a popular practice. The diet involves eating only during specific times and fasting for longer periods between meals. Is intermittent fasting right for you? Understanding fasting and an evaluation of your current state of health can help you know.
Some benefits of intermittent fasting
Reduces inflammatory reactions in the body.
Reduces blood sugar levels in Type 2 diabetes.
Lowers blood pressure, cholesterol, and triglycerides.
During fasting, the body’s function changes from “production mode” to “survival mode,” burning sugar and fat stores. Fasting detoxifies the body, promotes clarity of mind, and boosts energy levels.
In ancient times, human beings hunted and gathered wild fruits and vegetables. Without continual food supplies, they were often in a state of fasting. The development of agriculture and animal husbandry created a stable food supply and the evolution of the three-meals-a-day concept.
Who should avoid fasting?
Intermittent fasting is not recommended for:
Children.
Underweight or weak people.
Pregnant or lactating women.
People with conditions such as anorexia, and bulimia.
People with stomach problems, hyperacidity, and cholecystitis
People with diabetes or low sugar blood.
People with hyperlipidemia, kidney disease, or gout should not do long-term fasting.
Introduce fasting gradually
After 12 hours of fasting, blood ketone levels will gradually rise, and the body will start burning fat. Modern people are well-fed and seldom suffer from hunger—thus a good choice for them is “intermittent fasting,” also known as “light fasting.” Light fasting means choosing a fixed period of the day to not eat and only drink water, or non-caloric beverages such as plain coffee or tea.
The practice of fasting should be introduced gradually and take into consideration your current state of health, your workload, how much time you have to rest, social commitments, etc. Adapt fasting as a lifestyle step by step—if hunger causes adverse effects, shift the number of hours not eating to fewer, and gradually increase as the body adjusts.
Light fasting should not cause muscle loss—if it does there may be insufficient protein intake. Replenishing premium protein and moderate exercise can ensure no muscle loss during fasting.
The discomforts of fasting
Fasting usually does not cause major discomfort. However, some of the following may be experienced:
Feeling cold or chilled. As fat is burned in the body thyroid function may drop temporarily causing cold hands and feet.
Mood swings may occur as a symptom of low blood sugar and are usually temporary.
Headache. Transitioning from burning sugar to burning fat may cause a temporary headache.
Burning sensation in the stomach. If the accumulation of stomach acid causes too much discomfort you can eat something to slow down the fasting pace.
Nausea. Usually, the symptoms of nausea will ease naturally.
Dizziness. Be sure to sit down, and consider eating a bit if it persists.
Hair loss or fatigue. Be sure that when you are in times of eating that your diet is balanced and healthy to meet all the nutritional needs of the body .
In short, if the symptoms are not serious, they will usually improve within a few days—but you should stop fasting if the symptoms are not relieved.
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