Sugar is a bigger threat to health than many suppose, according to a study which claims it causes metabolic diseases such as high blood pressure and heart diseasewhether or not we put on weight.

The study was carried out by Robert Lustig, a paediatric endocrinologist in San Francisco and author of the book “Fat Chance: the hidden truth about sugar”.Writing in the Guardian, he says the health of 43 obese children in the care of his clinic dramatically improved when the sugar in their diet was replaced with starchy foods like crisps. They ate the same number of calories, he says, and yet their metabolic disease, which can cause diabetes, was reversed within 10 days.

The study, published in the journal Obesity, is evidence, says Lustig, that “a calorie is not a calorie” – the claim he makes in his book and which has been contested by many scientists who say the damage sugar does is through its calorific content alone.

In the study, carried out at the UCSF Benioff children’s hospital San Francisco and Touro University, California, the 43 children, aged nine to 18, had all been referred to hospital because of their weight and significant related health issues, such as high blood pressure.

They were given nine days of food prepared for them by the clinic and told to weigh themselves daily. The added sugar in their diet was reduced from 28% to 10% and the fructose – a form of sugar believed to be particularly problematic – from 12% to 4% of total calories. Sugary food was replaced by starchy food such as turkey, hot dogs, crisps and pizza.

After nine days, the researchers say, most aspects of the children’s metabolic health improved – their diastolic blood pressure, “bad” LDL cholesterol and triglycerides dropped, fasting blood glucose went down and insulin levels were cut by a third. Their liver function test results improved.

But in spite of Lustig’s assertions that he has proved his thesis, the reaction to the study was muted. “The results are not convincing to me – this is a very small study, and it has not been statistically well-controlled,” said Naveed Sattar, professor of metabolic medicine at Glasgow University.

“Also, when people are losing weight, even if modest, their metabolic changes can seem larger than they actually are – one needs to see results once folk return to their habitual state after they’ve finished losing weight. Overall, this study is of modest interest but is far from convincing.”

Tom Sanders, professor emeritus of nutrition and dietetics at King’s College London, said the study needed to be viewed “with some scepticism” because it was uncontrolled. It did not compare the children with a similar group who continued to eat a high-sugar diet. The comparison instead was made with their weight and health before the study while on their usual diet. “But it is well known that obese children underestimate and under-report food intake, particularly of soft drinks and snack foods,” said Sanders.

“This is a fundamental flaw in the study. It is likely that the changes in metabolic outcomes observed can be explained by the experimental diet providing fewer calories than the children’s usual intake.”

Given that sugar and starch contain roughly the same amount of calories per gram, he said: “It is just inconceivable that isocaloric substitution of sugar from starch would have such a large effect on metabolism. In fact it denies the basic laws of thermodynamics.”