U.S. Army recruits taking a daily calcium and vitamin D supplement during basic training had no change in tibial microarchitecture compared with placebo, according to study data published in Bone.

“A calcium and vitamin D fortified food product prevented increases in biochemical markers of bone resorption, but there were no significant changes in tibial density or microarchitecture in men and women after military training,” Erin Gaffney-Stomberg, PhD, RD, division chief of the combat feeding division at the U.S. Army Research Institute of Environmental Medicine in Natick, Massachusetts, and colleagues wrote. “In addition, markers of bone formation increased, and parathyroid hormone decreased equally in both groups over the course of training.”

Researchers conducted a randomized controlled trial of 50 male and 50 female U.S. Army recruits aged 17 to 42 years in basic training at Fort Jackson in South Carolina between April and June 2015. Participants had not been pregnant or breastfeeding in the 6 months before the study and had no self-reported history of endocrine or bone-modifying disorders, kidney disease, renal calculi or glucocorticoid prescription within 2 years. Participants were randomly assigned to an intervention group receiving a snack bar fortified with 1,000 mg of calcium and 1,000 IU of vitamin D daily, or a control group receiving a placebo snack bar. A background questionnaire was conducted at baseline to obtain demographic information. Anthropometric measurements and fasting blood samples were collected at baseline and at an 8-week follow-up. High-resolution peripheral quantitative CT was conducted at baseline and follow-up to measure density, microarchitecture and strength at the distal metaphyseal and diaphyseal regions of the tibia.

There were 93 recruits who completed the study, including 45 in the supplement group and 47 in the placebo group. At 8 weeks, increases in total volumetric bone mineral density, trabecular volumetric BMD, trabecular number, trabecular thickness, and trabecular bone volume/total volume, and a decrease in trabecular separation were observed for all participants. Women had changes in all bone parameters, and men had a change in all parameters except for trabecular number and separation.

All participants experienced increases in stiffness and failure load at the distal metaphyseal site, with no significant difference between the supplement and placebo groups. Stiffness and failure load did not change among women, but men had increases in stiffness of 1.3% and failure load of 3.4% at 8 weeks compared with baseline. (P < .05).

Increases in cortical perimeter and cortical volumetric BMD were observed in both groups at 8 weeks compared with baseline, but there were no other parameter changes at the diaphyseal site. Women experienced a decrease in cortical porosity and cortical volumetric BMD at 8 weeks, but there was no difference between the supplement and placebo groups. Men in both groups had an increase in cortical perimeter and decrease in cortical volumetric BMD at 8 weeks. Small increases in stiffness and failure load at the diaphyseal site were observed in women for both groups, but not in men.

“These results indicate that bone density and microarchitecture were not impacted by calcium and vitamin D supplementation at the doses tested,” the researchers wrote. “However, post hoc power analysis indicated that the current study achieved 35.8% power to detect a difference by group, and 106 volunteers per group would be required to detect a statistically significant difference if one exists. Thus, underpowering cannot be excluded as a possibility.”

The researchers said studies powered to examine stress fracture outcomes along with bone density and microarchitecture changes at varying doses of calcium and vitamin D are needed to better understand the effects of supplementation. They said future studies should also involve diverse groups of participants and take place during different times of the year.