Cutting way back on calorie intake for six months doesn’t take a bite out of bones in people who haven’t yet reached middle age, a new study shows.
The finding, reported online and in the Sept. 22 Archives of Internal Medicine, thickens the debate over calorie restriction: A 2006 study had suggested that quick weight loss weakened bones in people over age 50.
The current study looks at volunteers with average ages in the late 30s. It’s possible that the reports’ different results stem from the age discrepancy, says Luigi Fontana, a physician and gerontologist at WashingtonUniversity in St. Louis who coauthored the earlier study.
Calorie restriction — eating fewer calories than necessary for maintaining a steady body weight — seems to offer health benefits and possibly a way to stretch a person’s longevity. But researchers are now investigating whether there might be a downside to the practice, apart from being hungry.
The new study focused on bone density over a short time period, as measured by X-rays before volunteers began dieting and again after the volunteers had dieted for six months. Leanne Redman, an endocrinologist at the Pennington Biomedical Research Center of Louisiana State University in Baton Rouge, and her colleagues randomly assigned 46 healthy but slightly overweight people to maintain one of four diets for that time period. Each diet group included roughly a dozen volunteers.
One group served as a control, eating a normal, healthy diet about equal to their energy output each day. Another group achieved a net energy deficit of 25 percent by eating fewer calories, while a third group achieved the same deficit through a combination of dieting and exercise. People in the fourth group drew the short straw, assigned to consume a very restrictive diet of only 890 calories a day — about half the daily average typically needed to maintain weight — until they achieved a 15 percent weight loss. At that point they switched to a diet that maintained a steady weight for the remainder of the study. All volunteers also took calcium and vitamin D supplements.
The controls lost 1 percent of body weight on average, whereas the two calorie-restricted groups, whether exercising or not, lost 10 percent of their body weight. People in the fourth group, who were on the strictest low-cal diet, lost 14 percent of body weight.
Despite these sharp losses, none of the groups showed a significant change in bone mineral density over the half-year study. Heavier people have more bone because the stress on their frame demands it, Redman says. The bone density remained unchanged in these people, whether adjusted for a change in weight, she says.
The 2006 study found some bone density losses during a one-year trial in which participants maintained slightly less arduous calorie restriction than in the new study. But those losses didn’t show up in people who also exercised. That trial also comprised slightly overweight volunteers, but they averaged about 20 years older than participants in the new study.
The two studies are part of the initial wave of calorie restriction studies in people, Fontana says. “Women over 50 who are post-menopausal are already losing bone due to the fact that they are estrogen deficient,” he says. Men also show a loss, although it’s less abrupt. To clarify the issue of calorie restriction and risk of bone loss, Fontana says, researchers will need to go beyond bone density measurements and actually record bone fractures over a longer time period. Such studies will reveal overall bone quality and not merely density, he says.