Exercising while limiting calories could be detrimental to bone health. Find UNC School of Medicine Study

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HILL CHAPEL, N.C .– (BUSINESS WIRE)–A new study published today in Journal of Bone and Mineral Research shows how bones in mammals are negatively affected by caloric restriction and in particular by the combination of exercise and caloric restriction. Maya Styner, MD, associate professor of medicine at the UNC School of Medicine, is the senior author of the study.

"These results were somewhat of a surprise to us," Styner said. "Previous mouse studies have shown us that exercise combined with a normal low-calorie diet and even a low-calorie diet is good for bone health. Now we are learning that this is not true for exercise along with a low-calorie diet ".

Styner's peer-reviewed research focuses on fat in the bone marrow of mice. Although the fat in the bone is poorly understood, it is now believed to be harmful to mammalian bones, including humans, because it makes the bone weaker. Less fat is generally an indication of better bone health. Styner's previous studies have examined the effects of calorie consumption on bone marrow fat, along with the role played by exercise. He discovered that in the obesity caused by excess calories, the amount of fat in the bone marrow increases. Exercise in both obese and normal weight mice reduced bone marrow fat and improved bone density.

The latest study looked at what happens to bone marrow fat and overall bone health when limiting calories. There were a total of four groups of mice: a regular diet group (RD), a low-calorie diet group (CR), an RD group that practiced and a CR group that practiced. Mice in the CR group ate 30 percent less than RD mice.

For the context in the man, according to the United States Department of Agriculture (USDA), a "moderately active" woman in her 30s should consume 2,000 calories a day. A 30 percent reduction would amount to a 1,400-calorie-per-day diet, which is about the amount suggested to most women trying to lose weight at a rate of one pound a week.

Styner found that mice in the CR group had lost weight, but also had an increase in bone marrow fat.

"This was a mild caloric restriction and we found a significant increase in fat in the bone marrow," Styner said. "This group also had a decrease in bone quantity – it had altogether fewer bones due to calorie cutting."

Both groups of CR mice received vitamin and mineral supplements to match the amount the RD group received from the extra food they ate. This, states Styner, is an indication that the effect on bone health was due to caloric restriction and not nutrient deficiency.

When exercise was introduced into the CR group, bone marrow fat decreased as in previous studies, but the overall quantity and quality of the bone also decreased. Instead of making the bones stronger, the exercise made the bones more fragile when combined with a calorie restriction.

"Considering this from a human point of view, even a low calorie diet that is very healthy from a nutritional point of view can have negative effects on bone health, especially when combined with exercise," said Styner. "This is important to consider for women because aging bone health starts to decline naturally. Calorie intake and exercise can have a great impact on bone strength and the risk of breakage or fracture. "

Styner says his team is now planning to conduct more research to understand the purpose of bone marrow fat and why it is influenced by diet and exercise. This study was funded by grants from the National Institutes of Health (NIH) and the National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS).

Information on the UNC School of Medicine

The UNC School of Medicine (SOM) is the largest medical school in the state, graduating about 180 new doctors every year. He is consistently ranked among the best medical schools in the United States, including 1st overall for primary care by US News & World Report and 5th for research among public universities. More than half of the 1,700 faculty members of the school were the principal investigators on the active research awards in 2018. Two members of the SOM faculty of the UNC won Nobel prizes.

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