Exercising While Restricting Calories Could Be Bad for Bone Health

Diet and exercise are staples of any weight-loss regimen. But new research from the UNC School of Medicine shows how the wrong combination of the two can harm your bones.

Maya Styner, MD, an associate professor of endocrinology and metabolism at the UNC School of Medicine, researches the role fat plays in bone health. Her past studies in animals have shown how changes in diet and exercise can increase or decrease the amount of fat in bone marrow, which is an indication of overall bone health. Although bone fat is poorly understood, it’s mostly considered to be harmful to the bones of mammals, including humans.

Dr. Styner’s latest study on mice, published in the Journal of Bone and Mineral Research, shows the impact on bones of calorie restriction by itself and when paired with exercise. Both had a negative impact on bones, but in different ways. The findings surprised her.

“Past studies in mice have shown us that exercise paired with a normal-calorie diet, and even a high-calorie diet, is good for bone health,” Dr. Styner says. “Now we’re learning this isn’t true for exercise along with a calorie-restricted diet.”

The Calorie-Exercise Connection

Let’s start with the reduction in calories. During the study, mice were divided into two groups: One group was allowed to eat as much as they wanted, and the other was fed 30 percent less than their counterparts. (The total daily consumption of the first group was weighed, and reductions to the second group were based on that measurement.)

For context, a moderately active woman around age 30 should consume 2,000 calories per day, according to the U.S. Department of Agriculture. A 30 percent reduction would equal a diet of 1,400 calories per day, which is around the amount suggested to most women trying to lose weight at a rate of 1 pound a week.

Dr. Styner found that the group of mice eating fewer calories lost weight but also had an increase in bone marrow fat. As fat accumulates in bones, it replaces bone marrow cells. Because fat cells are less dense, bones with more fat in their marrow also have a lower density, which makes them weaker. This means that though cutting calories may lead to weight loss, it can also make the skeletal system more fragile.

Meanwhile, adding exercise to the reduced-calorie diet impacted bones in two ways. It positively affected bones by decreasing the amount of bone marrow fat. But the study also showed that exercise with a low-calorie diet made bones smaller and more porous, which means they are more susceptible to fractures.

To make all this more interesting, the mice on calorie-restricted diets were given vitamin and mineral supplements to match the amount the other group of mice ate. Because both groups received the same vitamins and minerals, Dr. Styner says, the effect on bone health was from calorie restriction and not a lack of nutrients.

“Looking at this from a human perspective, even a lower-calorie diet that is very nutritionally sound can have negative effects on bone health, especially paired with exercise,” Dr. Styner says. “This is important to consider, particularly for women, because as we age, our bone health starts to naturally decline. Your calorie intake and exercise routine can have a great impact on the strength of your bones and risk for break or fracture.”

Dr. Styner and her team plan to conduct more research to understand the purpose of bone marrow fat and why it is affected by diet and exercise.

If you are thinking about changing your diet or starting to exercise, check with your doctor first. If you need a primary care physician, schedule an appointment with a UNC doctor today.