Calorie-restricted diets are something of a trend these days. This might mean consuming as little as 1,200 calories a day (the average in the U.S. is 2,745 for males, 1,833 for females). Tests have shown that limiting energy intake in this way can lower cholesterol, fasting glucose, and blood pressure — and possibly extend lifespan.

Of course, the benefits of exercise are well-known, too, so combining calorie restriction with strenuous physical activity has got to be a good thing, right?

Not necessarily, according to a study published yesterday in The Journal of Bone and Mineral Research. Focusing on the fat found in the bone marrow of mice, researchers found that exercising while on a restricted-calorie diet can negatively affect bone strength — a measure of the bone’s resistance to fracture.

The function of bone marrow fat is poorly understood and will be the subject of future studies, but it’s clear that the more fat that’s in the marrow, the weaker the bone becomes. A group of mice that was fed a restricted-calorie diet but didn’t exercise lost weight but experienced an increase in bone marrow fat. When exercise was added to the group, bone marrow fat decreased — but, surprisingly, their bones became more fragile.

Weakened bones, the condition known as osteoporosis, affects women disproportionately. Some 68% of the people most at risk are female, and one out of every two women over 50 will have an osteoporosis-related fracture at some point — twice the rate of fractures in men. A sedentary lifestyle increases the risk. These are 17 ways sitting too much is ruining your body.

According to the senior author of the study, Dr. Maya Styner, associate professor of medicine at the University of North Carolina School of Medicine, “[E]ven a lower calorie diet that is very nutritionally sound can have negative effects on bone health, especially paired with exercise. This is important for women to consider, because as we age our bone health starts to naturally decline.”

Most people have some history with calorie counting. These are 17 things nutritionists want you to know about calories.

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