In contrast, exercising while on a full calorie diet can benefit bone health, say the researchers.
They describe their investigation and its results in a recent Journal of Bone and Mineral Research paper.
“These findings were somewhat of a surprise for us,” says senior study author Dr. Maya Styner, an associate professor of medicine at the University of North Carolina at Chapel Hill.
“Past studies in mice,” she continues, “have shown us that exercise paired with a normal calorie diet, and even a high calorie diet, is good for bone health.”
“Now we’re learning this isn’t true for exercise along with a calorie restricted diet,” she adds.
Bone health and osteoporosis
Bone is not an inert material but very much alive; it is continually renewing itself. During childhood, new bone formation happens faster than removal of old bone, resulting in bigger, heavier, and denser bones.
Bone formation continues outpacing bone removal until around the age of 20–30 years, during which time it peaks in most people.
Most people can minimize the bone loss that begins around their third decade of life.
They can do this by getting regular exercise, not smoking, not drinking too much alcohol, and ensuring that they have a sufficient amount of vitamin D and calcium in their diet.
Osteoporosis occurs when bone formation is too slow, when removal is too quick, or both. The condition, which tends to affect females more often than males, weakens bones and makes them more likely to fracture.
According to the Centers for Disease Control and Prevention (CDC), osteoporosis affects around 25% of females and 5% of males aged 65 and older.
Scientists suggest that one reason osteoporosis is more common in females is because their bones tend to be smaller and thinner. Another reason could be because menopause brings on a sudden drop in estrogen, a hormone that can protect bone.
Dr. Styner suggests that the new findings could be particularly relevant for women because as they age, their bone health starts to deteriorate naturally.
“Your calorie intake and exercise routine can have a great impact on the strength of your bones and your risk [of] break or fracture,” she cautions.
Calorie restriction and exercise
In their investigation, Dr. Styner and colleagues focused on bone marrow fat. Scientists do not fully understand how this type of fat works. They suspect that it is harmful to bones in humans and other mammals.
Previous studies have suggested that lower levels of bone marrow fat are usually an indication of good bone health.
In earlier work, Dr. Styner had examined how calorie consumption relates to bone marrow fat and how exercise might influence this link.
Those studies showed, for example, that levels of bone marrow fat go up when excess calorie consumption leads to obesity.
They also found that when mice of a normal weight and mice with obesity exercised, it caused a drop in their bone marrow fat and improved their bone density.
The purpose of the new study was to find out what happens to bone marrow fat and bone health during calorie restriction.
The researchers split mice into two groups. They fed one group a regular diet and the other a calorie restricted diet comprising 30% less calories than the regular diet.
The calorie restricted mice received supplements of minerals and vitamins so that these nutrient intakes matched those of the mice on the normal diet.
The team then split the mice again, into sedentary and exercise subgroups, and monitored them for 6 weeks.
This created four groups of mice on four different patterns of diet and exercise:
- regular diet without exercise
- calorie restricted diet without exercise
- regular diet with running exercise
- calorie restricted diet with running exercise
‘Negative effects on bone health’
The results showed that although the calorie restricted mice lost weight, their bone marrow fat levels went up significantly. These mice also experienced a decrease in bone quantity.
The researchers conclude that the bone loss in the calorie restricted mice was due to calorie reduction alone and not lack of nutrients, since the mice had the same vitamin and mineral intake as their regular diet counterparts.
The team found that, as expected from previous studies, adding exercise to calorie restriction led to a reduction in bone marrow fat. However, it unexpectedly also led to a reduction in overall quantity and quality of bone.
The researchers were surprised to find that under conditions of calorie restriction, exercise appears to make bones more fragile — not more robust.
They are already planning further investigations to better understand the function of bone marrow fat. In particular, they wish to learn about the underlying mechanisms that cause diet and exercise to produce the effects that they found.
“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. Maya Styner