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alorie restriction, especially in
combination with exercise, can make bones smaller and weaker, according to new
research in mice.
Can restricting calories and exercising make bones smaller and weaker?
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."
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