| A Low Calorie Diet May Contribute
More To Longevity Than Exercise
A study investigating aging in mice has found that hormonal
changes that occur when mice eat significantly less may help explain
an already established phenomenon: a low
calorie diet can extend the lifespan of rodents, a benefit
that even regular
exercise does not achieve.
We know that being lean rather than obese is protective
from many diseases, but key rodent studies tell us that being
lean from eating less, as opposed to exercising more, has greater
benefit for living longer. This study was designed to understand
better why that is, said Derek M. Huffman, the studys
The study applies only to rodents, which are different in some
key ways from humans, cautions Huffman. However, at least two
studies which examined people who engage in high-volume exercise
versus people who restricted their calorie intake, had a similar
outcome: caloric restriction has physiological benefits that exercise
alone does not. Researchers expect that clues to the physiology
of longevity in mice will eventually be applied to people, Huffman
The study, Effect of exercise and calorie restriction on
biomarkers of aging in mice, appears in the May issue of
the American Journal of Physiology published by The American Physiological
Society (APS; www.The-APS.org). The study was carried out by Huffman,
Douglas R. Moellering, William E. Grizzle, Cecil R. Stockard,
Maria S. Johnson and Tim R. Nagy, all of the University of Alabama-Birmingham
(UAB) and funded by the UAB Center for Aging. Dr. Huffman is now
at the Albert Einstein College of Medicine in New York.
The study built upon previous studies that showed:
* Rats that exercise regularly will, on average, live longer
compared to a group that eats the same amount but does not exercise.
This is because exercise prevents some diseases, which allows
more individual animals to live out their expected life span.
* However, when comparing the rats in these two groups that eat
the same amount, the longest-lived animals in the exercise group
dont live any longer than the longest-lived rats in the
non-exercise group. Taken together, these findings indicate that
exercise can prevent an early death from disease in some rats,
but does not extend the maximal lifespan of any of the rats.
* When comparing rats that exercise to those that dont
exercise but eat much less, the longest-lived rats are from the
group that ate less.
Taken together, these findings indicate that caloric restriction
protects against disease better than exercise does, and has the
added benefit of extending the life span of some rats. Physiologists
have been trying to unravel the reasons for this, and two major
theories have emerged.
One theory is that exercise places stress on the body, which
can result in damage to the tissues and DNA. Another theory is
that caloric restriction leads to physiological changes which
benefit the body.
Huffman and his colleagues designed a study to examine the roles
of exercise and caloric restriction, singly and combined. They
controlled for factors such as weight and the amount of energy
expended versus the calories consumed.
* Mice allowed to eat as much as they wanted had higher insulin
levels, regardless of whether they exercised. That is, how much
the mice ate determined their insulin level, while exercise did
not have much effect. High insulin levels are associated with
a risk of diabetes.
* The animals that ate as much as they wanted and did not exercise
had the highest levels of insulin-like growth factor (IGF-1),
which plays a key role in regulating cell growth and cell death.
The animals on caloric restriction had the lowest levels of IGF-1.
Exercise also seemed to play an important role in regulating IGF-1
* There were some elevated levels of heat shock proteins, a measure
of oxidative stress and possible tissue damage among the exercising
mice. But total protein carbonyls, another stress measure, were
not significantly different.
* Both exercise and caloric restriction moderated the level of
8-hydroxyguanosine (8-OHdG), a marker of DNA damage. Among the
animals that ate all they wanted, those that did not exercise
had the highest levels of 8-OHdG and those that exercised had
much lower levels. The researchers concluded that DNA damage increases
with age and is accelerated by obesity but could be slowed by
caloric restriction and/or exercise. The researchers noted, however,
that the results may differ if they had used older mice or subjected
them to greater caloric restriction than the mild (9% fewer calories)
or moderate (18%) restriction this study employed.
Overall, these findings indicate that the physiological stress
of exercise did not produce enough damage to tissues or DNA to
explain why exercise does not lengthen life span. Instead the
study suggests that caloric restriction creates beneficial changes
in the bodys hormone levels which exercise does not. The
researchers concluded that these metabolic changes play a role
in extending life.
A handful of studies comparing calorie restricted people to people
who are avid exercisers, found similar hormonal benefits among
those eating less. However, calorie restriction studies are difficult
to carry out in people because participants often complain of
feeling hungry, lethargic, and cold.
Huffman also emphasized that the benefits of exercise may be
greater for humans than for mice because people are more prone
to develop cardiovascular diseases, and exercise is particularly
good at warding off those diseases. Mice tend to die of kidney
disease and cancer, Huffman said.
I wouldnt say this study has direct implications
for people right now, Huffman said. But it shows what
physiological changes caloric restriction and exercise produce.
We can continue to build upon these findings until we can get
a better understanding of how this works in people.