Not in Medflies
Authors: May Thomas S. 
Published: August 2003

In the field of aging, it's about as close to gospel as one can get: Reducing calories extends life-span. Cutting calories by one-third increases longevity in a variety of living things, including
yeast
,
nematodes
,
mice
, and perhaps
primates
(see
"Monkey in the Middle"
and

comment on Walford
et al.

). The diversity of species that live longer with less food suggests that the regimen could be a universal life-extender. But the ascetic approach isn't effective for all creatures: Dieting Mediterranean fruit flies don't live longer than their well-fed counterparts, according to new research. The results suggest that animals must curtail reproduction when food is scarce in order to enjoy longer lives.

Previous studies have shown that delaying reproduction extends life-span in some species, suggesting that investing energy in health maintenance rather than procreation delays death. But calorie-restriction (CR) experiments don't typically assess reproductive patterns. In the new work, James Carey, a biodemographer at the University of California, Davis, and colleagues measured the effect of CR on the life-span and reproductive capacity of the Mediterranean fruit fly,
Ceratitis capitata
; this insect is available in greater numbers than are other species, such as the common laboratory fruit fly
Drosophila melanogaster
, enabling researchers to identify subtle trends by performing experiments on many individuals.

The team fed 1200 male and 1200 female flies diets ranging from 30% to 100% of normal quantities. Each individually housed insect received a fixed amount of food, a methodological improvement on previous experiments in which investigators diluted rations, says Steven Austad, an evolutionary biologist at the University of Idaho in Moscow. As a result, the researchers knew the maximum amount of food consumed by each fly. None of the calorie-restricted animals lived especially long. Even insects fed 60% to 70% of their normal diet--regimens that extend the life of other species--did not outlive fully nourished individuals. These results are "certainly unexpected," says physiologist Andrzej Bartke of Southern Illinois University in Carbondale.

Below that amount of food, life expectancy fell sharply and the number of eggs laid by females plummeted, probably because the flies weren't consuming enough calories to operate both reproductive and self-preservation systems at full capacity, says Carey. In contrast, lifetime egg production declined by only about 15% when flies were fed 70% of their normal diet. And, with the exception of the lowest-calorie group, females started laying eggs at approximately the same age, regardless of diet.

The results suggest that life extension occurs only in species that reduce their reproductive rate significantly in response to CR, says Austad. Mice and rats, for example, produce much fewer young when they're underfed. Carey notes that most CR experiments on rodents build in a clamp on reproduction: The animals are typically kept in separate cages and thus can't mate. He predicts that a restricted diet might not lengthen their lives if they could mix freely with members of the opposite sex. If CR does work in humans, the study suggests that people might have to deprive themselves of more than just food to live longer.

--Thomas S. May

J. R. Carey, P. Liedo, L. Harshman, Y. Zhang, H.-G. Müller, L. Partridge, J.-L. Wang, Life history response of Mediterranean fruit flies to dietary restriction.
Aging Cell
1
, 140 (2002).
[Abstract]
[Full Text]