Sci. Aging Knowl. Environ., 26 October 2005
Caught With Their Proboscises in the Sugar Water
Flies on life-extending diet eat more than expected
Like people who sneak downstairs at midnight for a slice of cake, calorie-restricted flies cheat on their diets. The hungry bugs lap up more food than scientists thought, according to a new study. The authors argue that researchers need to take a closer look at the animals' dining habits.
Slashing food intake by 20% to 40% lengthens life in animals from worms to mice (see Masoro Review). Putting rodents on such a regime is easy: Just cut each animal's rations. But the standard method for depriving fruit flies--diluting their food--doesn't guarantee less food consumption, some researchers argue. A bug might scarf more of its weaker fare. Tracking the eating habits of individual flies is tricky, and scientists have tried various indirect means to gauge their gobbling, such as counting "proboscis prints" the insects leave on the food as they dine. In a study published earlier this year, for instance, evolutionary biologist Linda Partridge of University College London and colleagues tallied the amount of time flies spent touching food with their snouts, concluding that the bugs raised on lean fodder weren't eating more. In the new work, biogerontologist Pankaj Kapahi of the Buck Institute for Age Research in Novato, California, and colleagues sought to measure fly feeding directly.
They raised one group of flies on a watered-down mixture of sugar and yeast; other groups slurped meals that were 5, 10, or 15 times more nutritious. Food concentrations vary somewhat from lab to lab, but the 1x and 5x groups would count as calorie restricted, and the 15x group as well-fed. The researchers had tagged the provisions with radioactive isotopes, so they could determine how much each insect assimilated. If ill-fed flies weren't compensating, bugs in all the groups should eat about the same volume. However, insects on the dilute diet sucked up double the amount of the 5x group, which in turn snarfed about 60% more than the 10x group and about 130% more than the 15x insects. Moreover, if the insects weren't adjusting their feeding behavior, the 15x bugs should have obtained 200% more nutrients than the 5x group. But the real difference was only 40%. The animals' life spans reflect their consumption, not food concentration. The 5x insects buzz only slightly longer than the 10x and 15x Drosophila. Overall, the results suggest that calorie-restriction researchers need to apply methods like these to gauge how much the insects actually ingest, says Kapahi.
The paper is important because it demonstrates that scientists can't ignore animals' behavior when trying to understand what determines their longevity, says demographer James Carey of the University of California, Davis. However, Partridge contends that the study doesn't establish that the flies are altering their feeding. The radioactive technique indicates how much food the bugs have incorporated into their tissues, which depends not just on how much the animals eat, but also on how much they absorb from their digestive system and how much they excrete, she says. Researchers need to investigate how these variables change with food quality, she says, before they'll know whether flies give in to temptation.
October 26, 2005
Science of Aging Knowledge Environment. ISSN 1539-6150