Diet can change the rate of aging in fruit flies, even reversing the normal longevity advantage of females over males, according to a population study by researchers at the University of California, Davis.
The results support earlier findings by the same scientists indicating that life span is not necessarily fixed for all animals, as had been presumed for years by many gerontologists.
Fast-breeding Medflies are used as models to study basic population questions. The newly published study provides a statistical analysis of the life span of more than 400,000 Mediterranean fruit flies. It was conducted by UC Davis statistics professor Hans-George Müller and entomology professor James Carey and will appear in the March 18 issue of the Proceedings of the National Academy of Sciences.
"This is the first known study to demonstrate that the age trajectory, or path of mortality, during a specific period of life can be changed by manipulating environmental conditions, in this case through diet," said Carey, an authority on insect demography, who for several years has used the Medfly to study life-span theories. "It was previously thought that the mortality curves for most species were more-or-less genetically predetermined."
Medflies usually live for about two to three weeks. Those that make it to 60 days have lived the equivalent of 85 human years. If they receive a normal diet of sugar plus protein, females generally outlive males by about 1.5 days. But in this new study, the researchers report that, when fed a protein-deprived diet, males lived shorter lives overall, but outlived females by a little more than two days.
Carey and colleagues conducted this nutrition-manipulation study to determine why death rates level off sooner and at a lower level for female Medflies than for their male counterparts. This earlier flattening of the female mortality curve was observed in protein-deprived medflies studied in the 1992 "Oldest-Old" project, funded by the National Institute on Aging. That project used 1.2 million Mediterranean fruit flies to produce the largest mortality study of any non-human species and provided powerful evidence that longevity is not genetically fixed for a species, but can be environmentally extended.
In this new study, the researchers wanted to determine if the lack of protein in the diet of the earlier study might have a sex-dependent effect on life expectancy and mortality rates. They divided their Medfly subjects, including equal numbers of males and females, into two groups: half receiving a sugar-plus-protein diet and half receiving a sugar-only diet. Then they simply stepped back and counted as the medflies lived out their brief lives.
They found that life expectancy was reduced overall for both protein-deprived male and female Medflies. But females seemed to be harder hit by the dietary deficiency, their life expectancy plunging by more than 26 percent compared to only a 6 percent reduction among males. This, in effect, reversed the normal life span advantage of females over males.
Carey and colleagues suspect that the early surge in deaths of females deprived of protein occurred because the females were at that point in the process of producing eggs. If proteins and other essential nutrients were not being consumed, the Medfly might call upon nutrient reserves that normally would be used for body maintenance and repair. That notion is supported by the fact that the early-life surge in the death rate is not seen in sterile protein-deprived females.
"It's likely that this is a particularly vulnerable window of time for females because of the nutritional demands of reproduction and egg-laying," said Carey. "These changes may account for the leveling off of the death rate among the Medflies at older ages.
"The broad importance of the study is that the results demonstrate that the rate of aging, as measured by the rate of change in the age-course of mortality, can be altered by manipulating environmental conditions," he added.
Collaborating with Carey and Müller on the study were Jane-Ling Wang and William Capra, both of UC Davis, and Pablo Liedo of the Centro de Investigaciones Ecologicas del Sureste in Tapachula, Mexico. Funding for this study was provided by the National Science Foundation and the National Institute on Aging.