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Eat less, live longer? Study on black widow spiders suggests sex comes first

 

New research by a UTSC evolutionary biologist suggests the relationship between how much you eat and how long you live – traditionally understood by the mantra eat less, live longer – might not be so straight forward after all.

In a recent paper published in Functional Ecology, Maydianne Andrade (Associate Professor of Biology at UTSC and Canada Research Chair in the Integrative Behaviour and Neuroscience Group) and co-authors Jeff Stoltz and Ramez Hanna investigate the relationship between dietary restriction and lifespan in female spiders. Their findings support the theory that reduced-calorie diets do have the potential to increase longevity, but that this potential may depend upon the natural history and mating status of an individual spider.

Most studies of dietary restriction have suggested that reduced-calorie diets lead directly to longer life spans. These findings have been interpreted in media and in books to suggest that humans can find the “fountain of youth” simply by eating less. But there’s a catch, says Andrade.

“Most experimental work in this area has been done on typical laboratory animals – fruit flies, mice, rats and worms,” she says, “which represent only a small proportion of the varied life histories present in nature.” To help correct for this bias, Andrade and her colleagues designed a dietary restriction study using a somewhat unorthodox laboratory organism – Australian black widow spiders.

To determine whether the mating status of a spider can affect the impact of dietary restriction on its longevity, the researchers restricted the caloric intake of mated and unmated female redback spiders. Metabolic rates were monitored since these reveal underlying energy use patterns and life spans recorded. In the end, dietary restriction increased longevity in the mated individuals, but not in the unmated spiders. What’s more, overall lifespan was lower in unmated spiders.

These results support life history theories of longevity, which hold that dietary restriction should increase longevity only when the organism is prolonging survival to allow later reproduction. Metabolic measurements seem to confirm these findings, as mated females reduced their metabolic rate very quickly when subjected to caloric restriction. Mated female spiders can store sperm for later use, so prolonging life allows them to survive a famine and begin reproducing again when food is available and renewed egg production makes metabolic sense.

By contrast, the unmated females did not lower their metabolic level until much later—when they were effectively starving. This suggests that there is some physiological cost to remaining unmated for these spiders, and that caloric reduction, rather than increasing lifespan, is very harmful to a spider that is still seeking a mate.

The impact of dietary restriction on longevity is clearly a lot more complex than it may seem. And Andrade and her colleagues are quick to point out that we must take caution when applying the findings from controlled laboratory studies to investigations into human aging.

“Additional work on non-model organisms is critical,” says Andrade, “if we are to determine the extent to which longevity increases are expected to result from dietary restriction across the board, or only under the circumstances predicted by life history theory.”

(photo: Ken Jones)




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