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Caterpillars fed a high-carb diet evolved to stay thin.  (Photo courtesy Spencer Behmer)

Caterpillars fed a high-carb diet evolved to stay thin. (Photo courtesy Spencer Behmer)

10/10/2006

Bingeing Bugs Avoid the Bulge

By Liz Savage

When was the last time you saw a pudgy ant?  A plump grasshopper?  Fat insects are few and far between, and now researchers think they know why. 

In a recent study, caterpillars raised on a high-carb diet evolved to stay thin after only eight generations. The study’s authors believe their findings could apply to all animals, even humans.

"The basic pathways involved in fat and carbohydrate metabolism are shared by all animals," said Stephen Simpson, co-author of the study and professor of biology at the University of Sydney, Australia.
   
The team randomly selected 200 caterpillars (Plutella xylostella) from a group that had been bred for 350 generations on a diet consisting of equal parts of protein and carbohydrates. The caterpillars were split into two groups—one on a high-protein, low-carb diet, the other on a high-carb, low-protein diet—for eight generations, or about four months. The scientists measured each generation of caterpillars to see how much fat they gained.

To measure caterpillar body fat, the team removed the water from the caterpillar carcasses by drying them in an oven. The caterpillars were then washed with chloroform several times to dissolve the fat from their bodies. The researchers compared the weight of each caterpillar before and after the washes to determine how much fat the caterpillar had stored.

The caterpillars in the high-carb group evolved to store less fat than their ancestors after only eight generations, while their counterparts in the low-carb group were more likely to convert the carbs into fat. While eight generations may seem like too short a period for evolution to occur, Spencer Behmer, co-author and assistant professor of entomology at Texas A&M University, said this timeframe was not unusual. "Evolutionary biologists generally believe that if you don't see adaptation of some sort before 10 generations that you are unlikely to see it at all," Behmer said.

The researchers could not pinpoint what caused the metabolic change in the caterpillars. The culprit might lie in an increased ability to ratchet up metabolism without more physical activity, suggested David Raubenheimer, professor of nutritional ecology at the University of Auckland, Australia, and co-author of the study, which appeared last month in the Proceedings of the National Academy of Sciences.

The first person to figure out how it works would make a mint. With chronic obesity still on the rise—the U.S. Centers for Disease Control and Prevention estimates that 60 million Americans are obese—understanding how metabolism evolves has never been more important. Most humans consume a diet vastly different than the type our bodies evolved to process. Trans fats, processed sugars, and an abundance of fatty meats have made their way into our diet within the last fifty years—an eye blink on the evolutionary timescale. It took eight generations for the caterpillars to show any change; that equates to about 160 years for humans.

Is it possible then that in a few hundred years we could evolve to our current nutritional environment of fast food and giant portions? Simpson said it certainly is possible—given the right conditions. "Providing that those individuals with 'skinny genes' have a higher reproductive success than those with 'fatty genes', then we too could evolve like the caterpillars over future generations," Simpson wrote in an email.

But it's unlikely that humans would face the pressures of natural selection necessary for this sort of evolutionary change, said Jay Phelan, an evolutionary biologist at UCLA.  Just as humans have not evolved better eyesight because people can wear glasses, Phelan said we are not likely to evolve to our current diet because overweight people are just as likely to have kids and pass on their genes as skinny people.

But as obesity levels rise in children, they are more likely to develop other health problems. "If these develop to full-blown diseases which prevent or reduce reproduction, I suspect that we might well evolve resistance to high-carb diets as did our caterpillars," said Raubenheimer.


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