COLLEGE STATION -- Ever seen a fat insect? Probably not. Dr. Spencer Behmer may have the answer why, and that could have implications for what is billed as the current human obesity epidemic.
Behmer, an entomologist with the Texas Agricultural Experiment Station, and several other researchers conducted a series of experiments to find out whether caterpillars could adapt to extreme changes in their nutritional environment.
By manipulating the nutritional environment of the diamondback moth caterpillars, the researchers found that the insects evolved different physiological mechanisms related to fat metabolism. Which mechanism was used depended on whether the caterpillars were given carbohydrate-rich or carbohydrate-poor food.
The team's work was published recently in the Proceedings of the National Academy of Science.
The researchers theorized caterpillars – and animals in general – can evolve metabolically to adjust to extreme nutritional environments.
All animals need carbohydrates for energy and protein to build muscle and tissue, Behmer said. Different animals, however, need different amounts of these two macronutrients and sometimes it can be literally feast or famine for one or both of them.
"It's difficult to find in any environment a nutritionally perfect food," he said.
The researchers studied the insects over eight generations. In one experiment they fed caterpillars artificial diets that were rich in protein and low in carbohydrates (an Atkins-like diet); at other times the caterpillars received diets low in protein and high in carbohydrates (a high-carbohydrate diet).
In a second experiment caterpillars were allowed to freely eat one of two plants, an Arabidopsis mutant low in starch or an Arabidopsis mutant (plant) high in starch.
When the caterpillars were reared in carbohydrate-rich environments for multiple generations, they developed the ability to eat excess carbohydrate without adding fat to their bodies, Behmer said. On the other hand, those reared in carbohydrate-poor environments showed an ability to store ingested carbohydrates as fat.
Also after multiple generations on the low-starch plants, female moths preferred to lay their eggs on these same plants. This, Behmer said, is one of the first instances of a moth showing egg-laying behavior that is tied to a plant's nutritional chemistry.
Moths from low-starch plants might avoid the high-starch plants because these plants might make their offspring obese, he explained. Female moths reared on the high-starch mutant for multiple generations showed no preference for either mutant.plant.
Inferences can be made to humans from this work, he said. Looking back over human history, even as recently as 100 years ago, the diets of western cultures have undergone some radical changes.
Like insects, humans require carbohydrates and proteins. But, Behmer said, humans are not well adapted to diets containing extremely high levels of carbohydrates.
"Historically we haven't always had a lot of access to carbohydrates," he said, "and one of the biggest sources of carbohydrate in our current food is refined sugar. Our bodies tend to convert most of this excess carbohydrate to fat."
However, Behmer said other factors, such as a lack of exercise, might also be to blame.
Part of the research was done while members of the team were at the University of Oxford in England. Team members are Behmer, James Warbrick-Smith (currently pursuing a medical degree at Oxford University), Professor Stephen J. Simpson and Kwang-Pum Lee, now at the University of Sydney, Australia; and Professor David Raubeheimer, now at the University of Auckland, New Zealand.
Last reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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