Insect study provokes questions over obesity causes
21/09/2006 - The fact that you never see a fat insect could have implications for the current obesity epidemic, according to scientists.
From studies conducted with caterpillars, Dr Spencer Behmer, an entomologist with the Texas Agricultural Experiment Station, said that one factor behind the dramatic rise in obesity might be that humans have not evolved sufficiently enough to cope with our current high-carbohydrate diet.
Looking back over human history, even as recently as 100 years ago, the diets of western cultures have undergone some radical changes, he said.
Like insects, humans require carbohydrates and proteins. But 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, and one of the biggest sources of carbohydrate in our current food is refined sugar," said Behmer, who agreed that lack of exercise was likely to be another factor behind galloping obesity.
"Our bodies tend to convert most of this excess carbohydrate to fat."
Behmer conducted a series of experiments to find out whether caterpillars could adapt to extreme changes in their nutritional environment. The researchers theorised caterpillars and animals in general can evolve metabolically to adjust to extreme nutritional environments.
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 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. Moths from low-starch plants might avoid the high-starch plants because these plants might make their offspring obese, said Behmer.
Female moths reared on the high-starch mutant for multiple generations showed no preference for either mutant plant.
The team's work was published recently in the Proceedings of the National Academy of Science. 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.
Insects never obese 
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