Evolution trips up modern lifestyle

Genes give push toward obesity


The two-thirds of adult Americans counted as overweight or obese today should get some sympathy from the dwindling ranks of the skinny.

Scientists say the cards are stacked heavily against the fat, and the beverage industry's recent agreement to yank sugary drinks from school vending machines doesn't go nearly far enough.

With a genetic heritage exquisitely tuned to the food scarcity of the Paleolithic era, a bonanza of cheap, high-calorie 21st-century food, and a society designed to minimize physical exertion, some believe it's a wonder we're not all obese and sick.

"We are adapted for one environment, but the environment has changed," said Dr. Burt Humburg, a physician at the Penn State University Hershey Medical Center who writes and speaks about Darwinian evolution.

"Take bodies that are adapted for that kind of hunter-gatherer role and put them in an environment with Wendy's and refined cane sugar, and you end up with type II diabetes."

The removal of sugary soda pop from school vending machines will be a decent start to setting things right for kids, experts say. But some scientists worry that the larger problem - eating more calories than we burn off - may be intractable.

"I'm actually very pessimistic about it all because there are such powerful factors involved - powerful societal factors," said John de Castro, chairman of the psychology department at the University of Texas, El Paso.

The story starts on our tongues. They're packed with 10,000 taste buds - chemical receptors that respond to compounds that are sweet, salty, sour or bitter. The sweets generally signal the presence of glucose or proteins, and natural selection has ensured that we're attracted to them.

"From the very first single-cell organisms, glucose is the fuel we use for energy," said Andrew J. Petto, a bioanthropologist at the University of Wisconsin, Milwaukee.

Proteins provide us with the building blocks we need to create our own body tissues. Salts are vital to the nerves, kidneys and other biochemistry. Our aversion to sour or bitter tastes, on the other hand, may have protected our ancestors from bad food or natural poisons.

We need some fat in our diet, too. But for a long time scientists thought fats had no taste. "Evolutionarily, why wouldn't we have evolved the ability to detect fats?" asked Utah State University biologist and taste scientist Timothy A. Gilbertson.

Fats offer high-density energy - 9 calories per gram, or twice the energy in the same amount of protein or carbohydrates - definitely a bonus for people who had to chase their next meal across the savanna.

Gilbertson's lab discovered that certain essential fatty acids do activate the taste system. They didn't have their own dedicated taste buds but instead boosted the activity of all.

As a result, "Things that are sweet taste sweeter, and things that are salty taste saltier," he said. "That may be why we like potato chips so much. They enhance the pleasant aspects of other tastes."

But following their tongues to the foods that tasted good, and that also happened to be good for them, did not solve our ancestors' problems.

Evolutionary biologists say the survival of early humans depended on their ability to hunt down, gather and consume food before it spoiled - and store it in their bodies as fat. It had to be there to carry them through repeated cycles of exertion and rest, feast and famine.

In 1962, James V. Neel, a University of Michigan geneticist, proposed the "thrifty gene" theory - the seminal notion that natural selection favored the survival of people who could store as many calories as possible, and then burn them as slowly as possible.

In those days, people who ate all they wanted and never put on a pound would have perished. But if times have changed, unfortunately, our genes have not.

Once, Humburg said, the sweets on the table were chiefly fruits and vegetables - high in sugar but also high in vitamins, minerals and fiber: "A lot of bang for your hunting and gathering buck. It worked well to like sweet things."

"Now, the sweet thing is table sugar," or high-fructose corn syrup in prepared foods, he said. It satisfies our sweet tooth but is empty of other nutritional benefits. To get those benefits, we have to consume something else, with still more calories.

That's not necessarily a bad thing, scientists say, provided we also burn off the additional calories and don't store them all as fat.

In a 2002 paper in the Journal of Applied Physiology, Frank W. Booth and Espen E. Spangenburg of the University of Missouri noted the disparity. Although modern humans probably consume fewer calories than their active ancestors, the researchers said, today's consumption "is nevertheless high, relative to the corresponding larger decrease in caloric expenditure."

In other words, we eat too much for our paltry level of physical activity. Where pre-industrial adult men expended an estimated 3,000 calories a day, today's more sedentary variety burns off 2,000 calories.

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