Saturday, February 14, 2009

Cooking...the good and the bad

"Cooking Has Been Both Boon and Bane for Humans"


Michael Wall

February 13th, 2009


Raw-food devotees take note: Your diet is not in any way natural. Humans are as adapted to cooking our food as cows are to eating grass, or ticks are to sucking blood.

"Cooking is a human universal," said Harvard primatologist Richard Wrangham at the American Association for the Advancement of Science meeting here Friday. While cooking kills parasites and other pathogens, Wrangham believes this health benefit is not its primary contribution.

"The fundamental importance of cooking is that it provides increased sources of energy," he said.

And that boost may be what facilitated the leap in size between Homo erectus and modern Homo sapiens. But, cooking may also have helped some modern humans into an obesity epidemic.

Wrangham cited data showing that cooking increases the body's ability to digest starches (as found, for example, in bread, potatoes and bananas). Only about 50 percent of raw starches are digested, compared to 90 percent of cooked ones. The trend, and the numbers, are similar for protein: from 50 to 65 percent digestibility raw to better than 90 percent cooked.

The reason: Heat breaks down starch and protein molecules, making it easier for digestive enzymes to attack them.

Cooking also softens up food, meaning the body doesn't have to use as much energy to process it. We spend less time chewing cooked food, and we secrete fewer chemicals to break it down. A recent study gave rats identical diets, except half the animals were served softened pellets and the other half received hard ones. After 25 weeks, the rats eating soft food were significantly heavier and had 30 percent more body fat, Wrangham said.

We cook almost everything, except some fruits and vegetables, and we need to. People on raw-food diets lose weight almost without fail, because they can't access enough calories. Half of all women eating exclusively raw foods become so thin that they stop menstruating, he said.

And our bodies show the imprint of this dependence on cooking. We have small, soft teeth, and our guts are smaller than those of any other primate relative to body size.

These traits go back a long way. Humans have controlled fire for at least 200,000 years, and perhaps for much longer. In fact, Wrangham speculates that cooking may have spurred a huge leap in the evolution of H. sapiens' ancestor, H. erectus. He's written a book on the idea called Catching Fire: How Cooking Made Us Human, which will come out in June.

About 1.8 million years ago, the brain of H. erectus ballooned and its body got bigger. Its arms got shorter, its legs longer. It became, in short, less of an ape and more of a human. Mankind has not undergone anything approaching this anatomical transformation since.

"No really big changes are seen after H. erectus," Wrangham said.

He thinks cooking may have freed up enough energy and nutrients to feed H. erectus' burgeoning brain. And this is no trivial matter: Brain tissue burns through 16 times as much energy as a comparable chunk of skeletal muscle. But as yet, it is not known if H. erectus had mastered fire. Anthropologists have found what may be remnants of 2-million-year-old cooking fires, but the evidence is equivocal.

It's also possible that a change in climate keyed H. erectus' developmental quantum leap. Around 1.8 million years ago, Africa was drying out. Forests were changing into savannas. And savannas have many more herbivores — nearly three times as many by weight as forests do — said biological anthropologist William Leonard of Northwestern University, who also spoke at the AAAS meeting Friday. So increased meat on the hoof may also be part of the picture.

Whenever we began cooking, Wrangham's ideas bear on the United States' current obesity epidemic: 65 percent of Americans are overweight and 33 percent obese. We know that softened or otherwise processed foods are digested more easily, yet we do not take this into account in our diets.

"We're very bad at calorie-counting at the moment," Wrangham said. "Counting calories does not take into account the processibility of food. Biophysics is just as important as biochemistry in nutrition."

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