In the meantime, the research continues into biochemical reactions in "brown fat," a kind of tissue that produces heat when an animal is chilled. In people, brown fat is plentiful in infancy but little survives past the age of five. "White fat" makes up the significant portion of body fat, but doesn't produce heat.
When scientists removed the "fat" gene and created a defect, it turned up the activity of the animals' brown fat. That, in turn, diverted some of every meal's nutrients away from white fat, where it would normally have been deposited, to brown fat, where it was burned off immediately.
The effects were astonishing: The mutant mice were lean, with six percent body fat, despite being fed a 58-percent-fat diet. They were warm, with body temperatures 1.5 degrees Fahrenheit higher than normal. They were hungry, and ate more than their normal rodent colleagues. Their metabolism, according to Steve A. Thomas, a UW biochemistry and genetics fellow, was 25 percent higher than normal. "That baffled us," Thomas says. "We expected these mice to get fat." Moreover, McKnight says they were quite healthy, fertile and able to get around their cage like their normal brothers and sisters.
Normal mice with the same high-fat diet became obese, their livers clogged with fat, an abnormality associated with overeating. Most became diabetic and died shortly thereafter.
By finding success with the physiological and genetic systems of weight control, doctors have apparently found an important inroad to the long-running "genetics vs. environment" debate. In other words, is a person likely to get heavy because his or her folks were or because he or she lives above the Honey Bear Bakery on Green Lake and can't resist the killer cinnamon rolls?
Genetics are now thought to determine about 90 percent of a person's weight. Thus, people who struggle to shed excess pounds on a New Year's diet shouldn't blame themselves. "It isn't their fault," says Brunzell. "It's in their genes."
On the other hand, take a jaunt down to the Arizona desert to see the classic example of the role environment plays in the whole enchilada. The Pima Indians lived in the Valley of the Sun long before the arrival of Europeans and Miracle Whip. As classic hunter-gatherers, the Pimas were rugged, lean and strong. But once the U.S. government forced the Pimas onto reservations in the late 1800s, their lives changed forever. They were exposed to a high-fat diet and their lifestyle became sedentary. Now, 90 percent of all Pimas are obese. Their diabetes rate is staggering. While people from other cultures (such as Asia) have suffered increased cancer rates and other health problems after moving to the U.S., nowhere has an entire people been affected the way the Pimas have.
"Their genetics didn't change," says Schwartz. "They could have had a genetic predisposition to becoming obese. But this is clearly an environmental factor."
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