Because eating is a powerful, primal drive, one that trumps willpower or the desire to be thin in people genetically predisposed to obesity, argues Dr. Jeffrey Friedman, head of the laboratory of molecular genetics at Rockefeller University in New York City.
"The feeling of hunger is intense and, if not as potent as the drive to breathe, is probably no less powerful than the drive to drink when one is thirsty," Friedman writes. "This is the feeling the obese must resist after they have lost a significant amount of weight."
Friedman's article appears in the Feb. 7 issue of Science.
While Friedman's article argues that genetics is the primary determinant for obesity, an article by James Hill, a researcher at the University of Colorado Health Sciences Center in Denver, points to a different culprit -- the environment.
Supersized portions. Calorie-dense fast food. Too little exercise and too much to eat.
The rate of obesity in the United States increased from 23 percent in 1991 to 31 percent in 2000, Hill says, citing previous research.
Using data from the National Health and Nutrition Examination Survey (NHANES), Hill and his colleagues estimated that Americans are gaining, on average, 1.8 to 2 pounds each year, or 14 to 16 pounds in eight years.
If the current trend continues, 39 percent of the U.S. population will be obese in 2008, Hill says. Obesity is defined as having a body mass index (BMI) of 30 or more.
Hill has proposed a simple solution to halt the creeping weight gain: Eat 100 fewer calories a day. That's equal to the calories in a cookie or three bites of a fast-food burger.
It takes just a slight energy imbalance to cause a gradual increase in weight, Hill says. Say you ate just 50 excess calories a day, that is, you ate 50 more calories than you burned during physical activity. That means five extra pounds in a year.
"Even one extra LifeSaver a day, 10 calories, is a pound of weight a year," Hill says.
One pound of weight gain is about 3,500 calories.
Another option for "closing the energy gap" is walking another mile a day, which burns about 100 calories. Since it takes most people about 2,000 to 2,500 steps to complete a mile, people could fit this in fairly easily, he says.
"We believe that what we've been doing isn't working," Hill says. "Trying to tell people to get out and change their whole lives isn't helping. We have to stimulate people to think differently about this."
Most adults eat about 2,000 to 2,500 calories a day, Hills says. So a 100-calories difference is small. Restaurants could help the process by reducing portion size a tad.
Though some might argue that if you burn more calories through walking, you'd get hungrier and eat an additional 100 calories, Hill says previous research has shown a moderate increase in physical activity doesn't mean you eat more.
Adam Drewnowski, a spokesman for the American Dietetic Association and director of the Center for Public Health Nutrition at the University of Washington in Seattle, says he believes the environmental argument more than Friedman's genetic argument.
In his paper, Friedman argues that the tendency toward obesity is rooted in evolution. Survival meant being able to withstand periods of famine. Those who were able to store fat well had a better chance of surviving. And now, many Americans can thank (or curse) their ancestors for their battles with weight.
Friedman points to the hormone leptin as a key player in the biology of obesity. Some obese people, shown to lack leptin, lost dramatic amounts of weight when they were give leptin injections.
However, Drewnowski says the genetic argument doesn't explain why minorities, the poor, and people with low education have the highest rates of obesity.
"In the last 20 years, as obesity has doubled and tripled, the genetic pool has remained the same," he says. "You can't tell me all those people don't have enough leptin."
It will be lifestyle changes, not medical interventions, that will ultimately stem the tide of obesity in this country, he says.
"It all comes back to this issue of eating less and exercising more," Drewnowski says. "The Romans used to say, 'To avoid corpulence eat less and take physical activities.' After 2,000 years of research, we've come back to the same conclusion."
He favors Hill's 100-calorie-a-day approach. "It's simple. It's easy," Drewnowski says. "What he's saying is that you don't have to make drastic changes."