Obesity is our century's version of the Kennedy assassination: Everybody's got a theory. But even with blame perpetually shifting -- one day it's fast-food corporations, the next it's genetics -- and a $40-billion-a-year diet industry, our waistlines just won't stop expanding.

The prevalent belief is that the problem is merely a matter of willpower. If we could only acquire some, the thinking goes, we'd be able to eat less, move more, and maintain a reasonable weight. It's a position backed by common sense, of course, but it's also the kind of oversimplification that could be the reason we're not coming up with any lasting solutions. "There's a sense about obesity that we already know all the answers," says David B. Allison, a biostatistician at the University of Alabama at Birmingham.

But in truth we're only just beginning to reveal them. Faced with a mounting collection of research implicating unconventional factors like viruses, pollutants and the amount of sleep we get, even the National Institutes of Health has begun to explore alternatives to traditionally held beliefs about weight gain. "We realize that obesity is more complex than we thought, so it's necessary to explore all possible theories," says Jerrold J. Heindel, the health science administrator for the National Institute of Environmental Health Sciences, a division of the NIH.

Weird as they may seem, the following hypotheses are quietly transforming the way we think about and treat obesity.

Artificial Sweeteners Make Us Fatter

THEORY
Sugar substitutes may blunt the brain's natural ability to measure calories, causing us to overeat.

EVIDENCE
On the surface, it makes sense that America's consumption of products made with no- or low-calorie sweeteners would increase at about the same rate as incidents of obesity -- after all, don't zero-calorie sweeteners go hand-in-hand with dieting? They do, but perhaps not in the way you might think. "Most people have assumed that as people gained weight, they increased consumption of artificial sweeteners," says neuroscientist Terry Davidson of Purdue University. "Our data suggests that [the cause and effect] could go the other way."

Davidson and his colleague, psychologist Susan Swithers, published their findings last February in the journal Behavioral Neuroscience. They fed rats either artificially sweetened yogurt or sugar-sweetened yogurt in addition to their normal rodent chow. The animals that ate artificially sweetened yogurt not only gained more weight, they also appeared to lose their natural ability to keep track of the extra calories and eat less later on.

"It's a Pavlovian approach to obesity," Davidson says. "Animals learn to use taste to predict caloric consequences, and in nature, sweetness is almost always an indicator of calories." When we experience a sweet taste with no accompanying caloric intake, it confuses that calibration tool. Repeating that experience, as in drinking a diet soda every afternoon, might actually deprogram your calorie-counting mechanism for good. (In the rats, effects were seen in as few as 10 days.)

FRINGE FACTOR
Moderate. Even skeptics admit that the evidence is compelling, but causality has yet to be proven in humans. And although rats have similar taste receptors as us, they have a more limited diet and don't respond to all sweeteners the same way as humans do. (The Purdue study focused on saccharine, one of five artificial sweeteners approved by the Food and Drug Administration and one that rats do register like us.)

NEXT STEPS
More study is needed.