Our experts weigh in: Everybody has a body weight "set point," Ludwig says. "If your weight falls below that point, the body has compensation mechanisms to try to kick it back up." A diet with a low-glycemic load lowers the set point, he adds, "so you don't run into a brick wall so soon."
Susan Roberts of Tufts University thinks low-glycemic diets are a legitimate option for losing weight, but she doubts that high-glycemic load diets are a cause of the obesity epidemic. "Just think about the 1950s," she says. "Everyone was eating white bread, meatloaf (made with white breadcrumbs), potatoes and rice -- and were much thinner than today."
Barry Popkin of the University of North Carolina says there's plenty of evidence that a variety of dietary patterns will sustain weight loss -- low-fat and high in complex carbs; low-carb and high in lean proteins; diets higher in fat. "The issue is cutting calories," he says. "All do work for the short term and few work for the long term without strong self-control, family support, and exercise."
The theory: Chronic stress leads to weight gain, and chronic stress is at epidemic levels (just like obesity).
The research: Whole books have been written to explain how stress leads to weight gain -- "The Cortisol Connection: Why Stress Makes You Fat and Ruins Your Health -- and What You Can Do About It"; "Fat Around the Middle: How to Lose That Bulge for Good."
Cortisol is often called the "stress hormone" because it's part of the body's fight-or-flight response. It's a good thing when you're being chased by a lion (or chewed out by an angry boss), but many doctors and scientists believe that chronic stress is anything but good.
Studies have shown that cortisol makes people crave rich sweets in the worst way -- and pile on pounds in the worst place, around the middle, putting a body at risk for bad cholesterol, heart attacks and strokes. One study compared women with high waist-to-hip ratios to women with low waist-to-hip ratios and found that the former secreted more cortisol in stressful lab situations and self-reported more stressful feelings.
In 2007, researchers introduced a different notion of how stress is related to weight gain. Their study compared stressed mice (who had to live either in cold cages or with a bunch of mean cousins) and unstressed mice (who, relatively speaking, had the life of Stuart Little before all that bad stuff happened). When fed a high-fat, high-sugar diet, all the mice gained weight, but the stressed mice gained twice as much. The scientists found that a molecule in the stressed mice -- neuropeptide Y -- activated a gene in fat cells, causing the cells to grow in size and number. When that gene was blocked for two weeks, the mice lost 40% of the weight they had gained.
Are we more stressed these days? "I would say that modernity . . . provides more factors that are a source of stress," says Angelo Tremblay of Laval University in Quebec.
Our experts weigh in: Susan Roberts of Tufts University says lots of research shows changes in food preferences for animals under stress. And Dr. Julie Lumeng of the University of Michigan says studies have shown that obese people are less likely than others to be drug addicts or alcoholics -- "the thought being that if you use food to 'soothe your mood,' you will be less likely to need to use alcohol or drugs to 'soothe your mood.' "
STUDYING 'INFECTOBESITY'The theory: Certain viruses may put people at greater risk of becoming obese.
The research: At least 10 viruses are believed to cause obesity in animals, and two have been tenuously linked to people. Antibodies against one (SMAM-1, which causes obesity in chickens) were found in about 20% of a group of 50 obese people tested in 1992. (Scientists don't know how many non-obese people would have antibodies to this virus as well.) In 2005, in a study of 500 people tested, antibodies to Ad-36 -- a virus that causes symptoms similar to the common cold -- were found in about 30% of obese people, but only in 11% of non-obese people. And in a 2005 test of 89 twin pairs, if one had antibodies and the other didn't, the one who did was generally heavier.
After exposure to Ad-36, chickens, mice, monkeys and rats hardly act under the weather at all, but their body fat increases, sometimes even doubling. Strangely, though, their total cholesterol, "bad" cholesterol and triglyceride levels go down. People who have antibodies for Ad-36 also have better metabolic profiles than people who don't.
It's unknown whether more people are exposed to Ad-36 now than 30 years ago since no one was tested then.
Our experts weigh in: "Obesity due to infection is possible in some people. How big the group is I don't know. . . . Not all obesity is due to viral infection," says Nikhil Dhurandhar of Louisiana State University, who has studied SMAM-1 and Ad-36 and coined the term "infectobesity" for "obesity of infectious origin."