Here are three possible explanations:
Most people underestimate the amount they eat, studies show—and it's more likely to be true the heavier a person is. "Scientists have searched for people who eat very little yet weigh a lot," says James O. Hill, PhD, director of the Center for Human Nutrition at the University of Colorado and co-founder of the National Weight Control Registry (NWCR), which tracks people who have maintained a loss of at least 30 pounds for at least a year. "What they have found instead are people who say they eat very little but turn out to eat quite a bit when their food intake is monitored. Rigorous studies show that it's impossible to be a really large person and not eat that much."
Obesity researchers say this gap between perception and reality is not due to conscious lying; these people truly believe they're living on very little. For a study published 15 years ago in the New England Journal of Medicine, Steven Heymsfield, MD, and colleagues used a sophisticated technique to monitor nine women and one man who weighed, on average, nearly 190 pounds, even though they insisted they ate only about 1,000 calories a day. The results were startling, especially to the subjects. It turns out they were actually consuming about 2,000 calories a day—twice what they'd estimated. And though they guessed they were active enough to burn about 1,000 calories a day, the number was closer to 770.
Mary Schreiner, 61, a recently retired weight management counselor for the University of Colorado's Health Sciences Center, understands how this could happen. Barely more than five feet tall and 160 pounds as a young woman, she tried counting calories and eliminating fattening foods, but the weight just wouldn't come off. The problem, she realized later, was that "while there were 75 calories in the cookie I wasn't having, I didn't know how many calories there were in the orange juice I was guzzling." Many of her clients were like her—drinking a lot of lattes, for example, because "coffee has no calories, right?" But they never registered the fact that each latte can have 200 calories. Or those who said that sure, they walked 10,000 steps a day but, when given pedometers, clocked in at only 1,500.
Another reason people may feel they're starving themselves, says Hill, has to do with the metabolic drop caused by dieting: The lower your body weight, the fewer calories you need to maintain that weight. (Exercise, especially weight training, helps mitigate this unfair truth.) "Let's say you weigh 250 pounds and eat 3,000 calories a day," explains Hill. "Then you lose 50 pounds. To keep that off, you're going to have to eat only 2,300 calories a day—and it is very difficult to eat 700 fewer calories than you're used to."
As for simply being born with a slow metabolism, that may be another common misperception among the overweight. When Heymsfield carefully tested his subjects—several of whom claimed to have this problem—all 10 had metabolisms within the normal range. But instead of being relieved to discover that there was nothing medically wrong with them—they just needed to readjust their intake and output—"they were angry," recalls Heymsfield. "They said, 'No, you can't be right.' Some said, 'My metabolism really is slow; you just don't know how to find it.'"
Heymsfield's subjects might be onto something, according to new research by James A. Levine, MD, a professor of medicine at the Mayo Clinic College of Medicine in Rochester, Minnesota—but not in the sense that their bodies don't burn food efficiently. Levine says some people have a biological drive to beach themselves in a Barcalounger while others constantly flit around like hummingbirds. He calls this nearly unconscious physical activity NEAT, for "nonexercise activity thermogenesis." NEAT encompasses everything from sitting up straight to tapping your foot to gesturing with your hands when you talk. A hundred or so years ago, he says, people typically burned 1,500 more calories a day than they do now. Even those who would rather be relaxing, thank you, had to plow the fields or walk to town or take the laundry to the creek and slap it on rocks. But in our age, people born with the urge to sit find that the world is one big, comfy couch—an inert way of life that, Levine believes, is enough to explain the obesity epidemic.
For a study published two years ago in Science, Levine took 20 self-described couch potatoes—10 lean and 10 mildly obese—and dressed them in high-tech underwear that recorded their bodily movements every half second for 10 days. He discovered that his leaner spuds burned about 350 more calories a day through NEAT—or 33 pounds a year.
In an earlier NEAT study, Levine recruited 16 volunteers and for two months had them eat 1,000 calories a day over what they needed to maintain their weight. You would expect they'd all put on weight—1,000 extra calories a day is a lot. But at the end of the study, the gain per individual ranged from less than a pound to more than 9 pounds. And all the variation, says Levine, could be explained by the amount of NEAT.
The good news is that if you're not a natural-born fidgeter, you can consciously work at overriding your biology. When Levine noticed his body starting to thicken as he hit middle age, he put a treadmill in the living room, and every night when he came home and watched The Simpsons (some have their wine, others de-stress with Homer), he did it while walking. He lost 15 pounds over a period of nine months without changing anything he ate.
It may sound far-fetched, but the theory that a virus can make you fat is gaining credibility. In 1986 Nikhil Dhurandhar was treating obese patients in Bombay, India, while working on a PhD in biochemistry, when he had a conversation with a fellow scientist about an avian virus that was killing poultry. The scientist mentioned an odd effect the virus had on the infected chickens: Their abdominal cavities were full of fat, and the dead birds were far heavier than their healthy counterparts. A sick chicken should be a skinny chicken, Dhurandhar thought. He wondered what would happen if he exposed normal chickens to the virus. Sure enough, the ones that got infected developed significantly more body fat than the healthy birds and, paradoxically, lower cholesterol and triglycerides.
The findings were so compelling that he decided to test his patients for antibodies to the virus—and he discovered nearly 20 percent of them had been infected. Not only that, these were among the heaviest people in his practice, and they had lower cholesterol and triglycerides than most of his other patients.
Today Dhurandhar is a scientist at the Pennington Biomedical Research Center in Louisiana, studying a field he has named infectobesity. He and others have found nine viruses that cause obesity in animals, four of which also infect humans. He may have discovered part of the mechanism as well: After animals are infected with one particular human virus, their prefat cells mature and proliferate, increasing the number of fat cells in the body.
Dhurandhar says we are a long way from being able to tell some overweight people that their problem is a virus, or better yet, offering an obesity vaccine. But he points out that there is exploding research in the area of germs causing other chronic illnesses such as heart disease, autoimmune diseases, even depression. And he cites the experience of the two Australian researchers who suggested that a bacterium was responsible for stomach ulcers and were scoffed at for years—until they won the Nobel Prize for Medicine in 2005. Famously, one of those researchers swallowed a Petri dish of the bacteria to prove his case. Is the slender Dhurandhar willing to infect himself with one of his viruses to prove his thesis? He laughs and says if he did it and gained weight, "people would just say I ate too much."
Chicken viruses, NEAT factor—maybe these culprits explain why we're fat. Or maybe we get too little sleep. Or have too much stress. Or modern indoor temperature control protects us from the hot, sweaty, appetite-dulling days and shivering cold that used to keep people trim. Every few months, there seems to be a new theory. But whatever pans out among these ideas, science knows right now what works to lose weight and keep it off: Move more and eat less. That means making active choices whenever possible—getting up and changing the TV channel, taking the stairs, and being more conscious when it comes to diet (keep a food diary; look up calories at nal.usda.gov/fnic/foodcomp/search, and check out NWCR.ws). As for those people who holler that they eat like birds but complain they look like butterballs, whatever the cause of this dilemma, if you're mindful about what you put in your mouth, it feels as if you're eating a lot more.
Emily Yoffe is a columnist for Slate.com and the author of What the Dog Did (Bloomsbury).