"But I Don't Eat That Much!"

There's your friend in the size 6 jeans who always seems to be attacking a cookie-dough ice cream cone; there's the stick-figure colleague who lunches on burritos the size of her head. And then there's you. Day after day, you toss the bread from your turkey sandwich, nibble on a Baggie of carrots, and refuse desserts—yet you can't get the scale to budge downward. How is it that you're still heavy when you'd swear on a stack of pancakes, "But I don't eat that much!" Of course, many will acknowledge there's no mystery as to why they struggle with their weight: They eat more than they should. But a persistent minority of people recount tales of heroic food deprivation followed by a humiliating inability to lose a single pound. What's going on with them? Here are three possible explanations.

Second-helping amnesia

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, Ph.D., director of the Center for Human Nutrition at the University of Colorado and cofounder 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, M.D., 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 5 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.'"