What Is Your Weight Destiny?

Why Big Gains Are a Bad Idea

A few pounds are easier to deal with. "You can lose 5 or so percent of your body weight and with a little effort, keep that off," Dr. Greenway says. Dieting is key to reducing, exercise is key to maintaining.

A cinch in time saves nine. If you haven't gained a lot of weight, "you don't have to do as much as someone who has," says researcher James O. Hill, PhD. "It doesn't take 90 minutes of exercise a day to prevent weight gain, but it may take that much to keep pounds off once you've lost them. It's not fair, but that's the way it is."

The more you have to lose, the more vigilantly you'll have to diet -- permanently. Let's say you cut back from 2,000 to 1,500 calories a day. Dr. Heymsfield says that the pounds will come off, but that over the course of a couple of years, as your metabolism slows to conserve weight (and as a lighter person, you're burning fewer calories during any activity), you'll need only 1,500 calories a day to stay the same. Go back to eating 2,000 calories, and the weight returns. Adding muscle helps, but a pound of it burns only about 14 calories more a day.

Larger weight losses make your hormones go haywire. Dr. Proietto's research found that once you lose 10 percent or more of your body weight, the levels of certain hormones, including leptin and ghrelin, get all out of whack and stay that way for an unknown amount of time, so your brain tells you you're hungry even when your body doesn't need the calories.

When you have to maintain a diet for a long time, your mind plays tricks on you. As you first start dieting, says John R. Speakman, PhD, of the Institute of Biological and Environmental Sciences in Scotland, your body is blowing through its glycogen reserve and shedding the weight of the water that glycogen is stored with, so the scale shows a big drop. "Studies in the lab have suggested that if you stay on a diet, the weight loss after this initial drop is pretty steady and doesn't reach a plateau," he says. But in the real world, because weight loss appears to slow down, people tend to lose their resolve and become a little less strict with their diet than in those first weeks, thereby creating an actual plateau.