David L. Katz, MD
Photo: Mackenzie Stroh
Q: In February 2007, I weighed 247 pounds. I went on a liquid diet—900 calories a day—because my blood pressure was so high. After dropping 55 pounds, I switched to a 1,200-calorie Weight Watchers plan. In three and a half months, I regained 36 pounds. Why can't I lose weight on 1,200 calories a day?
— Carol Duesterhoeft, Mount Vernon, South Dakota

A: There are two possible reasons the 1,200-calorie diet didn't work. The first is that without realizing it, you were getting extra calories from sources like sauces, spreads, and dressings. This is a common pitfall for dieters, and it could put you well above 1,200 calories. (One thing that makes liquid diets so effective—besides their being incredibly low in calories—is that you get all your meals from one source. There's no way to unintentionally overeat when your entire menu consists of three shakes a day.)

The other possibility is that 1,200 calories is more than you burn in a day—if you happen to have a very slow metabolism or low activity levels. Genes determine your metabolic rate—the number of calories you burn while at rest—so you don't have a lot of control over that number (although extreme diets and weight loss can in fact slow your metabolism). But you can boost your activity levels by exercising daily, and a strength-training regimen that adds muscle should increase your calorie burn. It's been said many times before, but all weight loss comes down to a simple equation: You need to expend more calories than you take in. I'm guessing that you've been dealt a bad genetic hand—your calorie needs are much lower than that of most people. But you've already demonstrated your ability to lose weight; with some extra effort, you can turn your hand into a winning one.

David L. Katz, MD, is director of the Yale-Griffin Prevention Research Center and president of the nonprofit Turn the Tide Foundation.