You have a week to fit into that dress, and five pounds (O.K., 10) to drop. The plan? If you were a Hollywood star, you might eat nothing but baby food or grapefruit until then, or forgo meals in favor of liquids. If you were Kim Kardashian, you'd probably prefer a QuickTrim detox formula. Or if you were Michelle Obama, you would opt for a two-day vegetables-only "cleanse," as she calls the regimen in an interview in the September issue of Ladies' Home Journal.
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"People could eat nothing but jelly beans and if they were eating just a small amount, they would lose weight," says Donald Hensrud, chairman of preventive medicine at the Mayo Clinic and medical editor-in-chief of The Mayo Clinic Diet, a guide to healthy weight loss. "You might be able to get away with it for a period of time, but the more restrictive [the diet] is—and the longer you follow it—the greater the risks."
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Crash diets are a tempting way to lose weight fast, says Hensrud. But most experts agree that they're not worth the risk. Just one week of overly restrictive dieting can cause serious nutritional deficiencies, alter your metabolism, and undercut your emotional well-being. And most crash diets only set you up to regain the weight, since you haven't made any long-term lifestyle changes.
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"When people go on really rigid, low-calorie diets, they gain the weight back," says Katherine Tallmadge, a registered dietitian and author of Diet Simple: 192 Mental Tricks, Substitutions, Habits and Inspirations. "Their plan backfires. You might lose weight through severe dieting, but you don't develop the habits you need to keep it off, like getting the right amount of exercise."
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Short-term dieting becomes especially unhealthy below 1,000 calories a day, warns Hensrud. While dipping below that level is dangerous for anyone, the threshold for a particular person could be significantly higher, depending on age, height, weight, activity level, and body composition. The majority of women in their 30s and 40s, for example, need roughly 1,800 calories a day to stay healthy; for men in that age range, it's about 2,200. The American Dietetic Association (ADA) defines healthy weight loss as one to two pounds per week; for each pound you want to lose, you should consume 500 fewer calories a day—or burn them off through exercise. It's no trick to shed far more than a couple of pounds each week, but you could run up some serious nutritional deficiencies: It's hard to get enough calcium, vitamin D, or iron on a radically reduced number of calories. You could permanently damage your organs by not providing them with sufficient working fuel. And—to be blunt—crash dieting could kill you if you lose too much fluid and your electrolytes go out of whack, says Hensrud, who has treated several short-term dieters who were hospitalized for dehydration. One of them had alarmingly low levels of potassium, sodium, and other vital electrolytes, which could cause muscle cramps, dizziness, fainting, or even a heart attack.
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Even if a crash diet puts smaller numbers on the scale, the weight loss may be illusory or harmful. The first few pounds to go are usually water, and they inevitably return, says Cheryl Forberg, staff nutritionist for NBC's The Biggest Loser. You can lose muscle mass—on near-starvation diets, the body starts to feed on protein for sustenance. And don't be surprised if you're more snappish: Irritability, depression, and inability to handle everyday stress are travel companions of low-cal diets.
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