The promise: You can lower the risk of your getting heart disease and improve your cardiovascular function—all while slimming down.
The problem: When you drastically cut calories, your body adapts by learning how to do more with less. It lowers your metabolic rate, holds on desperately to fat stores and takes longer to execute basic functions like breathing; it literally slows down to survive, says Sasson. There's a hotly debated theory that this adaptive response also slows the aging process. But, while initial research on the link between cutting calories and adding years seemed encouraging, the latest studies are much less convincing. (For now, the jury is still out.)
The risks: Eating just a little bit more than your strict low-calorie usual (say, during a stressful business trip) makes you more likely to put on pounds—and, worse, it's tougher to get rid of them. You're also at a higher risk of losing muscle and bone mass (and testosterone, if you're male). And unless you're following this eating plan under the close supervision of a doctor, Sasson says, you will likely end up deficient in essential nutrients.
Next: The biggest mistakes women make when dieting