Q: I'm on a weight-loss diet and my friend keeps warning me about the risks I'm taking. That seems nuts since being overweight is what's really risky. Are there dangers to losing weight?

A: Achieving and maintaining a healthy weight is not risky. It's essential for improved health. But — and there is always a but —crash dieting or yo-yo dieting is a whole 'nother story. Weight loss should be done by adopting new habits for a lifetime, not following extreme or rigid diets that virtually guarantee failure.

Research on The Biggest Loser contestants showed that extreme dieting slows metabolism far more and for a longer time than ever realized. And a meta-analysis of studies found dieters regain over half of what they lose within two years.

Crash dieters can:

• lose muscle strength
• develop a slowed heart rate
• electrolyte imbalances
• decreased oxygen utilization
• restricted brainpower
• depression
• throw leptin (I'm full), ghrelin (I'm hungry) and insulin hormones off-balance, making weight regain even more likely

In extreme cases, crash dieters can see:

• hair loss
• sleep disturbances
• disruption or complete stopping of the menstrual cycle