Q&A: Is being skinny risky?

I don't smoke or have any disease. But my body mass index is in the underweight range. Is that unhealthy? —A.M., Elk Grove, Calif.

Almost surely not. Body mass index (BMI) was devised to measure obesity, and conclusions drawn about thinness have been misleading. The BMI is calculated by multiplying weight in pounds by 705, dividing by height in inches, then dividing by height again. A BMI of 30 and over is considered obese, 25 through 29 overweight, and 20 through 24 ideal. Scores below 20 are considered underweight because studies have linked them to increased risk of premature death. But that's almost certainly because disease, eating disorders, or malnourishment makes some people thin, not because thinness make people susceptible to deadly diseases.

And while a low BMI may slightly increase your risk of developing osteoporosis, you can counter that by getting enough exercise and calcium. Note that BMI may be misleading in another way: People who have bulky muscles may be classified as overweight even if they have little body fat.