It's no secret that being fat and out of shape is a health hazard. But what if you're overweight and still physically fit? Does the fact that you're active eliminate the health risks of those extra pounds? And what if your best friend is a skinny couch potato? Which one of you is healthier?

As it turns out, the "fat but fit" debate is a longstanding source of controversy among experts. Although there's almost universal agreement that just being skinny doesn't guarantee wellness (yes, your best friend still needs to exercise), some experts believe that carrying extra weight doesn't necessarily equal poor health, while others maintain that a person can't be overweight and still meet the guidelines for a healthy body.

Measuring Up

There are numerous ways to determine if our bodies are in good shape, and perhaps the most common measuring tool used at home is the scale. But scales can't distinguish fat from lean muscle, so they're not always the best barometers of a healthy physique.

Body Mass Index (BMI) measurements have gained popularity in recent years. According to the Centers for Disease Control (CDC), BMI is a number calculated from an individual's weight and height. This does not measure body fat directly, but research has shown that BMI correlates to direct measures of body fat, such as underwater weighing. BMI is an inexpensive and easy-to-perform method of screening for weight categories that may lead to health problems. For both men and women, a BMI ranging from 18.5 to 24.9 is normal, according to the American Heart Association.

Another method for measuring health is via blood cholesterol levels. Cholesterol is a waxy, fat-like substance present in the body. It is actually necessary for the body to function normally, but the body makes enough cholesterol for its needs. When cholesterol levels are elevated, due to either an unhealthy lifestyle or genetics, cholesterol is deposited in arteries, including those of the heart--which can lead to narrowing of the arteries and to heart disease. About 17 percent of adult Americans have high blood cholesterol, which the American Heart Association defines as total cholesterol levels of 240 of above.

The Verdict

So, there are definitely more ways to measure fitness that just body weight. But is it really possible to be fat but fit? The findings are mixed.

A 1999 study published in the American Journal of Clinical Nutrition followed nearly 22,000 men for an average of eight years to assess risk factors that predispose individuals to die prematurely. The study's authors found that lean, unfit people had a higher risk of death than their overweight but physically fit counterparts.

Another study, conducted by researchers at the Cooper Institute in Dallas, came to a similar conclusion. The study's authors measured the body composition of 22,000 men aged 30 to 83, then conducted treadmill tests. Over the next eight years, 428 of the men died, and the researchers found that those who were overweight but fit, as determined by the treadmill test, were two times less likely to have died than men who were lean but not physically fit.

But other scientists disagree. A 2004 study in The New England Journal of Medicine found that obesity predicted a higher risk of death regardless of the level of physical activity. Frank Hu, a professor and the first author of the New England Journal study (which tracked 116,564 women over 24 years), explained that active, obese people did indeed reduce their mortality risk. But they were still at 91 percent greater risk of dying than active lean people. Sedentary lean people had a 55 percent greater risk of mortality.

Findings seem to lean toward the advice that while it's better to be fit and fat than unfit and fat, in the long run, it's best to be fit and lean.