Health experts now think it's altogether possible to be overweight — but still fit.

They say measuring only an individual's BMI, which is a measurement of body fat based on height and weight, can be misleading. Muscle weighs more than fat, for one thing. Extremely muscular individuals could actually have BMIs that classify them as overweight or even obese. Increasingly, health experts say a better measure of overall health includes not only BMI but a test of "fitness," too.

Cardiologist C. Noel Bairey-Merz is one of those experts. She's the director of the Women's Heart Center as well as the Preventive and Rehabilitative Cardiac Center at Cedars-Sinai Heart Institute. As a result of her research, she says "fitness" now trumps "fatness." She says those who are fit are people who could walk 30 to 60 minutes without having to stop; who could climb two flights of stairs without becoming winded; or who could do some mild to moderate aerobic activity — a brisk walk or short jog, for example.

Such routine exercise strengthens heart and lung function, bones and muscles, as well as how the body processes oxygen. The greater the efficiency of these functions, the greater one's fitness.

Fitness can be measured with a simple questionnaire that asks patients about their level of physical activity. If a physician is particularly concerned about fitness, he or she can administer a stress test, which usually requires the patient to walk briskly or jog on a treadmill for a certain period of time while blood pressure and oxygen levels are measured.

Thin Does Not Equal Fit

In one study, Bairey-Merz found that women who were routinely physically active and overweight were less likely to suffer heart problems than their normal weight counterparts who didn't exercise.

And conversely, normal weight women who did not exercise, she says, increased their risk of cardiovascular disease.

"They don't have the cardiovascular reserve," she says. "They're fine sitting quietly, working on the computer, hopping on the Internet, watching DVDs. Everything's fine at rest."

But when they actually have to do something, or when they face an actual cardiovascular stresser — like a car accident, heart attack or pneumonia — then, Bairey-Merz says, they do not have a fitness level that is going to help them do well and survive.

They're going to be at more risk for premature death, disability and disease, she says, adding that there is some evidence that being overweight is protective when people get sick.

"Fat is our energy storage device. It's, in a way, a little bit like having something in the bank," she says.

When Pounds Come In Handy

A number of studies have found that overweight people demonstrate a lower death rate than their peers who are underweight, obese or, most surprisingly, normal weight.

According to Bairey-Merz, a few extra pounds may be beneficial.

"If you're suffering a critical illness in the ICU [intensive care unit], you're often on a ventilator. You can't eat, are fed water by vein, and stressful conditions like (a severe skin) burn, pneumonia or heart attack are consuming calories," she says.

But is an extra 10 pounds always OK as long as you're exercising? Health experts say not necessarily.

Problems With Packing Pounds

When metabolism slows down at around age 50, men tend to gain weight in the abdomen while women tend to gain weight in their hips and thighs. And it turns out, one is more dangerous than the other.

Abdominal fat is worse than fat elsewhere in the body. Geriatrician Arun S. Karlamangla is a professor and clinician at UCLA. In one study, he analyzed the health status of more than 4,000 men and women nationwide.

He found that men with a waist circumference larger than their hips experienced a 75 percent increase in death rate. For women, risk increased gradually with every inch of increased waist size.

Researchers don't know exactly why belly fat is particularly dangerous, but suggest it interferes with the body's ability to process sugar.

Abdominal, or visceral, fat increases insulin resistance, which increases levels of insulin floating around in the body, which increases the amount of glucose in the body, which eventually leads to diabetes. Diabetes increases the risk for heart attacks and strokes.

To keep abdominal fat down and to maintain or build fitness, the federal government recommends 30 minutes of moderate exercise every day. If you want to lose weight, exercise time increases to 60 minutes a day.

And health experts like Bairey-Merz suggest doctors pay as much attention to "fitness" as they do to "fatness."