Want a healthy body? It's all about that fat and muscle, not your BMI

You know your body weight. You may even know your BMI, or body mass index.  But do you know what your body is made of?

If the answer is “too much fat and not enough muscle,” that’s bad news — no matter what you weigh.

“Body composition is the key to truly understanding health,” says Lauri Wright, an assistant professor of nutrition at the University of South Florida and a spokesperson for the Academy of Nutrition and Dietetics. While most people who are “overweight” also are “over fat,” she says, it’s the fat that’s dangerous. You also can be “skinny fat,” she says — carrying too little muscle and too much fat at a normal weight.

All of this is well known to experts in obesity, diet and exercise. But the experts say the public continues to struggle with the concept of body composition, even as research makes it increasingly clear that fat and flabbiness, not pounds alone, are the enemy.

The latest evidence: a big Canadian study of more than 50,000 people, mostly women over age 40.  Researchers looked at BMI — a measurement of weight in relation to height — and at body fat percentages obtained from scans. The major finding, published in the Annals of Internal Medicine: People with the most body fat, more than 35% fat for men and 38% fat for women, were the most likely to die within a few years, regardless of weight and BMI.

“When people are having discussions with health care providers about their body weight, they have to focus on more than what it says on the bathroom scale,” says study co-author William Leslie, a professor of medicine and radiology at the University of Manitoba.

So here’s what you need to know:   

It’s still worth knowing your body mass index.
Because your doctor can calculate your BMI at a glance (by plugging your weight and height into a chart), it’s a good starting point, says Martin Binks, associate professor of nutritional science at Texas Tech University. But, Binks says, a high BMI alone does not justify a diagnosis of obesity or mean a person is at increased risk for diabetes, heart disease and other fat-related diseases. Likewise, a low BMI is not necessarily a mark of health.

Muscular athletes with little body fat may have very high BMIs and be perfectly healthy, he says. “If Arnold Schwarzenegger is sitting in my office, I’m not going to tell him he’s suffering from obesity,” Binks says.
Likewise, a frail, elderly person who has lost muscle and weight because of aging, illness and inactivity may have a low BMI but high body fat and many health risks. In fact, the Canadian study, like previous studies, found a link between low BMI and premature death.

Getting an exact body fat percentage is a little trickier

Some newer digital scales display a body fat percentage. These home scales, and more expensive devices in some doctors’ offices and gyms, use a technology called bioelectrical impedance (BIA) to detect fat levels. But they can give varying results depending on how hydrated you are, Binks says. Some professionals rely on other pricey technologies. And some fitness trainers still use hand-held fat calibers to measure body fat, but their accuracy depends on the trainer’s skill level, says Cedric Bryant, chief science officer of the American Council on Exercise.

When you do get a body fat percentage, what does it mean? While there’s no universally-accepted cutoff for healthy vs. unhealthy numbers, the exercise council says athletic or fit women maintain ranges of 14% to 24%, while athletic or fit men see ranges of 6% to 17%. One U.S. government study found average levels of 40% fat for women and 28% fat for men.

The location of your fat matters.
One reason your doctor may not insist on getting an exact body fat percentage: You can learn just about as much by putting a tape measure around your waist. “Waist circumference is going to correlate very highly with body fat,” especially dangerous abdominal fat, says Tim Church, professor of preventative medicine at Pennington Biomedical Research Center at Louisiana State University. Waist sizes of more than 40 inches in men and 35 inches in women are linked with increased risks for type 2 diabetes and heart disease, according the federal Centers for Disease Control and Prevention.

There’s no fat-burning secret. 
Cut calories and burn them, through diet and exercise, and you will burn fat. You also will burn some muscle, so you will do your body the most good if you include weight-lifting and other muscle-strengthening moves, Church says.
Binks says that “the general thing the public gets wrong is that there is some sort of exercise that spot-reduces body fat." Whether fat drops first from your waist or your hips is more a matter of genetics than work-out strategy, he says. 
Bryant says the best fat-burning exercise is the exercise you stick with. He also says it's a great idea to pump some iron — or use resistance bands, weight machines or other strengthening techniques — regardless of your weight goals or age. “It’s so important to preserve those lean tissues, muscle and bone mass,” to stay strong and keep functioning, he says.

As for diet, Wright says: “Claims of ‘fat burning foods’ are prevalent, but they are not true. The only way to burn fat is consume fewer calories than you expend.”