Rev It Up: How to Reboot Your Metabolism

What Affects Your Metabolism

I am lying on what looks like a cross between a jumbo Xerox machine and a tanning bed at the University of California, Los Angeles, Risk Factor Obesity Program as the big mechanical arm of a DEXA (dual energy X-ray absorptiometry) scanner moves over my head and then down to my feet. I came here to get the latest high-tech body-composition tests and to learn how fast my metabolism is.

Two minutes later a virtual relief map of the muscle, fat, and bone in my body starts to fill in on a computer screen.

"I never would've guessed," says Zhaoping Li, MD, PhD, the UCLA professor of clinical medicine analyzing my results, when she reads me the verdict: 40 percent body fat. As in obese. Except I'm a size 8. Here in Los Angeles, that alone can make you a plus size, but at five feet four inches and 148 pounds, I'm really just three or four pounds overweight.

To think that I had actually been looking forward to this visit. Me, the lucky girl who never dieted, never gained the freshman 15 and, until recently, never came close to being overweight. I chalked it all up to my speedy metabolism.

Electrodes are pasted onto my hands and feet for my second test, the bioelectrical impedance analysis. This one pegs my body fat at 32.7 percent, which, I am assured, is the better figure to go with. Translation: I'm not obese, just borderline unhealthy.

I am what is called skinny fat, explains David Heber, MD, PhD, director of the UCLA Center for Human Nutrition, who oversees the obesity program. "People can appear to be thin and fit, but their body fat is putting them at risk for diabetes, heart disease, and even breast cancer," Dr. Heber says. "A lot of models and actresses who don't exercise are actually skinny fat."

For the next eight weeks I would systematically follow the advice of leading scientists and trainers to reboot my metabolism. What I found out may be the key to keeping yours from ever flatlining.

I'm living proof that you can't judge a metabolism by its cover. You and I could be the same height and weight, have the same BMI, and even fit into the same J Brand jeans, but have wildly different flab-to-muscle proportions, making one of us the calorie-burning equivalent of a Bic lighter and the other of a blowtorch.

Metabolism, simply put, is the total number of calories your body burns each day. Sixty-five percent of those calories are used up for 24-7 functions like breathing and circulation­ -- the top burners are your brain, liver, heart, and kidneys -- with another 10 percent devoted to the process of digesting the very foods that may have given you that muffin top in the first place. The remaining 25 percent of the calories you burn can be chalked up to the physical activity you do in a day -- not just Spinning class but every move you make, including standing in line or tweeting your latest DEXA scan results.

You've heard it before: The more muscle you have, the more calories you burn. In fact, lean tissues, including organs and muscles, on average burn 14 calories a pound a day, while fat only burns about three calories per pound, Dr. Heber says.

Simply put, whether you're Kate Moss or Kirstie Alley, it's the absolute amount of muscle in you that determines the overall speed of your resting metabolism -- the amount you burn just sitting around -- and some of us are born with an edge in the amount of muscle fibers we've got. But don't blame bad genes for your extra flab. "It's your environment -- that is, food and activity -- that is extremely important in ultimately determining your weight," says Andrew G. Swick, PhD, director of obesity and eating disorders research at the University of North Carolina at Chapel Hill Nutrition Research Institute in Kannapolis.

And there's plenty you can do to not only add muscle but also maximize your metabolism along the way.