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Before you embark on the latest fad diet to lose those pesky holiday pounds, you might want to get to know your metabolism a little better. It’ll make all the difference.

It provides a convenient scapegoat, our metabolism. Some days, the mysterious biological process seems to provide the only explanation for why some people can burn off breakfast by running errands while others have calories that never truly vanish. Unfortunately, the metabolic reality appears to be complex and likely a bit frustrating.

Granted, the success of your battle with your waistline in 2009 rests largely upon basic math: Do you eat more daily calories than you expend? Still, some factors beyond your control, including age and genetics, may unfairly stack the metabolic deck. “Some people are naturally gifted in that they have a higher metabolism than other people,” says Tara Gidus, an American Dietetic Association spokeswoman.

The intriguing and hotly debated question is whether or not you can tweak your metabolism, at least to some degree. How many hours you sleep, how much muscle mass you maintain, how frequently you eat -- all may play a role, according to some researchers and clinicians. Even how much you putter around the house and yard may contribute.

“We all have to play the genetic cards we’ve been dealt,” says Tedd Mitchell, MD, chief executive officer of the Dallas–based Cooper Clinic and coauthor of Move Yourself (see “Metabolic Reading Material”). “But while a percentage of your metabolism has been ingrained genetically, a large percentage of your metabolism is behavioral.”

YOUR METABOLISM -- the burning of calories to fuel your body -- depends upon three primary components, beginning with how fast you burn calories at rest. The resting metabolic rate is influenced by age, genetics, and your body’s own hormonal makeup, including thyroid, estrogen, and testosterone levels, Dr. Mitchell says. Then there’s the energy your body consumes to actually digest that apple or steak dinner you’ve eaten; it’s a relatively small metabolic contributor. And lastly, you have to take into account your daily activity level, considering everything from fidgeting in meetings to swimming laps.

Right out of the gate, men usually enjoy a metabolic advantage because they pack more muscle than women of the same age do and muscle helps to burn calories, says Gidus, also a nutrition coach at the Human Performance Institute in Orlando, Florida. Muscle mass is so important that it explains another fact: Metabolism slows with age.

Once you reach your mid-30s, Dr. Mitchell says, your muscle mass starts “very slowly drifting down.” Specifics vary, but the rule of thumb is that people lose one percent of lean muscle mass for every year past age 40, a decline that accelerates to about three percent annually by one’s mid-50s, he says.

Strength-building exercises can somewhat offset that muscle loss, but they likely won’t forestall it entirely, says Charles Burant, PhD, MD, director of the University of Michigan’s Metabolomics and Obesity Center. “Even super athletes can’t do it.”

Prepared to give up? Not so fast. When combined with calorie control, retaining muscle can enable you to achieve better long-term weight loss, Dr. Mitchell says. “Muscle makes you a better butter burner -- period.”

Here are some other strategies that clinicians and researchers are studying, and debating, as they delve further into the mechanics of metabolism

SLEEP: If you’re prone to hitting the snooze button, consider that some recent studies have linked sleep loss with weight gain. Women who failed to sleep more than six hours nightly were 12 percent more likely to gain 33 pounds than those who slept seven or eight hours, according to a 2006 study in the American Journal of Epidemiology. The researchers, who tracked nearly 70,000 women for 16 years, found an even higher risk, 32 percent, when average sleep maxed out at five hours. One theory is that sleep deprivation affects the production of leptin, an appetite-suppressing hormone, Dr. Mitchell says.

EATING TIMETABLE: You’ve heard this advice before. Eat breakfast to jump-start your morning metabolism. And break your daily food consumption into smaller and more frequent meals and snacks. Skipping meals is particularly bad, Gidus says, because it encourages your body to shift into protective mode, storing calories as fat. Dr. Burant is less convinced of that, though. Smaller meals may blunt hunger, but research hasn’t shown any evidence that meal scheduling makes a huge metabolic difference. A calorie is a calorie, he says. “I don’t think your body, in general, really cares when you eat.”

METABOLIC PROFILING: Numerous diet books and related products relay various theories regarding the body’s ability to metabolize certain foods, most notably carbohydrates and proteins. Proponents of profiling typically maintain that some people respond better to protein and thus can achieve more energy and diet success on low-carbohydrate, high-protein diets, while those classified as carb types should eat a higher percentage of healthy carbohydrates. Dr. Mitchell doesn’t subscribe to any particular diet, but he suggests talking with your doctor about getting some baseline tests, including for blood sugar and cholesterol, if you’re struggling to lose weight. Some people, for example, may have elevated blood sugar -- albeit not to the point of being diabetic -- and thus may be more prone to elevated insulin levels in their body, which promote weight gain. Not surprisingly, there’s disagreement. Such screening tests can indicate your long-term risk of heart disease and other chronic illnesses but have “nothing to do with whether you’re able to lose weight or not,” says Samuel Klein, MD, director of the Center for Human Nutrition at Washington University School of Medicine in St. Louis.

THYROID CHECK: Consider getting your thyroid checked if you’re sticking to your diet and exercise regimen (really and truly) and your weight loss is slim to nil, Dr. Mitchell recommends. The thyroid gland, located in the base of the neck, helps to control metabolism. So an underactive thyroid, which is far more likely in women, can sabotage your best weight-loss efforts.

EVEN IF YOU’RE not hitting the gym, you can boost your metabolism over the long haul, Dr. Mitchell says. Bolster your daily activity in seemingly small ways: Take up gardening, walk to your child’s bus stop, straighten up the house, and park several rows back. People have strayed too far from their hunter-and-gatherer heritage. “We’ve systematically worked physical activity out of our daily routine,” he says.

Some people, though, may be prewired to expend energy without even realizing it. A research team at the Mayo Clinic has been studying what they call NEAT, short for non-exercise activity thermogenesis. (Think of that friend who can’t linger over a cup of coffee.) In a pilot study, researchers found that unlike lean individuals, people with mild obesity spent more than two hours sitting daily. By adopting the NEAT behaviors of their lean counterparts, they could burn 350 more calories a day.

For Sam Zaman, changing his meal habits became the cornerstone of the diet he launched in January 2008. Weighing in at 293 pounds, the 28-year-old real estate appraiser typically skipped breakfast, worked through lunch, and overindulged at dinner and beyond. Now Zaman preplans meals and packs snacks so he can eat frequently, and he strives to consume no more than 1,400 calories a day. He hits the gym at least five days a week, splitting an hour-long workout between cardiovascular exercise and weight lifting. And he typically gets seven-plus hours of sleep, rather than watching television into the wee hours with a snack nearby. In short, he’s a metabolic enthusiast.

“That’s what I really attribute my success to -- understanding how the body works and how it burns the food that I eat,” he says.

By early fall, the Queens, New York, resident had surpassed his 175-pound goal, carrying just 167 pounds on his five-foot-eight frame. His waist size has plummeted from 48 to 32 inches, and he has a new number fixed in his line of sight: 150 pounds.
Metabolic Reading Material
Want to learn more about better fitness and other metabolic contributors? Take a peek inside these books:

Break Through Your Set Point: How to Finally Lose the Weight You Want and Keep It Off by George Blackburn, PhD, MD (Collins Living, $25)
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The Complete Idiot’s Guide to Boosting Your Metabolism by Joseph Lee Klapper, MD (Alpha Books, $19)
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Move Yourself: The Cooper Clinic Medical Director’s Guide to All the Healing Benefits of Exercise (Even a Little!) by Tedd Mitchell, MD; Tim Church, PhD, MD; and Martin Zucker (Wiley, $25)
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Spark: The Revolutionary New Science of Exercise and the Brain by John J. Ratey, MD (Little, Brown and Company, $25)
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The Ultimate Metabolism Diet: Eat Right for Your Metabolic Type by Scott Rigden, MD (Hunter House Publishers, $15)