And how to be more like those lucky bastards who eat whatever they want.
It's impossible to pick up a health magazine or diet book without reading about "metabolism," and how to kick-start, ignite, rev up, or otherwise recalibrate yours to get a trimmer, leaner physique.
In most cases, diet promoters and fitness gurus use "metabolism" as fancy sounding short-hand for the amount of calories your body burns, whether it's during a workout or after a meal. And that's part of it, says Arthur Weltman, chair of kinesiology at the University of Virginia. "Metabolism is an indication of how many calories we expend each day," he says. "There are a number of things that go into [a person's metabolism], but it's basically three major components."
The first component is your resting metabolic rate, Weltman says, which refers to the amount of energy your body burns just to keep you alive and in a happy state of biological homeostasis. The second component is diet-induced thermogenesis, or the energy you burn eating and digesting food. "The third would be calories you expend during physical activity," he says.
So it's true that what you eat and how much you move can "crank" your metabolism—at least temporarily. "Some foods require more energy to digest and absorb," Weltman explains. (Think kale and fiber-rich fruit, as opposed to heavily processed snack foods.) Likewise, some forms of exercise—especially longer workouts that are fairly intense—can keep your metabolism elevated for a while even after you stop moving.
But together, the calories you expend digesting food and exercising may make up less than 25 percent of your total metabolism, according to a 2011 study from the University of Florida. The majority of the energy you use each day—somewhere between 60 and 80 percent of your total daily calorie expenditure—is wrapped up in your resting metabolic rate (RMR).
As you might expect, most of the scientific literature suggests raising your RMR is the key to sustained weight loss, Weltman says. But accomplishing that kind of shift is tricky. "When you eat less to lose weight, your body senses that you're consuming fewer calories, and there's a compensatory effect," says Bahram Arjmandi, a professor of nutrition, food, and exercise science at Florida State University. By "compensatory effect," he means your RMR may slow down in response to the reduced amount of energy you're consuming. The result: While going on a "cleanse" or cutting your meal portions in half may help you drop weight quickly, that weight loss will soon taper off, and the lost pounds will be hard to keep off in the long run.
The same kind of compensatory effect can limit the weight-loss benefits of exercise. The more you move, the more food your body may crave to satisfy your newer, higher energy needs, Arjmandi explains. Responding to that hunger with snacking or a few extra forkfuls at meal times can wipe out any weight-loss gains.
Also complicating matters: "There's a lot of person-to-person variability when it comes to metabolic rate," Weltman says. One 2005 study in the American Journal of Clinical Nutrition found one person may burn nearly 1,500 more calories per day than another, even if both eat the same foods and perform the same exercises. (That explains why some of your friends can eat whatever they want, never exercise, and still look fit.)
Weltman says there also seems to be a large genetic component when it comes to your RMR. But regardless of the metabolism you were dealt at birth, your RMR tends to slow by 1 to 2% each decade once you hit age 20. Eat the same foods at 40 as you did at 18, and you're probably headed for a larger pant size. None of this means dieting and exercise aren't helpful if you want to lose weight—they are. But if you want to adjust your RMR in ways that promote sustainable weight loss, you can't just run every day and cut your calorie intake.
What should you be doing? When it comes to exercise, your goal is to reduce fat and add muscle. "Lean tissue, and muscle tissue in particular, requires more energy to maintain than fat," Arjmandi says. "The more you can change the ratio of muscle to fat, the more you can increase resting metabolic rate." That means incorporating resistance training—not just cardio—into your exercise routine, he says.
Also important: Eating "high-quality" protein to maintain muscle mass. Especially if you're cutting back on your food intake in order to lose weight, it's essential to eat either meat or plant-based proteins that contain all the essential amino acids your body needs to maintain muscle, he says. (Some research from Harvard University's David Ludwig suggests adding healthy fats—stuff like avocados or nuts—to your diet may result in some beneficial metabolic changes.)
Other factors may also promote a higher metabolic rate. There's some research that spending time in cold environments can increase your body's amounts of metabolically active "brown fat," too, which may help you burn calories even when at rest. Fidgeting and other "non-exercise" forms of physical activity may also up your daily calorie burn in significant ways.
But both Weltman and Arjmandi say the real keys to stronger metabolism and sustained weight loss are a healthy diet and regular exercise that contains a resistance training component. "What you see people doing to lose weight on a show like The Biggest Loser is not a realistic approach," Weltman says. "Quick fixes and rapid weight loss usually lead to weight rapid regain."
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