By Howard Schneider
Tuesday, July 1, 2008

This week we will discuss whether there are things you can do to spend more time sitting on your ashcan. That is not the word the late George Carlin would have used. But even in the Health section, where unmentionable things are often displayed in detail, there are rules.

Why bring this up?

Because the fitness, diet and food industries barrage us with claims about methods or products or substances that "boost metabolism" and thus cause the body to use more calories even at rest. Whether it's the new secret-formula "fat burning" supplement, the "miracle" of green tea (or the Coca-Cola company's effort to capitalize on that with a green tea-based soft drink), or even the sometimes exaggerated claims for the metabolism-boosting benefits of exercise, this is the health equivalent of money for nothing -- a free lunch promise that would, if true, liberate us for more time on the Barcalounger.

Don't count on it.

One thing seems pretty clear from my own fitful experience: To a certain degree, we are what we are. Sure, you can improve the shape and composition of your body with diet and exercise. As you become more fit, you function more efficiently and become able to do more. Your muscles store more fuel and become stronger, your circulatory system gets better at processing oxygen, your resting heart rate and blood pressure might drop, and you can tap different energy systems and sources more effectively.

But the pace and extent of those changes -- the underlying way in which our bodies react and adapt to stress -- is pretty tightly circumscribed. We all know people who seem to eat and drink what they want without gaining an ounce, or who seem to easily add muscle or shave time off of their 10K pace; others of us need to work a lot harder to keep things in balance or make progress.

For those in the great mass called "average," it also seems the case that when the activity stops, some of the more widely advertised benefits taper pretty quickly as well.

Advocates of strength training in particular argue that building more muscle through exercise increases your resting metabolic rate: the number of calories the body uses at rest simply to sustain itself. The higher rate will act, they say, as an automatic form of weight control.

Well, yes, to support the extra muscle the body will use more calories all day long. But how many?

The textbooks are cautious.

Sharon Plowman and Denise Smith, in their introductory text "Exercise Physiology for Health, Fitness and Performance," write that studies of the effect of strength training on resting metabolism "are not definitive." The difference between a bit of extra fat and a bit of extra muscle might come down to just a few calories a day.

Although there are plenty of good reasons to lift weights or do other resistance training (think walk vs. walker), the "transform your metabolism" pitch may be a bit oversold, particularly given the difficulty of adding those extra pounds of muscle.

Strength training is great for the body and great for long-term health. But "can we say that every additional pound of muscle burns X amount of calories? It is very hard to separate it," said J.C. Santana, director of the Institute of Human Performance, a training and research facility in Boca Raton, Fla.

Likewise, there is "afterburn," a concept tossed around casually in gyms and by trainers with the idea that your lunch-hour workout will still be consuming calories during your two-hour commute that evening.

"Excess post-exercise oxygen consumption" -- the formal term of art -- may indeed earn you more calories after leaving the gym, but the benefit is proportionate to the work done. Afterburn occurs when the body accumulates an oxygen deficit during activity -- and that means working beyond the ability of your lungs to keep up. Oxygen deficit is the norm at the start of a workout, until the body is warmed up, but sustaining it over time is hard.

For the modest exerciser, Plowman and Smith estimate that a three-mile jog that consumes 300 calories might produce 30 calories or so of afterburn.

Diet, as might be expected, influences metabolism, but not in the way that the magic supplement or herb pitchmen might lead you to believe. Although some foods (certain spices, caffeinated beverages) do appear to tweak your system, the effect is modest. You can't counteract a Big Mac with a thermos of green tea.

But the energy used to digest food is substantial, accounting for perhaps 10 percent of a person's daily calorie expenditure. What you eat can push that either higher or lower.

Dense, whole grains and vegetables have a higher "thermic effect": They take more calories to digest, along with providing nutrition. Sugary, refined foods, by contrast, carry a triple whammy: They're high in calories, typically low in nutrition and broken down with little effort.

No doubt all of these small contributions add up to a metabolic plus: a few extra calories from muscle, a few from the afterburn, a few from digesting a big bowl of broccoli. But don't count on this alone to turn your body into a high-octane furnace.

There really is no free lunch. Which lets us end where we began: The only sure bet is to get off your . . . well, you know.