Exercise: of Math and Myth

Does exercise burn body fat? Of course it does!

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Of course exercise burns body fat.

Eat + Run -- David Katz
David Katz
If you are like most of us, you once knew this was true, intuitively, and perhaps from first-hand experience. But that was before you knew it was false, because you were talked out of believing the obvious.

You have been talked out of it by all sorts of competing claims for this, that, and the other dietary approach, or exercise regimen. You have been told that exercise does not cause weight loss. Generally, that is followed by the contention (which is true) that exercise is good anyway, because it promotes health even when it isn't making you thin. That leads invariably to discussion of the "fatness versus fitness" issue, which changes the subject altogether.

You may have been talked out of it by a personal trainer, fitness instructor, or author advancing a pet theory. You may have been told that aerobic exercise is passé, and it's all about resistance training now. Or that exercise increases appetite, and so invariably causes you to replace whatever calories you burn.

You may even have been talked out of it by exercise machines that offer certain settings for muscle toning, others for aerobic conditioning, and some other particular arrangement for fat burning. You may have been convinced that exercise only burns fat when perfectly titrated into the fat-burning zone, in conjunction with rare and elusive arrangements of the stars and planets.

I would like to approach all of this with scholarly erudition. So here I go: It's BS!

Of course exercise burns fat. But it only burns body fat when we do enough of it, and when it isn't fully fueled by what we're eating. And yes, that's the whole story.

There are three simple elements to the argument that exercise does, indeed, burn body fat; that it can, in fact, cause weight loss (although it usually doesn't); and that you do not need to calibrate the dials on a given machine in any very particular way to get there from here.

1. Logic

Logic and intuition don't always lead to the truth, but they often do. Certainly, the truth is less likely when arguments are trampling on logic rather than toting it.

Olympic swimmers are lean and muscular. So are Olympic gymnasts. I have never seen a truly competitive marathon runner or top ballerina who shops in the husky section. Logically, and observationally, there is an association between high levels of physical exertion and low levels of body fat. We see it all the time. Logic and observation converge to say: High levels of physical activity, be they on the ground, in the water, or leaping through the air, burn fat. Period.

2. Laws of physics

There is as well the simple fact that exercise is work, and work burns fuel of some kind. Newton either said this, or meant to. Let's move on.

3. Limb loppers

I consider the third argument the clincher, even though it could be denigrated as anecdote. I prefer to think of it as experiential, the example that proves the principle. I have lived it.

I have lived it courtesy of limb loppers; not to mention work gloves, wheelbarrows, axes, mauls, wedges, handsaws, and chain saws.

I have lived it many times, most recently this week—courtesy of a massive oak tree Hurricane Sandy brought down across my parents' driveway. I spent just about all of the daylight hours last Sunday working with my father and my son to cut up and haul away the unholy mess that magnificent tree had become (crashing through a couple of other lesser trees on its way down), and clear the driveway. We did it.

In my case, that meant working without a break for hours, using every muscle in my body to the limits of its tolerance. By day's end, I couldn't lift my hands to my head to wash my hair without the muscles in my arms going into spasm.

I muddled through that shower, and then had dinner. I don't know how many calories I consumed at dinner that night, but I would guess 4,000 at a minimum. I ate until I could fit nothing more into my stomach, then stuffed in a bit more—and finally stopped eating even though I was still hungry. My stomach was full, but my calorie deficit was not, and my body knew it. I had no room for any more food, but I was still hungry.

This has happened to me before, but only on days of comparably epic exertion, like building a stone fence from dawn to dusk, planting trees all day in rocky soil, hiking 7 miles through sand and biking 30 in hills, and so on.

Inevitably I wake up the next day feeling lean and light, with the flesh around my middle stretched a bit tauter than usual—and weighing a bit less. Monday morning was no exception.

The reason resides in physics, physiology, and some simple math.

Let's begin with my resting energy expenditure (REE). I'm not all that big, but I have a fair amount of muscle mass, so my REE is roughly 1.7 kilocalories/minute. That means in a 24-hour period, if I don't do any exercise, I need roughly (1.7 X 60 minutes X 24 hours), or 2,448 kilocalories to maintain my body weight and run my metabolism. That's just about right.

The energy expended at rest constitutes one metabolic equivalent, or 1 MET. As we exert ourselves, we can measure the effort in multiples of METs: How many times harder are we working, in terms of energy consumption, than at rest?

The work on this tree was intense, somewhere between the level of a 10 to 12 minutes-per-mile jog, which is roughly 12 METs, and an 8 minutes-per-mile run, which is about 15. Let's split the difference and say my exertion average was about 13.5 METs. That's pretty high, but I was working very hard.

I sustained that effort for roughly five hours without a break. How much energy did I burn?

My basal rate of 1.7kcal/min was increased by a factor of 13.5, and then maintained for 300 minutes: (1.7 X 13.5 X 300) = 6,885. Just shy of 7,000 calories!

Now, of course, my body didn't stop its usual work for the other 19 hours of that day. In fact, there was probably considerable spillover after I put down the chain saw and maul, with an increase in energy expenditure persisting. But let's be conservative, and say the other 19 hours were at my customary basal rate. So, during that time I burned (1.7 X 60 X 19), or 1,938 calories.

So, my energy expenditure for the day was 6,885 plus 1,938, or 8,823 calories.

Now, let's image that I ate a fairly typical 1,200 or so calories during the day. And then, after this work, I stuffed in a huge dinner of 4,500 calories, more than twice my usual intake. That still left me with an energy deficit of more than 3,100 calories. That's almost the amount stored in a pound of fat.

So here's the bottom line: By working this hard for five hours, I could have eaten like a horse, burned through all of those calories, and still needed to burn through almost a pound of body fat to make up for needed calories my food intake did not provide. Exercise burns fat.

And it does, indeed, burn fat preferentially when muscles are put to work. When you work muscles, they go into anabolism (building) mode, and out of catabolism (breakdown) mode. I don't need to make this case; the bulging muscles of everyone who pumps iron puts it on vivid display.

Competitive body builders combine intense workouts with calorie restriction and preferential consumption of protein (leaving out of it any drugs they may take). The result: huge muscles and extremely low body fat. When you work muscles hard, they grow, and the body burns fat. So that's where my body went for the extra calories it needed. This is over-simplified, but it reflects the gist of things.

So, I burned through all that food PLUS a pound or so of body fat. I woke up last Monday with my belly a bit pinched, and needing to tighten my belt an extra notch. No question, my body had burned fat.

My example, by the way, shows how weight-loss resistance can be very profound if resting energy expenditure is set low due to genetic predisposition. The 1.7kcal/min could be a much smaller number; which means 13.5 times a smaller number would still be a smaller number. In other words, the person with a low REE would do the same strenuous work as me for those same five hours, and have many fewer calories burned to show for it. Not fair, but it is what it is.

But now back to me.

I don't lose body fat every day, which means most days, the exercise I do does not burn body fat. My daily regimen includes a 40-minute hill routine at a high resistance level on a total-body elliptical machine; 5 kilometers on a rowing machine; chin-ups, pull-ups, and abdominal exercises. When my schedule permits, I lift weights; take my dogs on a 3.5 mile walk; and/or ride my horse—in addition, not instead.

Honestly, I think that's pretty good. It's certainly a lot more than most of my patients do. But it's not nearly enough to guarantee I will burn fat. On any given day, I eat enough calories to fuel all that exercise, so my weight and body fat stay constant.

Modern living has completely obscured the levels of exertion native to our bodies, and the quantity of calories required to fuel them. Even those of us who exercise daily are relatively sedentary by historical standards. Consider, for instance, early pioneers who spent day after day clearing trees from fields using only hand tools and beasts of burden.

When we don't like the energy-balance math of modern living, we invent myths to replace it. But all such tales simply obscure the basic truth.

Of course exercise burns body fat—or can. But most of us, most of the time simply don't exercise enough to outrun our calories, and readily out-eat our workouts.

Hungry for more? Write to eatandrun@usnews.com with your questions, concerns, and feedback.

David L. Katz, MD, MPH, FACPM, FACP, is a specialist in internal medicine and preventive medicine, with particular expertise in nutrition, weight management, and chronic-disease prevention. He is the founding director of Yale University's Prevention Research Center, and principal inventor of the NuVal nutrition guidance system. Katz was named editor-in-chief of Childhood Obesity in 2011, and is president-elect of the American College of Lifestyle Medicine.