The study tracked energy expenditure among one of the last remaining hunter-gatherer tribes on earth: the Hadza people of Tanzania. It found that these tribesman, though they often walk up to 20 miles per day in search of food, burn no more calories per day than the average American, when accounting for differences in body size, age and gender.
This suggests that exercise is not an effective way even to burn additional calories, let alone to lose or maintain weight.
I’ve found a very different result in my own exercise patterns, as I noted in my earlier post. And so I’m somewhat skeptical of this study’s findings. After all, they seem implausible on their face: The average American consumes 2700 calories per day and, unless he’s gaining or losing weight at a rapid clip, that means he burns 2700 calories per day as well. But if you exercise enough in a given day, you can burn more than 2700 calories simply by exercising. Never mind the calories you burn brushing your teeth, rolling out of bed, snoring, blinking, breathing, thinking and doing the dishes. So that would increase your daily energy expenditure over what you’d see in a sedentary lifestyle.
Granted, 2700 calories is a lot to burn by exercise in a single day. But it can be done. A man of my size burns 130 calories per mile run. At this rate it would take me 20.8 miles of running to burn 2700 calories. A lot, but not impossible. (In fact, that’s almost exactly the distance a typical Hadza tribesman often walks in a day.) I’ve run at least 20 miles in a day several times during this summer’s marathon training and plan to do so several more before I run the Philadelphia marathon in mid-November.
And I’m not alone in this. Michael Phelps apparently works out so much that he consumes 12,000 calories a day just to maintain his weight. According to the study’s conclusions, this should technically be impossible.
Now, that doesn’t mean the study is wrong. My estimated 130 calories burned per mile run is based on different studies, and I’m not certain they hold up for runs of more than 20 miles. It’s possible that as I run farther, the number of calories I burn per mile declines, so that I never hit 2700. Possible, though not terribly likely.
And Michael Phelps may be exaggerating, or misreporting. But the difference between 12,000 and 2700 is so large, it’s hard to imagine he could be that far off.
What’s more, few people actually exercise that much, and maybe none of the Hadza tribesmen studied put in Michael Phelps-like hours in the pool or walked 20 miles more than once a week, or once a month. (The study doesn’t say how often is “often.”) So maybe this study only holds true so long as people are engaging in somewhat moderated levels of physical activity.
But my own experience sheds doubt even on that. There’s no question that I lose weight when I average even only 6 to 8 miles per day by foot. Even when I start eating less healthy foods because I know I can eat more of them and still look and feel pretty fit because I’m running so much. And my guess is, it’s because I’m burning more calories. Certainly, when I’m training for a marathon, I feel as though I’m eating all the time. But could it be that I’m not actually eating, or burning, any more calories per day than usual?
Since I haven’t ever actually measured how many calories I consume in a day, I can’t be 100% sure that I’m eating more calories during marathon training, even though it certainly feels like it. And maybe I’m losing weight not because exercise burns more calories but because it somehow causes me to eat fewer calories, even when I let the overall quality of the foods I eat decline. This would explain the Hadza study’s findings, and I suppose it’s theoretically possible.
But that doesn’t jibe with Michael Phelps’s experience. Or my personal anecdotal experience. And the study itself does little to clear things up: the researchers don’t seem to know how hunters and gatherers could burn no more calories than a sedentary westerner. It seems as though they’d have to save calories during the times when they’re not exercising; and yet the study contends that they burn just as many calories while at rest as we do. So, if they burn more through increased exercise, and the same amount while at rest, then where do we make up the difference?
It’s all very unclear to me.
I’ve been looking for a third-party analysis of the study that may shed some light on what seem to be paradoxes in the study’s findings. So far, the only reports I’ve found simply summarize the researcher’s findings; they don’t explain or challenge them. And the study itself is difficult for me to pick apart because it goes into mathematical methodologies I would have to sit down with for days to muddle through. Maybe someone will come along and help me out.
In the meantime, I issue a call: Please, oh please let me know if you have a good explanation of these results, and how we can possibly explain Michael Phelps’ diet, and our understanding of how exercise works, if exercise doesn’t help us to burn more calories day by day than sheer inactivity.