People living as hunter-gatherers burn roughly as many calories per day as those in industrialized countries.
Human metabolic equality. I’m Bob Hirshon and this is Science Update.
The Hadza people of northern Tanzania live as rugged hunter-gatherers. Yet they burn about the same number of daily calories as Americans with desk jobs. This according to Hunter College biological anthropologist Herman Pontzer. His team wanted to learn about the metabolism of our prehistoric ancestors, and chose the Hadza as a model.
Based on other people’s estimates, we expected—we had no reason not to—that the Hadza would be burning many more calories a day than you and I do, so we were very surprised to see the result.
To explain it, he notes that most of our daily calories go toward basic cellular functions, not physical activity. And when we do increase our activity, our bodies adjust by spending fewer calories elsewhere. The findings suggest that exercise may not affect obesity as much as we thought, although Pontzer stresses that it’s still essential for overall health. I’m Bob Hirshon for AAAS, the Science Society.
Making Sense of the Research
There's a general consensus that many people today don't get enough exercise, particularly in industrialized countries like the United States. Certainly, compared to our ancient ancestors, or even our grandparents, we enjoy many luxuries that make our lives less physically demanding. Meanwhile, obesity rates in developed countries continue to skyrocket.
It seems logical that these two trends should be related, and certainly, our modern couch-potato lifestyle has put us at greater risk for a host of problems, from cardiovascular disease to mood disorders. But has it really made us heavier? According to this study, not very much.
In order to test the basic hypothesis that our ancient ancestors burned more calories every day—thereby keeping them slim—Pontzer's team had to find and study people who live like those ancient ancestors today. There aren't many to choose from, but the Hadza people of Tanzania proved to be as ideal a study group as they could find.
The Hadza, although aware of the modern technological world, choose to live as traditional hunter-gatherers. They can't drive to the grocery store to pick up dinner, since they don't have motor vehicles, grocery stores, or even farms. As Pontzer told the New York Times, the Hadza men often cover 15 to 20 miles per day hunting game with bows and arrows. The women spend the day foraging for berries, tubers, and wild plants, often while carrying water, firewood, and infants.
It sounds like a workout worthy of reality-TV competition, and there's no question that their lifestyle is far more strenuous than that of most modern Westerners. They are almost completely free from obesity and heart disease, and often remain strong and vital well into old age. Yet, Pontzer's team found that they burn about the same amount of daily calories as we do.
This flies in the face of many health and exercise programs, which often tie even small physical activities, like climbing a flight of stairs, to calories you can deduct from the food you consume. Although it may be perfectly accurate to measure the number of calories an activity burns, Pontzer's study suggests that those burned calories may not contribute to weight loss as we might hope.
Rather, he points to other scientific evidence, showing that most of our daily calories go toward basic cellular functions, pumping blood, maintaining organs and tissues, and other simple requirements for survival. If we burn extra calories on our physical activity, says Pontzer, our bodies have ways of re-budgeting the calories for essential functions so that we spend the same number of calories per day overall.
Of course, he stresses that exercise is still extremely important. Without it, our bodies can fall into all sorts of disrepair. There also may be ways in which exercise indirectly affects our weight. But when it comes to obesity, the work suggests that the calories that come in, in the form of food, may matter a lot more than the calories that go out when we exercise.
Now try and answer these questions:
- Who are the Hadza people? Why were they chosen for this study?
- How does the Hadza lifestyle differ from that of a typical person in a developed country?
- The Hadza have much lower rates of obesity than in the Western world. Why do the researchers conclude that differences in physical activity can't explain that difference?
- Suppose the Hadza actually burned three times as many calories as a typical Westerner. What would your conclusion be?
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- Obesity: The Science Inside
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- Artificial Sweeteners
The Science NetLinks lesson Heart 2: Changing Lifestyles and Heart Health explores how dietary changes since the Stone Age have affected human cardiovascular health.