Why Aren’t (and Weren’t) Hunter-Gatherers Fat?

Can we learn anything from the latest research on the energy expended by the Hadza?

If we listen to the public health authorities, we’re hearing that obesity, heart disease, and diabetes are now epidemics in this country because we’re either eating too much, moving too little, or it’s a combination of the two factors.

In a recent New York Times Opinion article titled “Debunking the Hunter-Gatherer Workout,” Herman Pontzer, an assistant professor of anthropology at Hunter College and a co-founder of the Hadza Fund, a nonprofit organization that supports the Hadza population, wrote:

“One reason that heart disease, diabetes and obesity have reached epidemic levels in the developed world is that our modern way of life is radically different from the hunter-gatherer environments in which our bodies evolved. But which modern changes are causing the most harm?”

Pontzer also noted that the World Health Organization, in discussing the potential causes of obesity, has cited a “decrease in physical activity due to the increasingly sedentary nature of many forms of work, changing modes of transportation and increasing urbanization.”

In other words, we move too little, or so this organization believes. Pontzer and his colleagues measured the daily energy expenditure among the Hadza people of Tanzania, “one of the few remaining populations of traditional hunter-gatherers.” They set out to determine whether the Hadza expended more energy on a daily basis than we Americans do.

What these researchers found, was that they didn’t. This may seem surprising in a group that reportedly engages in relatively greater physical activity:

“They have no guns, vehicles, crops or livestock. Each day the women comb miles of hilly terrain, foraging for tubers, berries and other wild plant foods, often while carrying infants, firewood and water. Men set out alone most days to collect honey or hunt for game using handmade bows and poison-tipped arrows, often covering 15 to 20 miles,” wrote Pontzer.

The researchers believe that the Hadzas’ bodies adjust to their higher activity levels required for their hunting and foraging lifestyle by spending less energy at rest. Even for people who are extremely active, physical activity accounts for only a small portion of daily living: Someone who engages in two hours of exercise a day, for example, has spent little more than 8 percent of a 24-hour period working out. “If the Hadza’s bodies somehow manage to spend less energy in those areas,” Pontzer noted, “they could easily accommodate the elevated energy demands of hunting and gathering.”

There is a common assumption that exercise increases the metabolic rate (we “speed up our metabolism”), and the more we exercise, the greater the effect. However, the data doesn’t support it.

In 1988, Dr. Stephen Phinney and colleagues enrolled 12 moderately overweight women into a study, where they spent five weeks in a metabolic ward (the gold standard for human research), and all consumed an 800-calorie low-carbohydrate diet. Six of the subjects were to remain sedentary, while the other six engaged in progressively increasing exercise, up to 14 hours in a week.

Weight loss was actually greater in the sedentary group. What Phinney found was the exercising group’s resting metabolism decreased by about 15 percent over the course of the study.

There are four well-controlled inpatient, metabolic ward studies published from 1982 through 1997 that showed statistically significant reductions in resting metabolic rate when overweight subjects performed 300-600 calories per day of endurance exercise for week at a time (1-4), noted Phinney, and Jeff Volek, PhD, RD, in “The Art and Science of Low Carbohydrate Performance.” There are no human studies that show the opposite.

While results vary from individual-to-individual, when overweight people do more than an hour of endurance exercise daily, resting metabolism, on average, decreased by 5-15 percent.

Our bodies compensate to fit our environment. “Our bodies are complex, dynamic machines, shaped over millions of years of evolution in environments where resources were usually limited,” wrote Pontzer in the Times article. “Our bodies adapt to our daily routines and find ways to keep overall energy expenditure in check.”

Pontzer later claimed: “We’re getting fat because we eat too much, not because we’re sedentary.”

When viewed from the lens of the “calories-in/calories-out” paradigm, this must be true, because we can logically deduce from the Hadza, and 30 years of research on energy expenditure, that physical inactivity did not cause the obesity epidemic, so, eating too much, i.e., “overeating,” is the cause.

But why would biology explain one phenomenon: energy expenditure, while physics (the Laws of Thermodynamics), explain the other, energy intake?

If our bodies are indeed “complex, dynamic machines” that “adapt to our daily routines and find ways to keep overall energy expenditure in check,” why wouldn’t this apply to "overall energy intake" as well?

The Hadza, according to National Geographic reporter, Michael Finkel, will “eat almost anything they can kill, from birds to wildebeest to zebras to buffalo. They dine on warthog and bush pig and hyrax. They love baboon ... If hunters come across a recently dead elephant, they will crawl inside and cut out meat and organs and fat and cook them over a fire.”

Do the Hadza pull away from the proverbial table as to not “eat too much” and grow fatter? Do they not “overeat” because the exact number of calories in the wild is hunted, killed, cooked and eaten to match energy expenditure?

Or is it more likely that when it comes to both energy intake and expenditure, they still are “complex, dynamic machines,” that regulate the amount of fat deposited in their adipose tissue, and energy intake and expenditure are side effects of the underlying drive of the human body to remain at a relatively stable weight.

In any case, even if the Hadza expended more energy than Americans, biology tells us that they would compensate by eating more to maintain the same weight. The same can be said of individuals who engage in daily heavy labor -- construction workers, for example -- compared to people who sit behind a desk all day.

“In most instances, energy intake can be interpreted as a crude measure of physical activity,” wrote Walter Willett and Meir Stampfer in their 1998 textbook Nutritional Epidemiology.

Put another way, Jeff Flier, dean of Harvard Medical School, and Terry Maratos-Flier, in an article in Scientific American called “What Fuels Fat said:

"An animal whose food is suddenly restricted tends to reduce its energy expenditure both by being less active and by slowing energy use in cells, thereby limiting weight loss. It also experiences increased hunger so that once the restriction ends, it will eat more than its prior norm until the earlier weight is attained. Likewise, after intentional overfeeding, an animal will start to expend more energy and exhibit reduced appetite, with both states persisting until weight falls to the previous level. (5)"

In other words, our bodies compensate. If we eat like a hunter-gatherer, and only eat the foods that we “can kill, from birds to wildebeest to zebras to buffalo,” for example, we may not be able to “overeat,” even if we tried. We will begin to expend more energy and reduce our appetite so that we remain at a stable weight for most of our lives.

Our bodies regulate our internal environment exquisitely so that we don’t get too hot, or too cold; we regulate the amount of blood sugar in our bodies, we regulate the amount of cholesterol we produce, we regulate the amount of fluid we keep around...and we regulate the amount of fat and lean tissue we carry. Only when there is an upset in the regulation of the fat tissue will we generally become obese or emaciated.

It’s actually naive to think that our hunter-gatherer ancestors would become obese if they had an abundance of animals to kill and eat (eating more); and it is likewise foolish to think that they would become corpulent by not having to cover as much ground (less activity) to find food.

It’s equally absurd to think this applies to modern day humans. Even if we’re driving our cars to the restaurant and being served “oversized” portions of dinner - as long as the food we are eating is food that we are well adapted to eat - our bodies should adjust to any periods of “overfeeding” or “underfeeding” in the long run and we will maintain our body weights within a very close range over time.

In a recent study of overweight and obese young adults, (6) Dr. David Ludwig from Boston’s Children’s Hospital and colleagues looked at the effects of different macronutrient (carbs, protein, and fat) compositions on the amount of energy expended during a period of “weight maintenance” after a recent weight loss. Basically, the study looked at what happened to a group of people who lost weight and then tried to keep it off eating one of three diets:

1. A low-fat, high carbohydrate diet: the “conventional advice” diet.

2. A low glycemic diet: carbs are more restricted and the carbs eaten are lower on the glycemic index, meaning they have less of an impact on blood sugar spikes.

3. A low-carbohydrate diet: a diet more restricted in carbohydrates and higher in fat.

What the authors found was that total energy expenditure decreased when the subjects tried to maintain their weight loss. In other words, it seemed the body was compensating to put the weight they lost back on. However, the diet lowest in carbohydrates saw the smallest decrease in energy expenditure while the diet lowest in fat and highest in carbohydrates saw the highest decrease in energy expenditure. This suggests the currently recommended diet would be the least effective in the long term in maintaining weight loss. It also suggests that a calorie is not a calorie: different nutrients have different effects on the hormones and enzymes that help regulate the amount of fat we accumulate in our bodies.

The moral to these stories is that we don’t get fat because we eat too much. We don’t get fat because we move too little. We eat too much and/or move too little because we’re getting fat: because there is something awry in the regulation of our fat tissue. Just as babies eat more and move less because they are growing (not the other way around) and teenagers eat more and move less because they are going through puberty (again, not the other way around): we eat more and move less because we are growing - in this case “horizontally,” and we need to address the root of the problem - the regulation of our fat tissue - and not the side effects - eating more and/or moving less - to fix the problem.

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1. Bouchard C, et al. The response to exercise with constant energy intake in identical twins. Obes Res. 1994. 2(5):400-410.

2. Heymsfield SB, et al. Rate of weight loss during underfeeding: relation to level of physical activity. Metabolism, 1989. 38(3):215-223.

3. Phinney SD, et al. Effects of aerobic exercise on energy expenditure and nitrogen balance during very low calorie dieting. Metabolism. 1988. 37(8):758-765.

4. Woo R, et al. Voluntary food intake during prolonged exercise in obese women. Am J Clin Nutr. 1982. 36(3):478-484.

5. Flier, J, Maratos-Flier, T.  What Fuels Fat.  Sci Am.  2007.

6. Ludwig, D, et al. Effects of Dietary Composition on Energy Expenditure During Weight Loss Maintenance. JAMA. 2012. 307(24):2627-2634.

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