Who can forget the fight between gangly Ace Ventura and his diminutive opponent from the !Kung San tribe who substituted fierceness for stature? Jim Carrey is freakishly thin and fit in our society but his opponent benefited from the superb physical conditioning of all his tribe (pictured). No obesity there.

All diets assume that we Americans are overweight because we eat too much. Yet, the real problem is not over eating. We are overweight because we move too little.

That big picture often gets overlooked. This is unfortunate because you have to understand the problem before you can devise real solutions. Diets don't work because nutrition is not the main underlying problem. The key problem is inactivity.

The evidence is plain in every subsistence society
The transition to urban life is everywhere accompanied by soaring obesity rates and accompanying diseases such as diabetes, and high blood pressure. Urbanites sit around in offices much of the time and do not get enough exercise to regulate their body weight at a healthy level.

For highly active individuals, overweight is not a problem, however much they eat. In experiments where volunteers upped their food intake by 50%, there was no increase in body weight among physically active individuals ("non exercise" activity). Of course the inactive individuals put on plenty of weight.

Physical activity of any kind is capable of raising metabolism at rest and also the warmth we experience after a meal (thermic effect of food). A lot of food energy is thus spent in producing heat instead of getting stored as fat. This means that physically active people expend much more energy than that required to do the work of movement.

Because they are so much more physically active, people in subsistence societies have very low levels of overweight and obesity. The transition to an urban lifestyle can be devastating.

The star exhibit here are the Native American Pima of Arizona, one of the fattest groups of people on earth. When occupied as farmers, they were as skinny as other subsistence peoples. This phenomenon has often been attributed to genes but the Mexican Pima who are genetically similar, do not have a high incidence of overweight. Shockingly, the Mexican Pima eat significantly more than their overweight American counterparts.

The same paradox of the skinny peoples of the world eating more than the obese ones is found repeatedly. Among hunter-gatherers, such as the Ache of Paraguay, the average man, at 150 lbs weighs substantially less than the average American man but consumes 3,300 calories (i.e. kilocalories) compared to just 2,700 for the average American male.

How do the Ache manage to eat so much without getting overweight? The answer is that they are very active compared to us, using three times as much energy in physical activity as we do (about 1,800 calories compared to 600 for us). When we lead an active life, we are good at regulating our weight, regardless of how much we eat.

Farmers have long known that if you want to fatten a cow before slaughter the fastest method is by confining her in a small space with plenty of high-energy food. By restricting our own movement via a sedentary lifestyle we are doing the same to ourselves.

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