Few medical problems are as complex as that of obesity. What’s the cause? Reduced to the basic, one can say that it’s due to a caloric (energy) intake exceeding caloric (energy) consumption, i.e., for an adult, a stable weight is a floating zero-sum game.
The energy in food fuels many functions such cardiac and respiratory muscle contractions, enzyme and hormone synthesis, down to the digestion of the food itself. The rest, fuels the activity of our skeletal muscles, which is why we use physical exercise to control weight. Any excess energy is stored as fat; this, if repeated, causes weight gain.
What makes us want to eat? Imagine what life would be like if eating were entirely voluntary? How about every breath we take? Humankind shares with most animals a system that controls all our vital functions without our needing to think about it. Just imagine a kind of command and control center, like the one in the Kennedy Space Center. Located in the midbrain, it’s called the Hypothalamus. Fido’s is essentially the same. Now imagine that each one of those consoles represents a cluster of highly specialized cells receiving, via nerves and blood vessels, a stream of information from all over the body. Each one of these centers controls a vital function such as nutrition, blood pressure, body temperature all the way to one that tells us when to put up our dukes or run like mad (“flight or fight”). Several, have relays with cells in the “thinking” part of our brain, to which they send messages perceived as thirst, hunger, or satiety. Because we are thinking creatures, we can override these instructions, to a greater extent than Fido can. Otherwise, they function continuously without our realizing it, right until the very last.
Here’s where it gets interesting. Overweight people, when presented with a meal soon after eating one, will resume eating. They also have a positive response to food cues such as TV ads for tasty meals, even after having eaten recently, a behavior leading to pernicious late-night snacking. They override the satiety center.
When Australopithecus Afarensis left the forests of western Africa 4 million years ago to cross the savannas, this two-legged, strong ancestor was a hunter-gatherer. Similar societies exist in areas such as the Kalahari Desert. They searched for food, guided by a sense of taste that recognizes foods that are sweet, salty, bitter, and sour. The fact, that the sweet and salty tastes are pleasant, was an asset, since sweets often provide energy, and salt is essential for life. Never knowing when the next meal would come, their survival dictated that a kill or a find had to be consumed entirely, before it rotted or was snatched by another predator. Persistence of this primal behavior may explain obesity in some people.
Obesity tends to run in families. For various reasons, like attracts like. Therefore, in a nuclear family, both spouses are often overweight, a major factor underlying childhood obesity, whose incidence is 80% when both parents are fat, 40% when only one is and 10% when both are lean. Is it a genetic thing or is it the environment? Although the environment provided by two obese parents may play a role, studies of identical twins, separated after birth, have shown that each one of a pair of separated twins has the characteristics of the parents, rather than those of the new environment.
There’s another aspect to the genetic factor. Have you noticed that some people remain lean, despite eating a lot, while others, “only have to look at food” to gain weight? The explanation lies in something called the basal metabolic rate (BMR) (more about that later). One’s BMR is like an engine’s idling rate. Set high, the engine idles faster, burning more gas; the converse occurs when it’s set low. Areas of the world, such as the Kalahari Desert, the Congo and the southwest US, yielding few calories per acre, may have weeded out, via a process of natural selection, individuals with a high BMR. What happened to the others? The excess calories, stored as fat around the buttocks and thighs, a phenomenon called steatopygia gave their bodies a characteristic appearance: large buttocks protruding like a shelf over thick thighs, and one typical of the Bushmen, a race that once occupied large parts of Africa.
It’s still found in the pygmy tribes living in Congo the rain forests. The fat depots in the buttocks and the upper thighs, more pronounced in females, also occur in males. Primitive ivory sculptures found in caves of southern France, show the same silhouette, suggesting that, at one time, it was widespread in Europe.
We know now that fat stored around the buttocks is not unhealthy. Obesity is harmful when the fat stores lie within the abdomen, as in the “beer belly”. Moreover, it did not interfere with hunting and foraging. Also, and more importantly, since fat yields eight times more energy than carbohydrate or protein, it allowed for the accumulation of energy reserves without an unmanageable weight gain. Access to abundant food has rendered this trait, acquired through natural selection, counter-productive. This is why African Americans have a higher incidence of obesity.
To be continued
 Excerpted from the author’s book on obesity.