When I started the Atkins diet a year and a half ago [in early 2002], I wondered why it worked (I lost 50 pounds). Then I thought about it a bit and came up with what I think is the answer. So here it is:
Human metabolism evolved when early hominids survived by hunting small animals and gathering indigenous plants. Particularly in colder climates, the primitive human body without an insulating cover of fur required special metabolic cycles to bridge between the intermittent availability of food and the constant need to maintain energy and body heat.
Two such cycles evolved in human metabolism: a short-term gorge-and-fast cycle designed to operate between animal kills, and a long-term annual cycle designed to bridge between summer’s plenty and winter’s scarcity.
During the short-term cycle, fats are stored temporarily and metabolized within days. The transport mechanism for these fats is low-density lipids (LDLs). During the long-term cycle, fats are stored deeper in the body tissues, using high-density lipids (HDLs). The LDLs, being more reactive, are easier to convert into energy. The HDLs, being more stable, are easier to retain for months at a time.
As a result of evolution, the human body learned to trigger the short-term cycle simply by reacting to sudden floods of fats in the stomach. Body recognition of the long-term cycle (distinguishing between summer and winter) was more difficult. The mechanism that finally evolved was to treat an abundance of carbohydrates as an indicator of summer, because that was the time of year when plants grew.
So a useful algorithm evolved. When carbohydrates are plentiful, the human body burns them for immediate needs; while doing so, it tends to convert fats in the stomach to HDLs and puts them into deep storage. When carbohydrates are scarce, the body takes HDLs out of storage and burns them. Evolving this mechanism may have helped human groups emigrate from the tropics to the temperate zones.
Human beings no longer subsist by hunting and gathering. For the last ten thousand years we have lived primarily by growing crops. Human metabolism, however, has not evolved significantly during this time. Today, our bodies interpret the diet of civilized life, high in carbohydrates year-around, as perpetual summer. They react by converting the fats in our diet to HDLs and sending them through the circulatory system to remote body tissues. The result is arteriosclerosis and obesity.
The short-term cycle still seems to work fairly well -- a fatty meal or snack will get burned off over the next day or so. It’s the long-term cycle that is no longer relevant. Since we don’t really need to cope with an annual cycle of plenty and scarcity, the solution is to regulate our carbohydrate intake. By removing the “summer fattening trigger,” we can reduce our blood HDL levels and minimize body fat. When we maintain a diet heavy in carbohydrates, our bodies forever try to build up reserves against a winter that never comes.