Scientific American reports on research showing that a diet high in saturated fat causes the brain to become resistant to leptin and insulin, hormones that let us know when our need for food has been fulfilled.
The research in question was done on rats, and it's always tricky to extrapolate such work to humans; and there are serious ethical issues with killing rats to find our why humans become such pigs when they eat cows. But the phenomenon in question is expected to apply to humans as well.
The researchers also performed in vitro experiments where they directly observed palmitic acid (a common saturated fatty acid) inhibiting the signaling of nerve cells exposed to insulin.
On the other hand, oleic acid, a monounsaturated fatty acid, did not produce this result.
What evolutionary mechanism might produce such a result, that too much fat in the system actually tells the body to increase rather than decrease food uptake? Here's what one leptin expert quoted in the SciAm article says:
"There is a basic mystery here," says William Banks of the Saint Louis Veterans Affairs Medical Center. A leptin expert, he has shown that high levels of saturated fat in the bloodstream block the hormone's passage into the brain, further blinding it to those extra pounds below the neck.
One hint at an explanation for these counterintuitive effects comes from the physiology of starvation. When we starve, our body begins to break down its blubber for energy. As a result, the blood gets flooded with fat, just as it does in obesity and overeating. Apparently erring on the side of caution, our brain interprets free fat (no matter its source) as a starvation alert—had it done otherwise in our evolutionary history, we probably wouldn't be around to worry about it. Says Banks: "In the history of evolution, we've been faced with caloric shortages and starvation much more than we've ever been faced with a wealth of calories."
And that's not the only effect of dietary fat on the brain: recent research (also in rats, see disclaimer above) shows that dietary fat has an effect on the brain much like that of addiction. Rats on a high-fat diet developed a tolerance to food-derived pleasure, and pursed food even under circumstances that drove non-addicted rats away:
In the study, published in the journal Nature Neuroscience, Kenny and his co-author studied three groups of lab rats for 40 days. One of the groups was fed regular rat food. A second was fed bacon, sausage, cheesecake, frosting, and other fattening, high-calorie foods--but only for one hour each day. The third group was allowed to pig out on the unhealthy foods for up to 23 hours a day.
Not surprisingly, the rats that gorged themselves on the human food quickly became obese. But their brains also changed. By monitoring implanted brain electrodes, the researchers found that the rats in the third group gradually developed a tolerance to the pleasure the food gave them and had to eat more to experience a high.
They began to eat compulsively, to the point where they continued to do so in the face of pain. When the researchers applied an electric shock to the rats' feet in the presence of the food, the rats in the first two groups were frightened away from eating. But the obese rats were not. "Their attention was solely focused on consuming food," says Kenny.
Dr.Gene-Jack Wang, M.D., the chair of the medical department at Brookhaven National Laboratory, notes that there is a similarity between the way that coca, a beneficial herb, was refined into the addictive and troublesome drug cocaine. According to Wang, "We purify our food...Our ancestors ate whole grains, but we're eating white bread. American Indians ate corn; we eat corn syrup." He add that these highly refined foods cause people to "eat unconsciously and unnecessarily," and will prompt animals to "eat like a drug abuser [uses drugs]."
Perhaps this addictive property helps explain the amazing rhetorical gymnastics that many people engage in to justify their "need" to eat saturated-fat-rich animal flesh, milk, and eggs. (To be fair, I also find the occasional vegan who just "needs" to eat a lot of food containing coconut oil...)