It's one of the most intriguing discoveries about diet. People who consume a diet low in fats and especially low in cholesterol are at risk for depression and suicide.
The first clue that low-fat diets might have anything to do with depression or self-directed violence turned up a few decades ago, quite by surprise. Large community-based studies of heart disease prevention strategies showed that among persons with the lowest cholesterol levels, there was a increased incidence of death not caused by illness, primarily to suicide, accidents, and violence.
At first, it defied comprehension. And to a considerable degree it still does, twenty-something years later.
But the link between cholesterol lowering and suicide may run directly through brain serotonin pathways, with side stops to depression, irritability, impulsiveness, and aggression. Or it may run more indirectly through metabolic pathways of brain serotonin, the neurotransmitter most associated with depression. Whatever the link is, it's complex, because it hasn't been easy to pin down.
Not that there is a whole industry of people trying to hunt it down. To some degree, the issue pits hearts against minds, or the interests of cardiologists against those of psychiatrists and neuroscientists. Cardiologists, for the most part, along with many public health experts, are focused on the need for much of the American population to reduce their risk of heart disease by cutting their intake of dietary fats. They have lots of evidence to back them up. And they don't seem over-eager to vigorously pursue whether there may be an unusual behavioral hazard associated with cholesterol-lowering strategies.
Several studies have shown that low cholesterol is linked to depressed mood and to impulsivity, although it isn't clear whether the link to depression is as true for people whose cholesterol levels are lowered by diet as in people with naturally occurring low cholesterol levels. Behavior is a very tricky thing to study, and many different factors have to be taken into account.
But some problem in serotonin function seems always to be at the center of the story.
In nonhuman primates, high cholesterol levels enhance serotonin function. They lower levels of overt aggression. And they promote social behavior.
In people and other animals, serotonin dysfunction is implicated in major depression and, independently, with suicide. Serotonin is seen as a neurotransmitter of restraint. It remains inactive until called upon to inhibit some impulsive command, such as, say, the thought of suicide in the face of some extra stress.
In healthy people, serotonin turns on to suppress impulses and keeps them in check. In those whose serotonin system is not responsive, for whatever reasons, it fails to suppress an impulse, resulting in an impulsive act. Like hostility or acting on a suicidal thought.
Low-fat diets are known to alter serotonin function. They might decrease the fats in nerve-cell membranes, impairing serotonin receptors.
Where does that leave you?
With some awareness, hopefully, that there are few absolutes. Dietary fat isn't all bad, and some is absolutely necessary. Did we say moderation? Absolutely.