The human brain needs a lot of cholesterol to wrap around nerves, to serve as components of cell membranes, and to aid in communication between neurons. While cardiologists have been racing to lower serum cholesterol more and more (and drug companies keep coming up with fancy new cholesterol-lowering drugs*), the importance of cholesterol in the brain relative to cholesterol and heart health has been mostly ignored.
It was felt that lowering serum cholesterol wouldn’t have much affect on the brain for a couple of reasons: most of the cholesterol used in the brain is made in the brain (cholesterol from the blood doesn’t really get into the brain, which is separated by the blood-brain barrier), and most of that has a pretty low turnover. The cholesterol that wraps around nerve sheaths tends to stay where it is and not float around and be recycled like the cholesterol-carrying particles in the blood.
Despite these reasonable suppositions, many studies over decades have (for the most part) consistently linked low total serum cholesterol with suicide, violence, and depression. Total cholesterol levels below 160, and especially below 130, correlate with a higher risk of mental problems. And despite the blood-brain barrier and little movement of cholesterol from the blood into the brain, brain and serum cholesterol do tend to go up and down at the same time. There are other curious findings as well…cholesterol tends to be lower in Alzheimer’s Disease**, and cholesterol has been found to be lower during a manic episode in bipolar disorder and tends to pop up again when the episode gets better.
Now none of these findings prove that low cholesterol causes problems in the brain. Those affected with Alzheimer’s are known to eat less, for example, due to the cognitive impairment. Depression is known to affect appetite as well. Most of the early studies linking suicide and depression and low cholesterol only checked total cholesterol levels, which we now know is an unreliable indicator of overall health (the subfraction that is HDL or triglycerides compared to the total are much better indicators of cardiovascular health), so it was hard to know what to make of the findings.
However, a Mexican study was recently published*** that can shed some light on these many questions. In this study, patients hospitalized with depression, many after suicide attempts, were compared with age and body-weight matched healthy controls in the community. In order to remove some confounding variables, anyone with known medical conditions that affect the blood lipids, such as diabetes or metabolic syndrome, and anyone on a statin was excluded from the study. These researchers also measured the clinical lipid panel that is routinely used to measure cardiovascular risk, including HDL, LDL, triglycerides, VLDL, and total cholesterol. They used a term I’ve never heard before (but I like a great deal), “hypocholesterolemia” to mean folks with a total cholesterol of under 150. Over the course of several years they managed to study nearly 500 people, enough to give the researchers some decent statistical power.
Here is the gist of what they found: those with hypocholesterolemia were over four times more likely to have major depressive disorder and over five times more likely to have attempted suicide. In fact, half the suicide attempters had a total cholesterol less than 150 compared with 38% of the total group of depressed individuals and 14% of healthy controls. Triglyceride and VLDL levels, on the other hand, were higher in the depressed and suicide-attempt group.