Want to live a healthy life and increase your longevity? Then get your total cholesterol below 150, right?
One piece of evidence that is true is that the healthiest and longest-lived societies on planet earth, as documented in John Robbin’s Healthy at 100, all have cholesterol below 150. These cultures have virtually no heart disease or cancer. Contrasted with the typical modern, industrialized society where both cholesterol and mortality rates are much higher, one can’t help but wonder if cholesterol is causative.
Unfortunately, examining whole cultures is not really a good proof, since after all, there are many lifestyle factors that are significant health-determining factors. For example, all of the above cultures exercise for many hours of the day and all of these cultures also tend to be low stress and low calorie as well. It is difficult to separate out one variable from another. The famed China Study attempted to do this and offered additional evidence for those interested.
Complicating matters were several earlier studies that showed that all cause and cancer mortality tended to go up with lower cholesterol levels below about 150. In other words, some studies seemed to show the very opposite of what the above supercultures showed. The Paleo and Low Carb diet crowds took advantage of the situation and a tidal wave of internet articles and pages presented the idea that a low chosterol diet was actually dangerous.
Some researchers have even jumped on the bandwagon. One Japanese scientist recently went so far as to state that “cholesterol is an essential component for the creation of cell membranes and hormones. It’s not recommended to lower LDL figures by means of dietary intake and medication.”  In other words, the argument was that one could not get enough cholesterol on a low fat diet to adequately supply the body with needed fatty acids.
The burning question, of course, remained though, “Why did the studies show that low cholesterol led to decreased longevity and increased mortality?” Well, researchers had long ago answered this question. One study from 1981, for example, showed that
What they found was both important and interesting: low cholesterol did not increase overall cancer rates significantly IF you took out those with “preexisting conditions”. They did this by removing those with known cancer and those who died within the first two years. Let’s face it: most of those who die within two years from cancer likely had it at the time of the study and this will lower cholesterol levels unnaturally.  (NOTE: The researchers did find that colon cancer had a higher risk factor with low cholesterol once these other factors were taken into account, but this is the lone outlier.)
A 1995 British Medical Journal study found an additional explanation that verified this conclusion: they found that if you looked far enough into the future, all cancer risks were eliminated.  Again, this shows that a number of individuals had lower cholesterol levels due to prexisting cancer and, when eliminated, becomes irrelevant. A similar study of Finnish men reached the exact same conclusion. 
Thus, the studies show the following: lowering cholesterol dramatically and linearly decreases heart disease and is largely neutral with respect to cancer. This results in an impressive overall decrease in mortality and increase in longevity when all factors are combined.
2) Am. J. Epidemiol, 1981, 114(1):11-20, “SERUM CHOLESTEROL AND MORTALITY IN A JAPANESE-AMERICAN POPULATION THE HONOLULU HEART PROGRAM”
3) BMJ, 1995 Aug 12, 311(7002): 409 413, “Low serum total cholesterol concentrations and mortality in middle aged British men”
4) Am. J. Epidemiol, 1992, 135(11):1251-1258, “Short- and Long-term Association of Serum Cholesterol with Mortality The 25-year Follow-up of the Finnish Cohorts of the Seven Countries Study”