ntury. Over the last several decades, our understanding of the predictive power of various blood lipids has gone through many refinements as our ability to measure finer and finer detail has advanced.
In the early years, it appeared that high levels of total cholesterol carried some risk of heart disease in many cases. However, it is now well established that total cholesterol by itself is a weak predictor 2, 3, 4.
The reason is quite simple. The different subtypes of cholesterol work together in an intricately balanced system. There is a wide range of total cholesterol levels that are perfectly healthy, so long as the proportions of the subtypes are healthy ones. By the same token, a given level of total cholesterol, even if it is perfectly normal, could be pathological when examined by subtype. Strong evidence from recent decades suggests that the best known blood lipid measures for predicting future risk of CVD are HDL, triglycerides, and related ratios (see below).
One reason for this is that like total cholesterol, LDL is not uniform. Just as we distinguish between HDL and LDL, the so-called “good” and “bad” cholesterol, LDL itself is now known to have two important subtypes with opposite risk implications. Having more large, light LDL particles (also called Pattern A), does not indicate high CVD risk, but having more small, dense particles (Pattern B) does 9, 10, 11, 12, 13. Therefore high LDL by itself is not necessarily indicative of CVD.
Low HDL cholesterol is strongly associated with CVD
Having high blood levels of HDL is now widely recognized as predicting lower levels of heart disease. The proportion of total cholesterol that is HDL cholesterol is a particularly strong predictor. In 2007, a meta-analysis was published in the Lancet that examined information from 61 prospective observational studies, consisting of almost 900,000 adults. Information about HDL was available for about 150,000 of them, among whom there were 5000 vascular deaths. According to the authors, "the ratio of total to HDL cholesterol is a substantially more informative predictor of IHD mortality than are total cholesterol, HDL cholesterol, or non-HDL cholesterol." 14
This is consistent with many other studies, for example this very recent analysis from the COURAGE trial 15.
High triglycerides are strongly associated with CVD
There has been drawn out controversy in the medical community as to the relationship of triglyceride levels to CVD. There are two parts to the controversy: whether or not triglycerides are an independent predictor of CVD, and whether or not triglycerides play a causative role in CVD.
In both cases, however, it doesn't matter in which way the controversy is resolved! Whether or not triglycerides independently predict CVD (and there is at least some evidence that they do), and whether or not they cause CVD, there is no controversy about whether they predict CVD. The association between triglyceride levels and CVD still holds and is strongly predictive 16, 17, 18. In fact it is so predictive that those who argue that triglyceride levels are not an independent risk factor, call it instead a “biomarker” for CVD 19. In other words, seeing high triglycerides is tantamount to seeing the progression of heart disease.
HDL-to-Triglycerides Ratio: compounding evidence
Triglycerides and HDL levels statistically interact. That means it is a mistake to treat one as redundant with respect to the other. If you do, you will miss the fact that the effect of one on your outcome of interest changes depending on the value of the other. Despite the fact that most heart disease researchers who study risk factors have not used methods tuned to find interactions between triglycerides and HDL, many studies have at least measured both. This has allowed others to do the appropriate analysis. When triglycerides and HDL have been examined with respect to each other, that is, when the effect of triglycerides is measured under the condition of low HDL, or when the effect of HDL is measured under the condition of high triglycerides, this combination o