HDL and Triglycerides: A Complicated Story
Dr. Mills noted that HDL in the periphery is “very complicated,” and the idea that HDL, as a measure on its own, is “necessarily ‘good’ isn’t particularly informative.” Rather, HDL is “extremely heterogeneous, very diverse, has different lipid compositions, different classes, and different modifications.” For example, like oxidized LDL, oxidized HDL is also “bad,” preventing the HDL from having protective functions.
Similarly, the apolipoproteins associated with HDL can affect the function of the HDL. “Our understanding of the HDL-like particles in the CNS is limited, but we do understand the APOE4 link,” Dr. Mills said. “It seems that the HDL-like particles containing APOE2 or APOE3 are larger and are more effective at transferring the lipids and cholesterol linked to them relative to APOE4-containing particles.”
Because HDL is more complex than simply being “good,” measuring HDL doesn’t “give you the full story,” said Dr. Mills. She speculates that this may be why there are studies suggesting that high levels of HDL might not have protective benefits and might even be detrimental. This makes it difficult to look at population studies, where the different subclasses of HDL are not necessarily captured in depth.
Dr. Mills pointed to another confounding factor, which is that much of the risk for the development of AD appears to be related to the interaction of HDL, LDL, and triglycerides. “When you look at each of these individually, you get a lot of heterogeneity, and it’s unclear what’s driving what,” she said.
An advantage of observational studies is that they give information about which of these markers are associated with trends and disease risks in specific groups vs others.
“For example, higher levels of triglycerides are associated with cardiovascular risk more in women, relative to men,” she said. And the triglyceride-to-HDL ratio seems “particularly robust” as a measure of cardiovascular health and risk.
The interpretation of associations with triglycerides can be “tricky” and “confusing” because results differ so much between studies, she said. “There are differences between middle age and older age, which have to do with age-related changes in metabolism and lipid metabolism and not necessarily that the markers are indicating something different,” she said.
Some research has suggested that triglycerides may have a protective effect against dementia, noted Uma Naidoo, MD, director of nutritional and lifestyle psychiatry, Massachusetts General Hospital, and director of nutritional psychiatry at MGH Academy.
This may be because the brain “runs mostly on energy from burning triglycerides,” suggested Dr. Naidoo, author of the books Calm Your Mind With Food and This Is Your Brain on Food.
In addition, having higher levels of triglycerides may be linked with having overall healthier behaviors, Dr. Naidoo told this news organization.
Dr. Mills said that in middle-aged individuals, high levels of LDL-C and triglycerides are “often indicative of more atherogenic particles and risk to cardiovascular health, which is a generally negative trajectory. But in older individuals, things become more complicated because there are differences in terms of clearance of some of these particles, tissue clearance and distribution, and nutrient status. So for older individuals, it seems that fluctuations in either direction—either too high or too low—tend to be more informative that some overall dysregulation is going on the system.”
She emphasized that, in this “emerging area, looking at only one or two studies is confusing. But if you look at the spectrum of studies, you can see a pattern, which is that the regulation gets ‘off,’ as people age.”