TriglyceridesTriglycerides are the form in which the body stores fat (our body fat is mainly made up of triglycerides.) When we talk about someone's triglyceride level, however, we usually mean the amount of triglycerides that show up in the blood when it is tested. A high triglyceride level is a risk factor for heart disease and stroke.
Numerous studies find that low-carbohydrate diets cause high triglyceride levels to fall; in fact, the results are quite consistent and dramatic. Many physicians now recommend reducing carbohydrate as the first line of defense against high triglyceride levels, and this is often successful.
High Density Lipoprotein Cholesterol (HDL) -- "Good Cholesterol"HDL cholesterol seems to protect against heart disease; it becomes a risk factor for heart disease if it's low. Scientists think it carries excess cholesterol back to the liver, where is it broken down. There is also evidence that some aspect of HDL is involved in the initial response after injury or acute illness, and that people with higher levels of HDL have improved recovery.
Low-carbohydrate diets tend to raise HDL cholesterol levels, so this is a good thing.
Low Density Lipoprotein Cholesterol (LDL) -- "Bad Cholesterol"Although there is some controversy on this point, LDL cholesterol is considered "bad" in terms of heart disease risk. The relationship between low-carb diets and LDL cholesterol is more complex than with triglycerides and HDL cholesterol. There are some studies in which LDL is reduced on a low-carb diet, some in which it doesn't change, and some in which it goes up. But there is one thing about LDL changes which is consistent with low-carb diets, and that is that it causes a change in cholesterol particle size.
What has particle size got to do with it? Evidence is accumulating that the size of cholesterol particles has a lot to do with risk for heart disease. Basically, the smaller the particles are, the greater the risk -- it is thought that perhaps the small particles lodge in the walls of blood vessels more easily.
The good news for those of us following a low-carb way of eating is that studies of diet and cholesterol particle size have consistently shown that low-carb diets produce larger-sized cholesterol particles. However, a larger-sized particle weighs more than a smaller one. When LDL does go up on a low-carb diet, it may be due to the larger particles, since weight is what's being measured. (A total cholesterol of 200, for example, means 200 mg per deciliter.)
On the other hand, high-carb diets seem to produce a greater percentage of smaller cholesterol particles in some people. So the total LDL goes down (particles are smaller, so the total is lighter.) While the reading may be low, it can be deceiving as risk goes up in those cases.
A good way to sort out risk? LDL particle size seems to be strongly correlated with triglyceride level (high triglycerides go with small particle size and vice versa). So if your triglycerides are low, your LDL particles are probably larger.
The Bottom LineReducing carbohydrate in the diet generally has a positive effect on both HDL and LDL blood cholesterol and triglycerides. Still, there are different reasons for high cholesterol in different people. As with almost everything going on in our bodies, there is much individual variation. There is almost certainly a strong genetic component to the all the different correlations between blood lipids and risk for disease.
Lamarche, Benoit, et al. “A prospective, population-based study of low density lipoprotein particle size as a risk factor for ischemic heart disease in men.” Canadian Journal of Cardiology 2001 Aug;17(8):859-65.
Siri, Patty, and Krauss, Ronald. “Influence of dietary carbohydrate and fat on LDL and HDL particle distributions.” Current Atherosclerosis Reports 2005 Nov;7(6):455-9.
Volek, Jeff et al. “Modification of lipoproteins by very low-carbohydrate diets.” Journal of Nutrition 2005 Jun;135(6):1339-42.