What is Cholesterol?Cholesterol is a waxy substance related to fats. Although LDL cholesterol is often referred to as "bad", I think it's important to understand that cholesterol, in and of itself, is not at all bad -- in fact, it's necessary for our bodies to run. It's not overstating the case to say that we would die without cholesterol. Cholesterol is a part of our cell membranes and the myelin sheath insulating our nervous systems. It is also used in making many of our hormones (like estrogen and testosterone), vitamin D, substances that are vital to digestion, and much more. Our bodies manufacture cholesterol: depending on our dietary habits, anywhere from 80-100% of the cholesterol in our bodies is made "in-house".
What is LDL Cholesterol and What is So Bad About it?It's common to talk about different "types of cholesterol", but this isn't technically correct. All cholesterol molecules are exactly the same; it's just that they are packaged in different ways to be transported through our blood. The "transport structure" is called a lipoprotein. HDL is short for "high-density lipoprotein" and is sometimes referred to as "good cholesterol". LDL is short for "low-density lipoprotein" and is often called "bad cholesterol". (There is a third, called VLDL for "very low density lipoprotein", which could be called "worst cholesterol", but there is much less of it.) It is the LDL which is most closely correlated with heart disease related to plaques in the arteries.
But wait, there's more! HDL and LDL both are further divided into different types, and more is being found out about the different "categories within categories" of lipoproteins. In the case of LDL, the biggest difference in the types is the size and density of the particles of LDL. Many people have pointed out that the total LDL number isn't all that great at predicting who will go on to get heart disease, but the discovery of the patterns of particle size has given physicians much more information about how dangerous the LDL of a particular person is. But more about that in #2 below - first I want to briefly mention:
1. LDL Elevation is Often TemporaryThere have been quite a few studies now tracking LDL on a low-carb diet. In some of them, there is a tendency in some people to have elevated LDL at the 3-4 month mark, but by month 6 or 8, it tends to go back down. I have heard a lot of stories along these lines from low-carbers as well. So it seems prudent to keep your hand off the panic button in the early months of your low-carb eating program. (The reason for this phenomenon, as far as I know, is unknown, but it has been suggested that dissolved cholesterol in fat cells may be released into the blood stream during weight loss, and could be especially significant in the first few months, when the weight loss is most rapid.)
2. Particle Size Patterns Usually Improve on a Low-Carb DietSo why do we care about the size of the particles of LDL? It turns out that when you have more small, dense LDL particles (called Pattern B), it is much more predictive of coronary artery disease than when the particles are large and less dense (Pattern A). In every study I am aware of that looked at the LDL particle size when people switch to a low-carb diet, the patterns changed in the favorable direction.
Why are the larger-sized particles safer? I recently heard an interesting illustration by author Dr. Mark Houston (hear the interview here - it is from the August 25, 2012 show of "Ask Dr. Dawn"). He explained that rather than the common idea of cholesterol simply building up on the inside of an artery, it actually begins in the layer of the artery below the epithelium (which is like the "skin" of the artery) when there is damage and inflammation in the epithelium. Cholesterol (or, more accurately, lipoprotein) particles from the blood then lodge in this sub-epithelial layer and build up from there to form the plaque. As it grows, the plaque begins to block the artery. In explaining the effect of particle size, Dr. Houston provided this illustration: Imagine the net on a tennis court. A tennis ball will not go through that net, but if you throw a bunch of golf balls at that net, some of them will go through. This gives a rough idea of the thinking about why the small, dense LDL particles are more dangerous than the large, "fluffier" particles -- the smaller, heavier ones are more apt to lodge in the damaged area of the artery. (Dr. Houston also says that the number of LDL particles may also be important.)
You won't find out about LDL particles on a standard cholesterol test. Your doctor would need to order a special test. These are the two most common ones:
- Vertical Auto-Profile (VAP) (More about this test from the About.com Guide to Cholesterol)
- Gradient Gel Electrophoresis (GGE)
3. LDL Usually Isn't Measured Directly, Which Can Be ProblematicIt turns out that it's difficult to measure LDL directly, so labs usually use a formula to compute LDL from other blood lipid measures. It is called the Friedewald equation and this is it: LDL = Total Cholesterol - HDL - (Triglycerides/5). For most people, this formula is pretty accurate, although the mathematically inclined will realize that one way to "improve" your LDL score would be to simple raise your triglycerides by, for example, eating a bunch of sugar. (Not a good idea!! This is merely to point out a potential problem.) Now, it is known that when triglycerides are very high (over 400 mg/dL) the equation does not produce accurate results. But there is also growing suspicion that when triglycerides are unusually LOW (under 100) the LDL may be overestimated by using this formula. There have been a couple of studies about this, and also reports from physicians who use carbohydrate restriction as part of their treatment for obesity and diabetes. People on low-carb diets who are following the diet faithfully almost always find that their triglycerides drop to below 100 (at least one physician, Dr. Mary Vernon, uses triglycerides as one of the signs to tell her whether the patient is following the diet). So if triglycerides are low and LDL is high, it could be a good idea to request a test that measures LDL directly instead of using the Friedewald formula.
One of the groups that has studied this (Ahmadi et al) has developed an alternative equation, often called the Iranian equation. Although it's important to realize that this equation is not widely accepted in the U.S. medical community, it can be interesting to compare your LDL derived by the different equations. There is a calculator that compares the numbers here.
Interestingly, having triglycerides under 100 is also correlated with Pattern A LDL particle size (the healthier pattern). So if you're following a low-carb diet and have low triglycerides, there are two reasons not to worry as much about your LDL cholesterol, at least until you get additional testing.
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Krauss, Ronald, et al. Separate effects of reduced carbohydrate intake and weight loss on atherogenic dyslipidemia. American Journal of Clinical Nutrition 2006 May;83(5):1025-31.
Lamarche, et al. The small, dense LDL phenotype and the risk of coronary heart disease: epidemiology, patho-physiology and therapeutic aspects. Diabetes Metab. 1999 Sep;25(3):199-211.
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