Friday, June 26, 2009

When Friedewald Attacks

I don't get very excited about nitpicking blood lipids. That's not to say they're not useful. There's definitely an association between blood lipids and certain health outcomes such as cardiovascular disease. The thing that tires me is when people uncritically interpret those associations as evidence that lipids are actually causing the problem.

Low-density lipoprotein, or LDL, is the cholesterol fraction that typically gets the most attention. High LDL associates with heart attack risk in Americans and some other groups. Statins reduce LDL and reduce heart attack risk in a subset of the population, and this has been used to support the idea that elevated LDL causes heart attacks. This is despite the fact that lowering LDL via diet doesn't seem to reduce heart attack risk (typically by reducing total fat and/or saturated fat). Statins may in fact work because they're anti-inflammatory, rather than because they reduce LDL. But both explanations are speculative at this point.

The fact remains that if you want to know if Mr. Jones is going to have a heart attack in the next five years, measuring his LDL will give you more information than not measuring his LDL. This association doesn't seem to apply to all cultures or to Americans eating atypical diets. Then you can get into the fractions that associate more tightly with heart attack risk, such as low HDL, high triglycerides, small dense LDL, etc. Triglycerides vary with HDL (that is, when trigs go up, HDL generally goes down) and the ratio also happens to be a predictor of insulin sensitivity. Total cholesterol is virtually useless for predicting heart attack risk in the general population. This is something I'll discuss in more detail at another time.

When you walk into the doctor's office and ask him to measure your cholesterol, the numbers you get back will generally be total cholesterol, LDL, HDL and triglycerides. All of those except LDL are measured directly. LDL is calculated using the Friedewald equation, which is (in mg/dL):
LDL = TC - HDL - (TG/5)
Low-carb advocates have known for quite some time that this equation fails to accurately predict LDL concentration outside certain triglyceride ranges. Dr. Michael Eades put up a post about this recently, and Richard Nikoley has written about it before as well. The reason low-carb advocates know this is that reducing carbohydrate generally reduces triglycerides, often below 100 mg/dL. This is the range at which the Friedewald equation becomes unreliable, resulting in artificially inflated LDL numbers that make you have a heart attack just by reading them.

I had a
lipid panel done a while back, just for kicks. My LDL, calculated by the Friedewald equation, was 131 mg/dL. Over 130 is considered high. Pass the statins! But wait, my triglycerides were 48 mg/dL, which is quite low. I found a paper through Dr. Eades' post that contains an equation for accurately calculating LDL in people whose triglycerides are below 100 mg/dL*. Here it is (mg/dL):
LDL = TC/1.19 + TG/1.9 - HDL/1.1 - 38
I ran my numbers through this equation. My new, accurate calculated LDL? 98 mg/dL. Even the U.S. National Cholesterol Education Panel wouldn't put me on statins with an LDL like that. I managed to shave 33 mg/dL off my LDL in 2 minutes. Isn't math fun?

*This equation was designed for individuals with a total cholesterol over 250 mg/dL.


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