The Low-Down on Cholesterol

Cholesterol is the diva of the medical world.

Pursued relentlessly by the media, cholesterol gets more press than any other molecule and is constantly being brought down by drug companies. Cholesterol has been implicated as a major risk factor for heart disease and most people try to keep their blood cholesterol as low as possible. But is this the best way to lower your risk of heart attack and stroke? And are drugs or diet the way to go about it? Or neither? These are questions I have recently been asked and will answer here and in the next Health-e-Byte.

Cholesterol is essential to life. It’s a component of every cell membrane in your body and a precursor for making oestrogen, progesterone, adrenal hormones, Vitamin D and bile acids.

Cholesterol is also a vital part of the myelin sheath that enables signalling between brain and nerve cells. It plays many and varied roles in the body and is carried around in the blood by lipoproteins of varying size, weight and density.

And here is where the controversies start.

Traditional methods of measuring blood cholesterol didn’t differentiate between the different lipoproteins that transport cholesterol. We now understand that this is of critical importance.

In the 1980s we believed that the only thing that mattered was your total blood cholesterol level. This was a big mistake.

In the 1990s we believed that HDL-C (High Density Lipoprotein-Cholesterol) was good (prevented heart disease) and LDL-C (Low Density Lipoprotein-Cholesterol) was bad (caused damage to blood vessel walls leading to heart attack). This is partly true.

HDL transports cholesterol to the liver for excretion or hormone production. This means that cholesterol is not floating around in the blood with nothing to do. The devil makes work for idle cholesterol. High levels of HDL-C show that cholesterol is being put to good use.

With respect to LDL, in the last decade we’ve discovered even more subclasses that have differing effects on heart health. Small dense LDL-C is potentially harmful while light, fluffy (also described as large, buoyant) LDL-C is not.

Small dense LDL-C particles are atherogenic (damaging to blood vessels) because they circulate in the blood with nowhere to go. They eventually get oxidised and taken up by cells called macrophages which become engorged and go on to form foam cells. These foam cells then become trapped in blood vessel walls and contribute to plaque formation. As the plaques enlarge they bulge into the blood vessel cavity causing reduction in blood flow and eventually complete blockage. A blockage in an artery to the heart causes a heart attack. A blockage in an artery to the brain causes a stroke.

Furthermore, blood triglyceride (TG) levels are more predictive of heart disease than LDL-C. Body fat is mainly made up of triglycerides.

So where does this leave us?

In 2012 the National Vascular Disease Prevention Alliance published new guidelines for measuring the fats in your blood. Instead of focusing on HDL and LDL alone, the tests that best predict a healthy heart are the following:

  1. HDL-C – above 1.0 mmol/L
  2. fasting triglycerides – below 2.0 mmol/L
  3. non-HDL-C – below 2.5 mmol/L
  4. Total cholesterol / HDL-C ratio – below 2.2

Non-HDL-C includes cholesterol in all atherogenic (harmful) lipoprotein subclasses.

So here are the two take-home messages:

  1. Next time your doctor suggests you have a blood test for cholesterol, ask for the above four tests. If your numbers lie within the cut-offs listed, you have nothing to worry about in relation to your cholesterol. If your numbers are not what you want, read next week’s HEB.
  2. As we learn more about fats in the blood, different tests may be developed. Nothing in medicine stays static. For now, the above is your best bet.

A FEW MORE DETAILS ABOUT CHOLESTEROL

The cholesterol within each lipoprotein is the same – it is the carrier molecule that differs. The higher the protein to fat ratio, the more dense the lipoprotein. Hence the terms HDL (High Density Lipoprotein) and LDL (Low Density Lipoprotein.

However the picture is far more nuanced than that.

HDL and LDL are only two of many lipoproteins involved in cholesterol transport. In decreasing order of density there are also Intermediate Density Lipoproteins (ILD), Very Low Density Lipoproteins (VLDL) and chylomicrons. Each lipoprotein is designed to carry its cholesterol to a different location in the body to be used for a different purpose.

The total amount of cholesterol in your body is around 35 grams, mainly found in your cell membranes. Cholesterol is also a component of bile which is stored in the the gall bladder. When bile is released into the small intestine it helps to digest dietary fats and absorb the fat soluble Vitamins A, D, E and K. Bile also neutralises stomach acid before the acid enters your duodenum and it kills any pathogenic bacteria that might be lurking in your food.


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