Q. I understand that exercise raises the level of HDL in the blood. Can you explain how this happens? I haven’t been able to find the answer anywhere.
— Estero, Fla.
A. First, here are a few definitions for readers who don’t know what HDL is, much less LDL (its evil twin, so to speak).
LDL, which stands for low-density lipoprotein and is sometimes called “bad cholesterol,” is the protein-cholesterol package that carries cholesterol throughout the body via the bloodstream. When levels of LDL cholesterol are high, some is deposited in artery walls. Such buildup forms plaques that harden and narrow those blood vessels, especially the coronary arteries that supply the heart.
HDL, which stands for high-density lipoprotein and is sometimes called “good cholesterol,” is the protein-cholesterol package that can pick up cholesterol from those blocked arteries and take it back to the liver for re-use in more productive ways. Thus the higher the level of a person’s good cholesterol, the lower the risk of developing coronary artery disease and having a heart attack.
Exercise can indeed have a beneficial effect on HDL levels. Although we’re still learning about the exact mechanisms, it appears that several processes may be involved. Here’s what we think happens:
Triglycerides, a type of fat that circulates in the bloodstream, serve as potential fuel for the muscle cells. Most of the triglycerides are carried in the blood by particles called very-low-density lipoproteins (VLDL), which carry some cholesterol too. When you exercise, there is an increased need for fuel in the muscle cells, so an enzyme called lipoprotein lipase (LPL) goes to work breaking down the triglycerides in VLDL. After this, another enzyme transfers cholesterol and other molecules — phospholipids and proteins — that were carried by VLDL to HDL, thus raising the level of HDL cholesterol in the blood.
Because the all-important LPL enzyme resides in the capillaries and exercise increases muscles’ capillary density, exercise also raises the LPL concentration and heightens its activity. So the more exercise the better, as greater demand for fuel leads to corresponding gains in amounts and potency of LPL with subsequently greater reduction in triglycerides and transfer of cholesterol to HDL. LPL activity is thus strongly correlated with the HDL cholesterol level.
The HDL-raising benefit of exercise accrues mostly to people with “metabolic syndrome” — a combination of low HDL, high triglycerides, high blood sugar or insulin resistance and excessive fat around the waist. These individuals can raise their HDL levels by exercising, provided they are losing weight at the same time and also improving other metabolic parameters.
By contrast, people with low HDL who do not have metabolic syndrome will be less likely to address the problem through exercise. A low level of HDL can be largely genetically determined and is a risk factor for premature coronary-artery disease. These individuals are more likely to be helped by medications, chosen in consultation with their doctors.
A word of caution to those who are trying to lower their weight by dietary means: As you decrease your calorie intake, aim to reduce saturated fat (in beef, pork and dairy products, for example) and trans fats (in stick margarine and shortening) from your diet not only because they are high in calories but also because they tend to raise the level of LDL. But don’t eliminate all fat. Eating certain vegetable fats (monounsaturated fats), such as those contained in almonds and many other nuts, canola oil, avocados and olives, will help to keep your HDL levels from decreasing as you lower your intake of saturated fats and trans fats. A fish-oil supplement also may help to raise your HDL.
— Thomas G. Allison, PhD, MPH cardiovascular diseases, Mayo Clinic, Rochester, Minn.
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