High-density lipoproteins (HDL) form a class of lipoproteins, varying somewhat in their size (8–11 nm in diameter), that carry cholesterol from the body’s tissues to the liver. About thirty percent of blood cholesterol is carried by HDL.
Understanding HDL cholesterol
Cholesterol is carried through your blood attached to proteins. The cholesterol-protein package is called a lipoprotein.
- Low-density lipoproteins. LDL, or “bad,” cholesterol carries cholesterol throughout your body, depositing it along the walls of your arteries. Cholesterol buildup forms plaques that make arteries hard and narrow — ultimately increasing the risk of coronary artery disease.
- High-density lipoproteins. HDL, or “good,” cholesterol picks up excess cholesterol in your blood and takes it back to your liver for disposal. The higher your HDL cholesterol, the less bad cholesterol you’ll have in your blood.
The message to lower LDL cholesterol is loud and clear — but it might not be enough for people at high risk of heart disease. So doctors are beginning to turn their attention to HDL cholesterol.
Structure and function of HDL
HDL are the smallest of the lipoproteins. They are the densest because they contain the highest proportion of protein. They contain the A class of apolipoproteins. The liver synthesizes these lipoproteins as complexes of apolipoproteins and phospholipid, which resemble cholesterol-free flattened spherical lipoprotein particles. They are capable of picking up cholesterol, carried internally, from cells they interact with. A plasma enzyme called lecithin-cholesterol acyltransferase (LCAT) converts the free cholesterol into cholesteryl ester (a more hydrophobic form of cholesterol) which is then sequestered into the core of the lipoprotein particle eventually making the newly synthesized HDL spherical. They increase in size as they circulate through the bloodstream and incorporate more cholesterol molecules into their structure. Thus it is the concentration of large HDL particles which more accurately reflects protective action, as opposed to the concentration of total HDL particles. This ratio of large HDL to total HDL particles varies widely and is only measured by more sophisticated lipoprotein assays using either electrophoresis (the original method developed in the 1970s), or newer NMR spectroscopy methods, developed in the 1990s.
HDL particles are not too inherently protective. It is only the HDL particles which become the largest (actually picking up and carrying cholesterol) which are protective. There is no reliable relationship between total HDL and large HDL, and more sophisticated analyses which actually measure large HDL, not just total, correlate much better with clinical outcomes.
In the stress response, serum amyloid A, which is one of the acute phase proteins and an apolipoprotein, is under the stimulation of cytokines (IL-1, IL-6) and cortisol produced in the liver and carried to the damaged tissue incorporated into HDL particles. At the inflammation site, it attracts and activates leukocytes. In chronic inflammations, its deposition in the tissues manifests itself as amyloidosis.
Men tend to have noticeably lower HDL levels, with smaller size and lower cholesterol content, than women. Men also have an increased incidence of atherosclerotic heart disease.
Epidemiological studies have shown that high concentrations of HDL (over 60 mg/dL) have protective value against cardiovascular diseases such as ischemic stroke and myocardial infarction. Low concentrations of HDL (below 40 mg/dL for men, below 50 mg/dL for women) are a positive risk factor for these atherosclerotic diseases.
Data from the famous Framingham Heart Study showed that for a given level of LDL, the risk of heart disease varies 10-fold as the HDL varies from high to low. Conversely, for a fixed level of HDL, the risk varies 3-fold as LDL varies from low to high.
The American Heart Association, NIH and NCEP provides a set of guidelines for male fasting HDL levels and risk for heart disease.
|Level mg/dL||Level mmol/L||Interpretation|
|less than 40||less than 1.03||Low HDL cholesterol, heightened risk for heart disease, less than 50 is the value for women|
|40–59||1.03–1.52||Medium HDL level|
|>60||>1.55||High HDL level, optimal condition considered protective against heart disease|
More sophisticated laboratory methods measure not just the total HDL but also the range of HDL particles, e.g. “lipoprotein subclass analysis”, typically divided into several groups by size, instead of just the total HDL concentration as listed above. The largest groups (most functional) of HDL particles have the most protective effects. The groups of smallest particles reflect HDL particles which are not actively transporting cholesterol, thus not protective.
What measures can be used to increase HDL levels?
- Aerobic exercise. Many people don’t like to hear it, but regular aerobic exercise (any exercise, such as walking, jogging or bike riding, that raises your heart rate for 20 – 30 minutes at a time) increases the HDL levels.
- Lose weight. Obesity results not only in increased LDL cholesterol, but also in reduced HDL cholesterol. If you are overweight, reducing your weight should increase your HDL levels.
- Stop smoking. If you smoke, giving up tobacco will result in an increase in HDL levels. (This is the only advantage I can think of that smokers have over non-smokers – it gives them something else to do that will raise their HDL.)
- Alcohol. With apologies to the American Heart Association, which discourages doctors from telling their patients about the advantages of alcohol: one or two drinks per day can significantly increase HDL levels. More than one or two drinks per day, one hastens to add, can lead to substantial health problems including heart failure – and there are individuals who will develop such problems even when limiting their alcohol intake to one or two drinks per day.
- Add soluble fiber to your diet. Soluble fibers are found in oats, fruits, vegetables, and legumes, and result in both a reduction in LDL cholesterol and an increase HDL cholesterol. For best results, at least two servings a day should be used.
Now, let’s have a deeper look on what we can do to raise HDL levels naturally:
Fats that increase HDL
Whereas saturated fat (found in butter, cheese, etc.) can increase a person’s level of “bad” LDL cholesterol, monounsaturated fat can increase the level of “good” HDL cholesterol. Monounsaturated fat can also reduce levels of LDL cholesterol and triglycerides. The net effect of eating monounsaturated fat (in moderation) is a lower risk of heart disease.
Foods that contain monounsaturated fats include:
- Olive oil
- Other vegetable oils
- Nuts (for people who are not allergic)
The American Heart Association recommends that people consume less than 30 percent of their daily calories from any type of fat and choosing monounsaturated fat over saturated fat or trans fat is generally helpful when working to improve cholesterol levels. For more information about saturated vs. unsaturated fats, see Fats & Oils.
Impact of triglycerides on HDL
Research has shown a strong inverse relationship between HDL cholesterol and fat in the blood called triglycerides, which have been linked to increased risk for heart attack and heart disease. If a person’s triglyceride level is high, the HDL cholesterol level is probably low. If the triglyceride level is low, the HDL cholesterol level is probably high. Therefore, lowering the triglyceride level can help to improve the HDL cholesterol level.
The National Heart, Lung and Blood Institute classifies triglyceride levels as “normal” if they are below 150. Strategies for reducing triglyceride levels include:
- Decrease the amount of saturated fat in the diet.
- Eat a balanced, heart-healthy diet in which carbohydrates are eaten in proportion to proteins, vitamins and minerals, essential fatty acids and fiber.
- Favor the complex carbohydrates over simple carbohydrates. Simple carbohydrates, such as sugar, are absorbed quickly and can cause a sudden rise in insulin production (which in turn can increase triglycerides).
- Limit use of alcohol.
- Start a regular exercise program (see How to Exercise Safely).
- Achieve and maintain and a healthy weight.
- Control diabetes.
- Control high blood pressure.
Impact of phytosterols on HDL
Phytosterols are vegetable fats or “plant” cholesterol. They are found only in plants and have a similar chemical structure to human cholesterol. A number of studies over the past two decades have suggested that intake of phytosterols lowers LDL cholesterol levels, although scientists are still trying to understand exactly how this occurs. This discovery has led to the creation of several phytosterol-enhanced food items, usually in the form of butter alternatives.
One phytosterol-rich food that has received a great deal of attention is soy. Researchers have learned that soy contains chemical compounds called isoflavones, which have a significant impact on cholesterol levels. Soybeans (and flaxseed) also contain phytoestrogens, which are natural compounds that mimic the effects of estrogen in the body (e.g, raising HDL levels).
However, a woman with a personal or family history of estrogen-dependent health problems (e.g., breast cancer) should speak with her physician before consuming large amounts of phytoestrogen-rich foods such as soy – particularly if she is already taking estrogen in birth control pills or hormone replacement therapy. Also, although the benefits of foods containing soy have been well documented, other studies have shown that soy supplementation does not have the same range of benefits.
Soy products include:
- Soy milk (available in regular and nonfat forms)
- Chocolate made from soy milk instead of dairy milk
- Tofu or textured soy as meat alternatives
- Soy flour that can be used for baking
- A soy version of peanut butter
Other foods in which phytosterols occur naturally include:
- Sesame, corn, sunflower and canola oils
- Sesame seeds
- Sunflower seeds
- Rice bran
- Green peas
Other strategies for increasing HDL
In addition to the dietary strategies involving types of fats, phytosterol intake or reducing triglyceride levels, there are several other strategies that can help increase HDL cholesterol levels:
- Quit smoking, which can dramatically increase HDL cholesterol levels.
- Start a regular exercise program.
- Maintain a healthy weight.
- Drink alcohol only in moderation (no more than one serving daily for women or two drinks daily for men). Moderate alcohol use may increase HDL cholesterol levels. Large quantities of alcohol, however, have the opposite effect and can cause damage to the heart muscle.
Studies of postmenopausal women have found that calcium supplements (calcium citrate) can increase HDL levels. This effect is attributed to calcium’s effect on the introduction of saturated fats into the bloodstream. Calcium is seen to enhance the release of these fats from the body as waste products. Always check with a physician first to see whether a particular over-the-counter drug or supplement is appropriate (and safe) to take.
If these strategies are unsuccessful, a physician may recommend medications that have shown to raise HDL levels, such as fibrates, nicotinic acid or some other cholesterol-reducing drugs, including statins.
Questions for your doctor on good cholesterol
Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions about increasing good cholesterol:
1. What are my cholesterol levels?
2. What should my HDL level be?
3. What happens if I do not have enough HDL?
4. How can I raise my HDL level?
5. Which fats are good and which are bad?
6. Is it possible to have too much HDL cholesterol?
7. How often should I have a cholesterol test?
8. Are there any medications that can help increase my good cholesterol?
9. Are there any lifestyle changes I can make to increase my good cholesterol?
What about drugs for raising HDL cholesterol?
If these strategies are unsuccessful, a physician might prescribe some type of cholesterol-reducing drugs.
Drug therapy for raising HDL cholesterol levels has, so far, been less successful than for reducing LDL cholesterol. Statins, in particular, are often quite poor at increasing HDL levels. And while some newer statins do reliably increase HDL, these drugs might not be optimal for people whose LDL cholesterol and total cholesterol levels are normal in the face of low HDL cholesterol levels. Of the drugs used to treat cholesterol, niacin appears to be the most effective at raising HDL levels. Niacin is one of the B vitamins. The amount of niacin needed for increasing HDL levels are so high, however, that it is classified as a drug when used for this purpose, and should be taken only under a doctor’s supervision.
Now that HDL levels are attracting more and more attention, several drug companies are attempting to develop new drugs aimed specifically at increasing HDL.