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Is blood like your waistline — the thinner, the better?

The old adage “Blood is thicker than water” makes sense for family ties. For the heart and circulatory system, though, thinner, more watery blood might be better.

Some threads of evidence suggest that people with thicker (or more viscous) blood have higher chances of developing heart disease or having a heart attack or stroke. The more viscous the blood, the harder the heart must work to move it around the body and the more likely it is to form clots inside arteries and veins.

Mind you, the evidence isn’t strong enough to put viscosity on a par with high cholesterol or blood pressure as heart hazards. It is strong enough, though, to make sure you drink enough water and stick with heart-healthy habits.

Blood thickeners

How thick or thin your blood is depends on many factors.  Red blood cells have the greatest influence on the blood's viscosity, since they account for up to half its volume. A normal hematocrit (a measure of both the number and the size of red blood cells) for men is 41%-53%. This means that red blood cells account for 41%–53% of blood volume. In women, it is 36%-46%.

Blood fats such as low-density lipoprotein (LDL, “bad” cholesterol) affect viscosity. The more LDL, the thicker your blood.

Chronic inflammation increases the viscosity of blood. So do smoking, diabetes, homocysteine, the stickiness of your platelets, and, of course, your genes.

So far there haven’t been any research “home runs” making the connection between the viscosity of blood and heart disease. Lab studies generally link blood viscosity with markers of heart disease. In one European study, people with the thickest blood were more likely to develop heart disease or die over an eight-year period. Not all the research is positive, with some studies showing no connection between blood’s mechanical properties and heart disease. But findings have been encouraging enough to fuel more research.

Water cure?

The Blood Thinner Cure, a book by cardiologist Kenneth R. Kensey, proposes a theory of how thickened blood damages the heart and blood vessels. Dr. Kensey offers a seven-step program for stopping heart disease and stroke by decreasing blood viscosity. The steps are

  • not smoking
  • eating a healthier diet
  • exercising
  • reducing stress
  • taking low-dose aspirin every day
  • donating blood
  • drinking 10–12 glasses of water a day.

The first four steps are good things to do for your heart. Each has been shown to help blood flow more freely. Is that why they reduce the risk of cardiovascular disease? Maybe. The heart healthy benefits of the last three steps are briefly explained below.

Aspirin makes it harder for platelets to form clots. Donating blood improves viscosity by removing red blood cells from circulation and stimulating the body to make flexible replacements. And, what about drinking 10–12 glasses of water a day? This is more myth than sound advice. Make no mistake — you need to take in as much water every day as you lose. According to the Institute of Medicine, it can come from food (particularly fruits and vegetables), or from beverages. Let thirst be your guide. While water could make blood less viscous, its effect on the heart isn’t known.

Sensible approach

We don’t really know if thin blood is better for you than thick blood and we don’t have a dipstick for checking blood viscosity. So in the meantime, it makes sense to exercise, eat healthy, avoid smoking, and reduce stress. These steps do much more for you than merely thin your blood.

What about drinking more water? It’s always a good idea to keep yourself well hydrated. Chronic mild dehydration has been linked with cardiovascular and non-cardiovascular problems. The bottom line is that you have to find the amount of water that’s right for you.

December 2005 Update

Help prevent coronary artery disease with this heart health report
Click to enlarge

Beating Heart Disease

If you follow the news about heart disease closely, it’s easy to be overwhelmed or confused about what puts you at risk and how you can protect yourself. This report helps you identify the risk factors you can control, which range from medical conditions such as high blood pressure to lifestyle choices such as an unhealthy diet or lack of exercise. Read more

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