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

An interesting theory proposes that watering down your blood can prevent heart disease. Don't overdo it.

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 tantalizing 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. Viscosity measures a fluid's resistance to flow; honey, for example, is more viscous than water. 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.

This is a concern for people with disorders such as polycythemia vera, in which the body makes too many red blood cells, or multiple myeloma, a type of cancer that creates too many white blood cells. Extra-viscous blood may — emphasis on the may — also be a problem for the rest of us.

Mind you, the evidence isn't nearly strong enough to put viscosity on a par with high cholesterol or blood pressure as heart hazards (although that didn't stop someone from writing The Blood Thinner Cure — more about that later). It is strong enough, though, to keep viscosity in the back of your mind as another reason for drinking enough water and sticking with heart-healthy habits.

Here's what we know about blood viscosity, how it might affect the heart and blood vessels, and what you can do to keep your blood flowing smoothly.

What's in blood?

The clear fluid known as plasma makes up blood's salty "base." It carries red and white blood cells, platelets, proteins, nutrients, hormones, dissolved gases, and wastes. Red blood cells make up to half of the volume of blood.

Blood thickeners

Blood is a complex soup (see "What's in blood?"). 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. Your hematocrit is a measure of both the number and the size of red blood cells. In men, a normal hematocrit is between 41% and 53%, meaning red blood cells account for 41%–53% of blood volume; in women, it is between 36% and 46%.

Blood fats such as low-density lipoprotein (LDL, "bad" cholesterol) affect viscosity. The more LDL, the thicker your blood. The same holds true for fibrinogen, a soluble protein that can be transformed into stringy, insoluble fibrin, which forms the semi-solid base of blood clots.

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

Over the years, researchers have looked into possible connections between the viscosity of blood and heart disease. It's an active field, with enough work to support a monthly journal (Clinical Hemorheology and Microcirculation). So far there haven't been any research "home runs."

Lab studies generally link blood viscosity with markers of heart disease. A few long-term studies have looked at its connection with heart attacks, strokes, and other manifestations of heart disease. In one European study, people with the thickest blood (highest viscosity) were more likely to develop heart disease or die over an eight-year period than those with the thinnest blood. A similar connection was seen in one of the original statin studies (along with a decrease in viscosity with long-term statin use).

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?

Whether the evidence is strong enough to support The Blood Thinner Cure, a book by cardiologist Kenneth R. Kensey, remains to be seen. He has spun a theory of how "the sludge factor" — thickened, sticky blood — damages the heart and blood vessels, and offers a seven-step program for stopping heart disease and stroke by decreasing blood viscosity. The steps are

  • not smoking

  • eating a healthier diet that lowers the amount of LDL in your bloodstream

  • exercising

  • reducing stress

  • taking low-dose aspirin every day

  • donating blood

  • drinking 10–12 glasses of water a day.

The first four steps should make anyone's list of 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. But each of these does other things that are probably far more important.

Aspirin makes it harder for the blood fragments known as platelets to clump and form clots. That's why an aspirin a day is a good strategy for protecting yourself against a heart attack or stroke if you've already had one, or are at high risk for one. Healthy people, though, especially healthy seniors, have to weigh the risks of taking aspirin against the benefits.

Donating blood improves viscosity by removing red blood cells from circulation and stimulating the body to make new, flexible replacements. Does it reduce the risk for heart disease? Although blood donation is truly the "gift of life," there's little proof it provides heart-health benefits for the donor.

What about drinking 10–12 glasses of water a day? That's a lot of water. While it could certainly make blood less viscous, its effect on the heart isn't known (though its effect on the kidneys certainly is!). One study of Seventh Day Adventists in California showed that those who said they drank five or more glasses of water a day were less likely to have died of heart disease over a six-year period than those who drank two or fewer glasses. It's possible drinking more water could be good for the heart and circulation; it's equally possible that something other than water drinking was at work here.

8x8 under fire

How often have you heard that you need to drink eight 8-ounce glasses of water a day to stay healthy? You won't hear it here, since that nostrum is more medical myth than sound scientific advice.

Make no mistake — you need to take in as much water every day as you lose through breathing, sweating, and elimination. According to the Institute of Medicine, it can come from food (particularly fruits and vegetables), or from beverages such as coffee, tea, and soda and, of course, water. Let thirst be your guide, advises the Institute.

An exhaustive search for the scientific root of the eight-glasses-of-water-a-day rule came up empty. Kidney specialist Heinz Valtin of Dartmouth Medical School found no studies supporting it, while several surveys showed that most healthy adults came up short on the eight-glass scale but didn't suffer from their supposed shortfall.

Of course, if you're physically active at work, exercise a lot, or live in a hot climate, you need more fluids. But don't overdo it. As Dr. Valtin points out in his review, too much water can also be a bad thing.

When it comes to slaking your thirst, you can't beat water. It has 100% of what you need — plain old H2O — for far less than a penny a glass. But if water just isn't the beverage for you, there are other ways to get as much as you need:

  • Eat plenty of fruits and vegetables. They're full of water, and have loads of the minerals, vitamins, and fiber you need.

  • Drink a variety of low-calorie beverages to quench your thirst. Sparkling water, skim or low-fat milk, coffee, tea, and low-calorie soda are good choices. (The calories from whole milk, regular soda, juice, and other sugary beverages can really add up.)

Sensible approach

There's something appealing about the idea that thin, fluid blood is better for you than thick, gooey blood. There are just a few things that stand in its way. First, we don't really know if it's true. Second, we don't have a dipstick for checking blood viscosity. Dr. Kensey hopes that clinicians will buy a device he invented called the Rheolog that can measure blood viscosity in the doctor's office. Whether this adds anything beyond traditional tests like cholesterol and blood pressure measurements remains to be seen.

In the meantime, it makes sense to exercise, eat a healthy diet, avoid cigarette smoke, and reduce stress. These steps do much more for you than merely thin your blood. If your cholesterol is high, taking a statin makes sense for reasons beyond reducing blood viscosity. Talk with your doctor about whether aspirin is right for you.

What about drinking more water? It's always a good idea to keep yourself well hydrated. Chronic mild dehydration has been linked with mitral valve prolapse and noncardiovascular problems such as bladder cancer. It's a common reason for daytime sleepiness and constipation. Among people over age 65, dehydration is one of the most frequent causes of hospitalization.

Blood thinner misnomer

One of the most commonly used heart drugs is warfarin (Coumadin), a so-called blood thinner. Warfarin doesn't really alter the thickness (viscosity) of your blood. Instead, it makes it harder for blood to form clots. Warfarin does this by blocking the action of vitamin K, a key player in the body's clotting cascade.

You have to find the amount of water that's right for you. The "rule" that we each need eight 8-ounce glasses of water a day turns out to be as much fiction as fact (see "8x8 under fire"). If you can manage an extra glass or two of water a day, so much the better. But if prostate trouble, an overactive bladder, or other problems make urination a chore, then drinking more to theoretically ward off a heart attack or stroke isn't a good tradeoff.