Blood may be thicker than water, but that's not necessarily a good thing. Some evidence suggests that people with thicker (more viscous) blood have a higher chance of developing heart disease or having a heart attack or stroke. Thick blood may also be a risk factor for type 2 diabetes.


According to the Harvard Medical School Family Health Guide, thick blood may be due to several factors, including:

  • The volume of red blood cells, which make up half the volume of blood
  • High LDL (bad cholesterol) levels
  • Chronic inflammation
  • Smoking
  • Diabetes
  • Genes
  • The stickiness of platelets (the component in blood responsible for clotting)

Problems Associated With Thick Blood: Artery Damage and Insulin Resistance

The heart has more difficulty pumping viscous blood, and thick blood may be more likely to form clots in arteries and veins. Ralph E. Holsworth, Jr., DO, and Jonathon V. Wright, MD, of the Tahoma Clinic in Washington, believe blood viscosity is behind some damage to the walls of the arteries (the tubes that carry blood from the heart to the rest of the body). Thick blood increases friction against artery walls, potentially causing abrasion, injury, and inflammation. Artery damage raises your risk of heart disease, and it seems to occur in specific arteries close to the heart, which bear the brunt of the force of blood as it's ejected from the heart.

Holsworth and Wright say that the largest blood viscosity study, which followed almost 1,600 middle-aged men for five years, found blood viscosity significantly higher in patients who experienced heart attack or stroke. In fact, the 20 percent of individuals with the highest viscosity had 55 percent of the major heart events.

Viscous blood may also be associated with insulin resistance, a risk factor for type 2 diabetes in middle-aged men and women. People with insulin resistance have trouble using the insulin (the hormone that regulates blood sugar levels) the body manufactures; they produce ever greater amounts of insulin to overcome the cells' resistance to the hormone. Because viscous blood decreases blood flow, the delivery of insulin, glucose (sugar), and oxygen to skeletal muscles is reduced, and the body must dilate blood vessels and elevate blood pressure to compensate. Because viscous blood decreases blood flow, the delivery of insulin, glucose (sugar), and oxygen to skeletal muscles is reduced. The body compensates for this by producing more insulin, perpetuating a dangerous cycle of increased insulin production.

Are You at Risk?

Individuals with these risk factors should be tested for blood viscosity:

  • People with a personal or family history of heart attacks, stroke, hypertension, or diabetes, or who show signs of decreased cognitive function.
  • Women who took oral contraceptives or who've had a hysterectomy, pregnancy-induced hypertension (high blood pressure), or pre-eclampsia, a condition that affects pregnant women and is characterized by swelling, high blood pressure, and protein in the urine.
  • Patients with the eye conditions macular degeneration or glaucoma.

Tips to Keep Your Blood (and Body) Healthy

William A. Tansey, III, MD, a New Jersey-based cardiologist, says blood viscosity apparently increases with dehydration. This raises the risk for heart attack and stroke, so it's important to keep well hydrated, especially in hot weather.

In addition, you can keep your blood (and your body) healthy by:

  • Not smoking
  • Eating a healthy diet
  • Exercising regularly
  • Minimizing stress

William A. Tansey, III, MD, reviewed this article.



Harvard Medical School Family Health Guide. "Is Blood Like Your Waistline - the Thinner, the Better?" Web. December 2005 Update. Page accessed 13 August 2013.

Tamariz Leonardo J., Young J. Hunter, Pankow James S et al. "Blood Viscosity and Hematocrit as Risk Factors for Type 2 Diabetes Mellitus The Atherosclerosis Risk in Communities (ARIC) Study." American Journal of Epidemiology 2008; 168(10): 1153-1160. Web. 17 October 2008. Page accessed 13 August 2013.

Gudmundsdottir, Helga, Høieggen, Aud, Stenehjem, Aud, et al. "Hypertension in Women: Latest Findings and Clinical Implications." Therapeutic Advances in Chronic Disease 2012; 3(3): 137-146. Web.

Holsworth, Ralph E., Jr., and Wright, Jonathan V. "Blood Viscosity: The Unifying Parameter in Cardiovascular Disease Risk."  Holistic Primary Care 2012; 13(1). Web. Page accessed 13 August 2013.