Blood pressure, the measurement of force applied to the walls of the arteries, is given as two readings, for example, 120/80 millimeters of mercury (mm Hg). The top number is the systolic pressure, the maximum force created when the heart pumps blood to the rest of the body. The bottom number is the diastolic pressure, the minimum pressure in the arteries when the heart is relaxed and filling with blood. Normal systolic blood pressure is 90 to 119 mm Hg, and 60 to 79 mm Hg for the diastolic pressure. Blood pressure is considered pre-hypertension when the top number is 120 to 139 or the bottom number reads 80 to 89. Mild high blood pressure is when the top number is 140 to 159 or the bottom number reads 90 to 99, and moderate-to-severe high blood pressure or hypertension is a systolic pressure of 160 or more or a diastolic pressure of 100 or over. When the cause of high blood pressure is unknown, it is called essential or primary high blood pressure. If it results from another medical problem or medication, it is called secondary high blood pressure.
When measuring blood pressure, it is important that the correct arm position is used (see image). Be aware that the reading may be invalid if the arm is not supported at mid-sternum level and horizontal to the body as there can be an overestimation or underestimation of pressures of about 10 mm Hg. Correct blood pressure measurements are essential to diagnosing and treating hypertension, particularly in the elderly, as it can significantly increase kidney and heart disease and especially stroke.
SYMPTOMS, INCIDENCE AND RISK FACTORS OF HYPERTENSION
Hypertension is under-diagnosed because it damages the body with mild to no symptoms. For the most part, you can’t tell if you have high blood pressure and for that reason, it is often called a “silent killer.” According to the American Heart Association, about 73.6 million people in the United States age 20 and older—that is one in three adults have high blood pressure.
The risk factors for hypertension are many and varied—age, ethnicity, gender, family history, smoking, activity level, diet, medications, kidney problems and many other medical problems. In turn, hypertension is a major risk factor for strokes and heart attacks, and in 2006, stroke accounted for about 1 of every 18 deaths in the United States. According to the American Heart Association, high blood pressure is the leading cause of stroke, a condition that develops when blood flow to the brain is interrupted. By lowering blood pressure, long-chain omega-3 fatty acids may reduce the likelihood of developing ischemic stroke—the most common kind—which occurs when there is an interruption in the flow of blood to the brain.
LONG-CHAIN OMEGA-3s USUALLY LOWER BLOOD PRESSURE
Increasing evidence suggests that omega-3s derived from fish and fish oils, especially from fatty fish like mackerel, sardines, salmon, trout and fresh tuna, play a protective role in heart disease through a variety of actions, including effects blood pressure. There is limited information on the role of foods in long-term blood pressure change. However, studies generally support the concept that diets higher in fruits and vegetables and lower in meats (except fish) may reduce the chance of developing high blood pressure. For example, long-term fish intake was associated with better systolic blood pressure. Another population that normally consumed 300–600 g of fish daily had lower blood pressure than a population with a vegetarian diet.
The beneficial effect of fish on blood pressure has often been ascribed to the increased intake of the long-chain omega-3s docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). Some studies indicated that the blood pressure-lowering effect of omega-3s was limited to hypertensive individuals and the reductions occurred at relatively high doses. One study showed that a daily consumption of salmon by healthy people significantly decreased systolic, diastolic and mean arterial blood pressure by 4%. In individuals with heart disease, the intake of lean fish at least four times per week also reduced blood pressure levels. Even a small reduction in blood pressure counts, and foods rich in omega-3s as part of a healthy diet may contribute to the prevention and control of blood pressure levels.
Dietary supplementation with polyunsaturated fatty acids—including linoleic, alpha-linolenic, arachidonic and docosahexaenoic acids—during infancy is associated with lower blood pressure in later childhood. Blood pressure is known to track from childhood into adult life, and lowering a population’s diastolic blood pressure can reduce hypertension, stroke and coronary heart disease significantly.
The effects of supplemental omega-3s on blood pressure continue to be inconsistent. Studies have shown significant benefits of omega-3s in lowering blood pressure in individuals with hypertension. Blood pressure was reduced in men with normal blood lipids and mild essential hypertension after consumption of 3.4 g of omega-3s/day for two months. Blood pressure was also reduced in people with essential hypertension after 16 weeks’ consumption of 4 g per day of fish oil containing 85% EPA plus DHA. In one 12-week study, men with mildly elevated blood pressure consumed fish meals or fish oil supplements providing about 3.5 g daily of long-chain omega-3s in high- or low-fat diets. The increased consumption of long-chain omega-3s from fish or fish oil supplements was associated with significantly lower blood pressure and heart rate. Similarly, diastolic blood pressure fell markedly in overweight and obese individuals who consumed salmon three times a week or 1.3 g daily of long-chain omega-3s from fish oil capsules for 8 weeks.
Meta-analyses—a systematic statistical analysis of data from several independent studies—have examined the relationship in randomized clinical trials between the consumption of long-chain omega-3s and blood pressure. One meta-analysis concluded that the evidence suggests an antihypertensive effect of high intakes of fish oil, especially in older populations and in populations with hypertension. In participants without hypertension, there was a small but non-significant reduction in systolic and diastolic blood pressures. This meta-analysis of 36 trials found that a median dosage of 3.7 g per day of fish oil reduced systolic blood pressure by only 2.1 mm Hg and diastolic blood pressure by 1.6 mm Hg. However, even a low dose of approximately 0.7 g DHA for 3 months lowered diastolic blood pressure significantly in middle-aged men and women. Overall, the evidence indicates that the blood pressure-lowering effects of omega-3s in food or supplements benefit individuals with hypertension to a greater degree than those with normal blood pressure.
Despite the seemingly small effects on blood pressure of rather high dose long-chain omega-3 fatty acids, the possibility that an increased intake of omega-3 fatty acids could have more than a limited role should not be discounted. Small reductions in blood pressure yield a very significant risk reduction. Decreasing systolic blood pressure by 3 mm Hg will decrease mortality due to stroke by 8%, cardiac disease mortality by 5%, and all cause mortality by 3%, suggesting that intervention to effect small changes might affect large differences in morbidity and mortality.
HOW DO LONG-CHAIN OMEGA-3s LOWER BLOOD PRESSURE?
A number of mechanisms have been suggested to explain the antihypertensive effects of omega-3s, including those involving vascular, cardiac and autonomic function. Thickening of the arterial wall, which is characteristic of hypertension, was reduced with DHA treatment in an animal model of hypertension. The blood pressure-lowering effects of omega-3s in the elderly were enhanced by sodium restriction and concurrent usage of antihypertensive drugs. In male patients with mild essential hypertension, the blood pressure-lowering effect of fish oil was comparable to that of a beta-blocker—a drug that regulates the activity of the heart and is commonly used to treat hypertension. The combination of the beta-blocker and fish oil was more effective than either alone.
Other potential ways that DHA might lower blood pressure include modification of sodium absorption by the kidney, changes in kidney arachidonic acid (a long-chain omega-6 polyunsaturated fatty acid) metabolism and calcium transport, and activation of potassium channels by metabolites of arachidonic acid that dilate blood vessels. EPA and DHA have differing effects on blood circulation. DHA is possibly more favorable in lowering blood pressure and heart rate and improving vascular function. One study showed that purified DHA, but not EPA, reduced ambulatory blood pressure and heart rate in men with mildly elevated blood lipids. Ambulatory blood pressure is that measured by numerous readings over a 24-hour period or longer. EPA in particular helps the body to produce anti-inflammatory hormone-like lipids and it thins the blood. Some scientists suggest that one way EPA could help reduce blood pressure is through this blood thinning effect. By allowing the blood to be pumped more efficiently throughout the body, there is less pressure exerted on the heart.
RECOMMENDATIONS FOR INTAKE OF OMEGA-3 FATTY ACIDS
Based on abundant findings showing the beneficial effects of long-chain omega-3s on health and disease prevention, numerous health agencies have made recommendations for intake of fish or omega-3 supplements. The typical Western diet provides only the equivalent of one fish meal per 10 days. This amount is equivalent to less than the U.S. National Institutes of Health recommendation of 650 mg/day of dietary EPA and DHA, or the American Heart Association recommendation of 300 mg/day for healthy people and 1000 mg/day for those with heart disease. In 2008, the Technical Committee on Dietary Lipids of the International Life Sciences Institute North America concluded that there is now a sufficient evidence to justify establishment of a Dietary Reference Intake recommendation for EPA+DHA between 250 and 500 mg/day. Because virtually none of the plant-based omega-3 alpha-linolenic acid is converted to long-chain omega-3s, protective tissue levels of EPA and DHA can be achieved only by consuming these omega-3s already formed in seafoods and eggs. Consumption of fish oil supplements or purified EPA and DHA is also an effective way to increase the intake of omega-3s for those who do not eat seafood.
*A fully referenced version of this article is available from the editor.