Healthnotes Newswire (March 1, 2001)—A diet low in salt, protein, and alcohol, and rich in potassium and calcium is associated with lower blood pressure, according to a study published in Monday’s Archives of Internal Medicine.¹ The authors examined the diets of over 17,000 women and men in the United States who participated in the Third National Health and Nutrition Examination Survey (NHANES III), making this one of the largest investigations of diet and blood pressure to date.

The Pressure is On

Researchers measured participants’ blood pressure, which typically goes up with age. Most people know that blood pressure is expressed as two numbers: the upper number (called systolic blood pressure) refers to the amount of pressure generated in the cardiovascular system when the heart contracts and fills the body’s arteries with blood; the lower number (called the diastolic blood pressure) is the system’s resting blood pressure between heart contractions. A normal blood pressure for a healthy adult is about 120/80 millimeters of mercury on a blood pressure gauge.

Historically, the lower (diastolic) number is the one doctors are most concerned about, since some reports have suggested that diastolic elevations are more closely associated with hypertension and its adverse consequences, such as stroke. The authors of the present study also measured pulse pressure, which is simply the difference between the systolic and diastolic numbers. However, pulse pressure may be more predictive of cardiovascular events, particularly heart attacks, than either of the other two blood pressure numbers.² This was the first study to report a relationship between dietary factors and pulse pressure. The researchers found that reducing dietary intake of salt (sodium chloride), protein, and alcohol could favorably modify this important risk factor.

Holistic Statistics?

Although many dietary factors have been independently associated with blood pressure changes in the past, relatively strong evidence exists for only a few of these associations. This is partly because the effect on blood pressure of a particular nutrient or food is modified by the intake of other nutrients and foods; these interactions among various dietary factors have complicated interpretation of results in past studies. To avoid mistakes such as this, it is important to assess the overall diet rather than any single nutrient in isolation.

The new study did just that. Researchers not only studied the relationship of individual dietary factors to blood pressure, but they also studied how these dietary factors interact, and used special analytical techniques that clarified some of the confusing effects of these interactions. The authors believe that their approach led to more valid results than previously reported.

Minerals are Key

Results of the NHANES III study show that the average participant ate too much salt and not enough potassium, calcium, and magnesium in relation to recommended dietary allowances (RDAs).³ These findings reflect the standard American diet, which is typically high in processed foods (the primary source of dietary salt for most Americans), and low in fresh fruits, vegetables, and whole grains (important sources of potassium, magnesium, and, to a lesser extent, calcium).

The average study participant consumed 8.25 grams of salt per day, and the average American eats between 8 and 10 grams per day. The U.S. Dietary Guidelines Advisory Committee and the American Heart Association recommend limiting intake of salt to 6 grams per day or less for the prevention of high blood pressure, with still more rigorous restrictions applying to people with existing hypertension. The average participant in the study consumed 2,700 mg of potassium; the RDA for potassium is 3,500 mg. Participants in the study consumed 761 mg of calcium and 280 mg of magnesium each day; many doctors who use nutritional medicine suggest that optimal intakes are approximately 1,000 mg and 400 mg, respectively.

Not only were participants’ intake of these crucial nutrients less than optimal, but the study also linked these aspects of the standard American diet with high blood pressure and provided convincing evidence that simple dietary changes emphasizing fresh, whole foods over processed and fast foods could have significant health effects for Americans.

Protein: Enough is Enough

Americans eat too much protein, a fact supported by the new study. The average participant in the NHANES III ate 79 grams of protein per day. Many Americans are currently enthused about high-protein/low-carbohydrate diets (e.g., the Atkins and Zone diets) and are consuming even higher amounts, often exceeding 100 grams of protein per day, an amount considered excessive by most doctors and nutrition experts. By comparison, the World Health Organization estimates that a safe level of protein intake for a healthy 150-pound adult is 52.5 grams per day. High protein intake increases the burden on the kidneys to eliminate nitrogen-containing waste products from protein metabolism. The kidneys regulate the balance of sodium and potassium in the body, and are the long-term regulators of blood pressure as well. So it is not surprising to learn that, in addition to salt and potassium intake, the intake of protein appears to influence blood pressure.

Earlier studies provided conflicting results about the association between protein intake and systolic blood pressure.^{4 5 6} The new study found an association between high protein intake and high blood pressure. The researchers also found that this effect could be reduced by higher intake of potassium and magnesium. Thus, people with high blood pressure who wish to try a high-protein diet (like Atkins or Zone) are advised to reduce their risk of further raising their blood pressure by either choosing a lower-protein diet as an alternative, or consuming adequate amounts of potassium and magnesium should they decide to consume a high-protein diet.

Alcohol and Heart Disease

Drinking alcohol was associated with higher systolic, diastolic, and pulse pressure in the new study. Consumption of more than about three alcoholic beverages per day appears to increase blood pressure.⁷ Whether one or two drinks per day meaningfully increases blood pressure has been unclear. Many studies have previously found an association between increased alcohol consumption and higher systolic blood pressure.^{8 9 10} This new study is the first to link alcohol consumption to rises in diastolic and pulse pressure, as well. These results solidify the case against alcohol consumption for people with high blood pressure. Although moderate drinking appears to be protective against some forms of heart disease (i.e., atherosclerosis and heart attack), it is probably best that people with high blood pressure avoid alcohol entirely.

The results of the new study present a clear course of action for those who wish to lower their blood pressure and prevent hypertension: Eat more fresh fruit and leafy green vegetables, whole grains, and legumes; reduce dietary salt intake by avoiding processed and fast foods, as well as the saltshaker; and drink alcohol in moderation or not at all.