Heart attacks and strokes account for about 1 million deaths per year. Heart attacks are the No. 1 and strokes the No. 3 cause of death in people over the age of 25 in the United States. And for every person who dies, at least two others will be seriously disabled as a consequence.

Both of these diseases are the results of the same process: arteriosclerosis, or hardening and thickening of arterial walls, followed by narrowing of the passageway through which blood flows, and finally total occlusion, or blockage. If the artery that is blocked nourishes the heart, the result is a heart attack; if it nourishes the brain, the result is a stroke. One of the most important risk factors for arteriosclerosis is high blood pressure, or hypertension.

High blood pressure is more common among blacks than among whites and is also more likely to occur at earlier ages and in a more severe form, probably accounting for the high incidence of strokes among younger blacks. High blood pressure often produces no symptoms. Thus it has been called the silent killer. The only way to find out if you have high blood pressure is to have your blood pressure measured.

As blood flows through our arteries propelled by the heart, pressure is generated. This pressure will fluctuate between a high level (systolic pressure), which follows each contraction of the heart, and a low level (diastolic pressure), which follows each relaxation. Normally, systolic pressure is about 120 and diastolic pressure is about 80.

High blood pressure (hypertension) is present if the systolic pressure increases to 140 or more, the diastolic pressure increases to 90 or more, or both. When the blood pressure reaches 160/95, most physicians consider it high enough to begin treatment. About 35 million Americans have pressures above 160/95.

A study in Framingham, Mass., has shown that men ages 45 to 64 who have pressures above 160/95 have two to three times the rate of coronary heart disease rate of as do those with pressures under 140/90. Among people with systolic pressures over 160, strokes are three times as common as among those with systolic pressures under 140.

There is no single cause of blood pressure. In a few cases, it can be caused by medical problems that can be cured, and therefore it is very important for anyone with high blood pressure to seek medical advice immediately. Most people with high blood pressure suffer from a form called essential hypertension, a type for which, at present, no cure exists. However, people with essential hypertension can often have their blood pressures lowered by drugs, proper diet, or a combination of the two.

Today we know that certain people are at greater risk for developing essential hypertension than others. Some risk factors are beyond a person's control. We cannot change our family history or our race or ethnic background. But we can change our life style. And an important component of our life style that contributes to our risk for developing hypertension is our diet. At least three nutrients have been associated with hypertension: sodium, calories, and calcium. There is evidence that implicates too much sodium, too many calories and too little calcium in the American diet in our high incidence of hypertension. This article will concern itself with the relation of sodium to high blood pressure. In a subsequent article I will discuss obesity (too many calories) and low calcium intake as they relate to high blood pressure.The first realization that high blood pressure might be related to sodium came from observations of physicians treating hypertension. They noted that a diet very low in sodium was useful in controlling the level of blood pressure in many hypertensive patients. Although this method of treatment has been largely abandoned with the availability of antihypertensive drugs, it provided the first clue to the relation of sodium to high blood pressure.

Further evidence came from several groups of scientists studying the control of fluid balance. Sodium plays a crucial role in regulating the division of fluid between the inside and outside of cells. Too little sodium can cause cells to accumulate water; too much can cause water to flow into the spaces between cells. Therefore, body sodium is very carefully regulated. Perhaps the most important organ involved in this regulation is the kidney. When there is more sodium than is needed, the kidney increases the amount excreted; when sodium levels in the body are too low, the kidney excretes very little.

One way the kidney increases sodium excretion is by releasing hormones that increase blood pressure. As the pressure increases within the small vessels of the kidney, more sodium can be filtered from the blood into the urine. But this increased blood pressure, if sustained for a long time, can cause such complications as heart attacks and strokes. Studies both in human populations and in laboratory animals have demonstrated an actual relationship between the amount of sodium consumed in the diet and the appearance of hypertension.

In the laboratory, special breeds of rats are available that will develop hypertension when fed a diet high in sodium but will maintain a normal blood pressure when fed a diet without added salt.

In countries where little sodium is consumed because salt is not added to the foods, hypertension is rare. By contrast, in countries using a lot of sodium in the food supply as salt or in other forms, such as soy sauce or M.S.G. (monosodium glutamate), the incidence of hypertension is high. Japan and China have an incidence of hypertension among the highest in the world. The United States also has a high incidence compared to those countries that do not process their foods with salt.

Finally, in populations consuming a low sodium diet, blood pressure does not increase with age. In the United States and other countries where sodium intake is high, there is a distinct trend for blood pressure to increase as people get older. Presumably the aging kidney responds to a lower sodium load by increasing blood pressure. Thus the younger kidney can excrete moderate amounts of sodium without raising blood pressure; the older kidney cannot. However, in countries where sodium consumption is low, even the kidneys of older people are capable of regulating body sodium without raising blood pressure.

Do Americans actually consume much more sodium than they need? We certainly do! On average five times as much or more. First, many of our foods are processed with salt. A can of peas may contain 100 times as much sodium as an equal quantity of fresh peas. Second, many Americans use a large quantity of salt in cooking, and some add even more at the table. Some people will salt their foods before they even taste it. Third, we are getting large amounts of sodium from the consumption of products other than salt. Alka Seltzer is sodium bicarbonate; headache pills may contain sodium salicylate, and other medications often use sodium as a vehicle for carrying the active ingredient.

So what can be done, particularly by a person who is at high risk for hypertension? Be aware of the sources of sodium in your diet. Read labels on foods. If sodium or salt is one of the first things listed, the food can be considered high in sodium. Smoked and pickled foods are very high in sodium. Use a teaspoon or less of added sodium per day in food preparation and eliminate or drastically reduce the use of the salt shaker. And read the label of any medication you take regularly. Often the same active ingredients can be obtained without sodium.

A reduced sodium diet is not difficult to follow. You need not eliminate any foods; rather you may need to change the form in which they are consumed. Some foods will be difficult to find without added salt. Potato chips, hot dogs, sour pickles and cold cuts fall into this category. Fast foods are also high in salt, and pizza, a relatively nutritious food, is no exception. For these foods, moderation is the key.

For those of you who feel that food just doesn't taste good without a lot of added salt, remember salt preference is a learned behavior. Just as it took time to learn to like salty foods, so it will take time to unlearn such a preference. For some it is helpful to cut back gradually. But however you do it, if you are at risk for high blood pressure and if you eat a typical American diet, you probably are consuming too much sodium.

And while there are no guarantees, a simple set of dietary changes may help prevent a serious chronic disease from developing.

Myron Winick, MD, is the Williams Professor of Nutrition at the College of Physicians and Surgeons, Columbia University, New York City.