FOR THE 60 million Americans with high blood pressure, including those whose blood pressure is only slightly above normal, a low sodium diet can possibly mean the difference between a lifetime of taking drugs and a drug-free life. At the least, it could mean a reduction in the amount of drugs they need to lower their blood pressure.

But many people who think they are on a low sodium diet really are not, mainly because they are unaware of the large amounts of sodium added to processed foods.

A few years ago, James Hunt of the University of Tennessee Medical School tried an experiment to see whether people know how to eliminate sodium from their diets. Sodium is most commonly found in table salt (the chemical sodium chloride), so most people think that all they need to do to reduce their sodium consumption is throw away their salt shakers and stop salting food during cooking.

Hunt's experiment involved a group of patients with high blood pressure who had been told to cut back on sodium to help bring their blood pressure down. They thought they were being model patients. As far as they knew, they had totally eliminated sodium from their diets.

But, Hunt discovered, they were inadvertently consuming at least 4,600 milligrams of sodium, which is as much as is in two teaspoons salt. This is more than twice as much as would be desirable in a low sodium diet to lower blood pressure. Their largest sources of sodium, Hunt says, were soups and condiments, such as ketchup.

The patients were fooled because they were unaware of the hidden sodium added with abandon by manufacturers of nearly all prepared foods. Most sodium that Americans eat comes in this hidden form.

To see how much sodium is added to foods, the Washington-based Center for Science in the Public Interest wrote to McDonald's restaurants for a list of the sodium content of their foods. What the center found was amazing. Excluding the sodas and the orange juice, the food with lowest sodium content is the french fries. There is more sodium in the shakes, the hamburgers, the cheeseburgers and the apple pies.

Among the processed foods that are enormously high in sodium are canned vegetables, American cheese, tomato juice, dill pickles, dried and canned soups and soy sauce. Other foods that contain large amounts of sodium are breads, ice cream, cookies and cakes and breakfast cereals.

For example, one cup of canned green beans has 925 milligrams of sodium. Two ounces of American cheese has 890 milligrams of sodium. Sodium is added not just as table salt but also as preservatives such as sodium benzoate, flavor enhancers such as monosodium glutamate and leavening agents such as sodium bicarbonate.

The problem with trying to cut down on sodium is that many of the foods containing huge amounts of the chemical do not taste salty. So people cannot use their taste buds as a guide to foods to avoid. Nor can they read the nutritional labels on the food containers.

There is no FDA requirement that the sodium content of foods be listed. This despite the fact that all sorts of micronutrients, such as thiamine and riboflavin, must be listed on these labels. Consumers can look for the word "sodium" in the list of ingredients on these labels. But the mere fact that sodium is listed does not necessarily mean that there is so much sodium present tha the food should be avoided.

Many medical experts are dismayed by the problems people have in following low sodium diets. Some doctors suspect, although they have not proved it, that certain people are extremely sensitive to sodium and that, in these people, excess sodium actually causes high blood pressure. Hunt, for example, says, "I think there is no doubt that sodium causes high blood presure."

Others agree with Adrian Ostfeld of Yale University, who says, "I think excess sodium in the diet has a kind of permissive effect. If sodium consumption is high, biger factors that tend to cause high blood pressure can operate."

Whether or not excess sodium causes high blood pressure, it seems likely that diets sufficiently low in sodium can alleviate it in many people. And even if these diets cannot cause the blood pressure to drop to normal, they can often allow patients to get by with fewer drugs. Most people who are being treated for high blood pressure take diuretics, which act by causing the body to excrete sodium. If these people consume large amounts of sodium they are counteracting some of the effects of the drugs.

The problem of reducing their sodium consumption is, or should be, of concern to as many as 25 percent of all Americans. This is the proportion of the population estimated to have higher than normal blood pressure. All these people are at an excess risk of dying of heart attacks, strokes, congestive heart failure and kidney failure if they do not get their blood pressure down to within the normal range.

Until now, doctors have placed most of their emphasis on treating the one-sixth of the population with definite high blood pressure, or hypertension. These are people were blood pressures greater than 160/95 (160 refers to the systolic pressure when the heart first contracts and 95 refers to the diastolic pressure when the heart relaxes between beats.)

Ten years ago, a Veterans Administration study conducted by Edward D. Freis showed conclusively that treatment of people with definite hypertension dramatically lowers their risk of dying. For these people, a low sodium diet alone usually cannot control their high blood pressure. They almost always must take drugs as well.

Elliot Richardson was the secretary of health, education and welfare when the results of the VA study were announced. So impressed was he with these results that he set up a national program to educate patients and doctors about the importance of treating people with definite hypertension. As a result, far more of these people are being treated now than 10 years ago. Statisticians attribute the 32 percent drop in deaths from strokes in the past 5 years to this education campaign.

But the VA study did not answer the question of whether it pays to treat the 25 million Americans whose blood pressure is above normal yet below the definite hypertension range. These people, whose blood pressures range from 140/90 to 160/95, are said to have borderline hypertension.

Lacking evidence that treatment is worthwhile, many doctors have told these patients not to worry about their blood pressure. They reasoned that the benefits of treatment were unproven but that the risks of taking anti-hypertension drugs, which is the only sure way to lower blood pressure, were clear. These risks vary with the type of drug, but it is a general rule of medicine that all drugs have risks as well as benefits.

Recently, the National Heart, Lund and Blood Institute held a press conference to announce the results of a new study that showed, for the first time, that when people with borderline hypertension lower their blood pressure, they reduce their risk of dying from strokes, heart attacks and other cardiovascular diseases.

As a result of this study, Robert Levy, director of the NHLBI, said he would first recommend that patients try reducing their sodium consumption and try losing weight if they are overweight. Only if these measures fail would he give them drugs. Hunt finds that more than 85 percent of these people can get their sodium consumption down to normal if they restrict themselves to consuming no more than one teaspoon of salt a day.

So now, more than ever, doctors will be emphasizing the benefits of low sodium diets. And there is some reason to hope that consumers will soon have a bit more information on the hidden sodium added to their foods.

An FDA panel has recommended that the amounts of sodium that can be added to foods be limited and that sodium labeling be required. The FDA is evaluating its panel's recommendations and should propose some actions in the near future. The public will then be invited to comment on the FDA proposals and, finally, something may be done to reduce the sodium content of foods -- or at least to let people know what it is.