Americans are more overweight today than ever before. According to a new study recently released by the National Center for Health Statistics, the average man is now 20 to 30 pounds over his desirable weight, compared to 15 to 30 pounds for the average woman. These figures represent a gain of four and one pounds respectively in the last decade.

This is especially bad news when you consider that overweight is implicated in some of our most common diseases. For example, a number of studies have shown that sustained weight reduction can bring mild hypertension - high blood pressure - back within normal levels.

In line with this, a new Israeli study at Tel Avis University illustrates how effective weight lose can be in controlling high blood pressure. Dr. Efraim Reisin and his colleagues followed 107 patients, who had been divided into three groups, for six months. All of the patients were more than 10 percent over ideal weight - as defined by the standards of the Food and Nutrition Board of the National Academy of Sciences - about 154 pounds for a 5-foot-9 man and 116 pounds for a 5-foot-6 woman. All had blood pressure consistently greater than 140/90, which is the cut-off point between normal and high blood pressure.

The first group of patients was treated only with a balanced weight reduction diet. The second group was composed of patients for whom the usual drug therapy had not succeeded in lowering blood pressure. This group was subdivided into groups Ila and Ilb. Those in Ila were continued on antihypertensive drugs and were put on a weight-reduction diet. Group IIb were continued on the drugs, but no change in diet was ordered.

For the first two months of the study, the patients were simply observed, while Reisin and his co-workers established a baseline of weight and blood pressure for each. Then, the patients in groups I and IIa were given individualized weight-reduction diets. Menus were tailored to each patient's dietary habits, financial situation and the season of the year. Interestingly, the diets were not restricted in salt, as is usually the case in hypertensive patients. In fact, the patients were encouraged to eat salty foods, provided they were low in calories. But they were told to drink about 2 1/2 quarts of water a day.

At the end of the next two months, the diets were adjusted to maintain the individual's weight at the new, lower level. Every two weeks the patients visited the clinic to have their blood pressure and weight measured. At each visit they were also checked by a physician and interviewed by a dietitian to be certain that they were following their prescribed diet and drug regimens.

It was found that 58 percent of the patients in group I and 44 percent in group II achieved a satisfactory weight loss. Perhaps more important, however, was the fact that normal blood pressure was achieved by 75 percent of the patients in groups I and by 61 percent of those in group IIa. The reduction was greatest in patients who had had the higher pressure readings at the beginning of the study.

And what about group IIb, whose members took drugs but were not put on a weight-reduction diet? They did not fare so well. Only one patient in this group showed a moderate weight reduction, and none achieved normal blood lressure. For about two-thirds, blood pressure stayed the same, and in 27 percent it rose slightly.

Of course, this does not mean that if you have hypertension and are taking drugs to control it, you should stop taking them. Only your doctor can make this judgment. Nor does it mean that you can indulge in all the salt you may want. There is evidence in both laboratory and human studies that salt may be a factor in the development of hypertension in susceptible individuals, and low-salt diets are important adjuncts to anti-hypertensive therapy.

But it is becoming increasingly clear that you should keep your weight down to its "ideal" level. And if you have a family history of hypertension, this may be the most effective preventive measure you can take.