WHEN A perfectly healthy 42-year-old from Bowie, Md., reported for his annual physical, he learned he had high blood pressure. An Air Force veteran, he wasn't overweight. He wasn't diabetic and showed no other signs of degenerative disease.

In addition to giving him medicine, his doctor advised a low-sodium diet, and after mentioning a few foods inappropriate for such a regime, turned him loose to fend for himself.

When the man's wife (the couple, by request, remains anonymous) began to cook according to that prescription, she didn't know what the diet entailed. The main family cook, she received no professional counseling or direction about special diets. It "was 'figure it out for yourself,' really," she said.

It was only over time that she quit buying canned soups and replaced regular bouillon with the low-sodium kind (which costs twice as much, she complains). She substituted table wine for cooking wine when she read that it had large amounts of sodium. And "our canned foods are down to about nothing now because I don't know what's in them."

According to an article in a recent "Journal of Family Practice," this experience is not unusual. Doctors apparently don't know many details about low-sodium diets, and don't use the services of those who do.

Two Seattle family practice physicians questioned other physicians, dietitians and members of the general public about their sodium knowledge and reached what were, to some, surprising conclusions.

First, the study found, family practice physicians know as much as the "average person" about the sodium level in foods. (Average people in this case were individuals coming to the University of Seattle to learn more about health-related issues. The researchers said "they may well have had an above-average interest in and knowledge of nutrition.")

Second, physicians' scores were about the same no matter how long they had been practicing. Some were medical residents (right out of med school); some had practiced more than 15 years. They had come from all over the country to attend a continuing education course.

Third, scores didn't differ much between the doctors who give general verbal advice directly to the patient and those who use handouts, refer patients to dietitians or actually prescribe a diet.

Last, the mean score for the physicians (6.4 out of 14) did not differ significantly from what would be achieved just from guessing.

On the other hand, dietitians did score significantly higher; their mean score was 10.3 out of 14.

One researcher, Dr. Fred Heidrich, of the Seattle Group Health Cooperative, says he had anticipated the test results. But, he adds, "the average physician doesn't have time to learn specifics about sodium or adequately educate the patient." He "leaves the instruction to the dietitian."

But only a minority of those doctors answering the survey depend on further patient counseling. Some, says Heidrich, are "from quite rural areas where they might not have access to dietitians or other professionals." Others don't realize additional expertise can help.

Without additional counseling, many of those who cook and shop according to low-sodium restrictions don't know how to approach the diet. Some don't realize, for instance, that bouillon cubes and soy sauce contain relatively high levels of sodium.

Washington cardiologist David Pearle says one reason physicians don't know the sodium content of common foods is that traditional medical school curricula do not stress nutrition education. "Physicians ought to know more," he maintains, since the "low-sodium diet is a very common prescription. Nutritional advice has to be very down-to-earth and concrete." Although he believes that patients need to take much of the responsibility for their own health, physicians must serve as a reference point for accurate information.

Dr. James Bergman, who helped devise the Seattle study, concurs with Pearle and says that a nutrition curriculum is just now being developed at the University of Washington. In addition, he says that it would be "nice if there was a method for the nutritionist to help the physician. It's good to have that kind of consulting going on." Physicians and nutritionists, he says, "don't talk enough."

Bergman calls for clearer sodium labeling--especially on foods high in sodium--as a simple solution to the problem. Such listings would quickly indicate to the hypertensive which foods are proscribed by sodium restriction.

For instance, on the Seattle test, physicians were asked to place certain foods in the small-, medium- or high-range of sodium content. Eight items were listed; very few physicians knew how to rate one teaspoon of soy sauce, one ounce of potato chips or one ounce of peanuts. A glance at some potato chip bags informs the consumer that a single serving contains 260 milligrams of sodium. Such clear sodium labeling, Bergman says, would make the diet easier for everyone.

Currently, Food and Drug Commissioner Arthur Hayes is encouraging food processors to label their products with sodium content. Most processors support this voluntary compliance program, and a spokesman for FDA says the department expects a third to a half of all foods under its jurisdiction to have sodium labeling by the end of 1982.

Many consumer advocates, on the other hand, support a bill introduced by Rep. Albert Gore (D-Tenn.) that would require sodium labeling.

The recommended intake of sodium is 1,000 to 3,000 milligrams per day. Among foods high in sodium are cheese, particularly processed cheese and spreads; cured and processed meats (bacon, bologna, hot dogs, sausage, etc.); smoked, pickled or canned fish; canned meats and canned meat entrees such as stews and spaghetti; heat-and-serve main dishes such as TV dinners, pot pies and pizza; condiments such as pickles, relish, olives, ketchup and mustard; canned and instant soups; crunchy snack foods; sauerkraut; and canned vegetable juices. SODIUM QUIZ

In each of the following pairs, one item has at least twice as much sodium as the other. Check the one with the most sodium for each pair. . 1 Heinz dill pickle 4 slices of Oscar Mayer bacon 2. 1 ounce Lay's potato chips 1 "Big Mac" 3. 1 serving Wishbone Italian dressing 1 ounce Planter's cocktail peanuts 4. 1 half cup Jello chocolate pudding 1 can beer 5. 1 cube instant bouillon 6 saltine crackers 6. 1 ounce Kellogg's corn flakes 1 slice Pepperidge Farm white bread Answers: 1. 1 Heinz dill pickle has about 1,137 mg. sodium, 4 slices of Oscar Mayer bacon has 452 mg. 2. A "Big Mac" has about 963 mg. sodium, 1 ounce Lay's potato chips contains about 191 mg. 3. 1 serving Wishbone Italian dressing has 293 mg. sodium, 1 ounce Planter's cocktail peanuts has 138 mg. 4. 1 half cut Jello chocolate pudding contains 404 mg. sodium, 1 can beer has 25 mg. 5. 1 cube instant bouillon has 960 mg. sodium, 6 saltine crackers have 198 mg. 6. 1 ounce Kellogg's corn flakes has about 216 mg. sodium, 1 slice Pepperidge Farm white bread has 117 mg.