TURN THE corner on 50 years and you see ahed the hazard signs of faulty diet. Friends enter the hospital for bypass surgery. Middle-aged shoppers, pushing a grocery cart with one hand, clutch low-sodium cookbooks in the other. Some scan labels looking for "palm oil" or "coconut oil" or other ingredients discouraged for low cholesterol diets.

High blood pressure. Elevated cholesterol and blood lipids. High blood sugar. Overweight. They all come tumbling down, right on top of people over 50 who find themselves worrying, all of a sudden it seems, about sodium, fat and calories.

Dorothy Bilick, a 53-year-old professor at the University of Maryland, says, "I don't have a serious case of anything." Still, if she salts her food, her blood pressure tends to rise. She finds she's a little more prone to "digestion problems" and notices the effects of coffee more now than she used to. At 5 feet 4 inches and 118 pounds, she says, "I've got to fight to keep weight off."

"Every calorie counts," says Phoebe Kurz, a 59-year-old Springfield resident whose husband just began watching his sodium consumption. "You find you have to cut down on your intake or you gain weight."

They are not alone, says Dr. Thomas Pozefsky, a Baltimore endocrinologist who teaches at Johns Hopkins. Americans can expect their blood pressure, cholesterol, blood sugar and weight to rise with age, he says, adding that all these problems are "independent and interrelated."

"If there was one way to attack all these problems simultaneously," he says, "all can be potentially reversed with weight reduction."

Pozefsky can rattle off some interesting, if not surprising, statistics about Americans.

The population as a whole has gotten heavier since 1959, when "ideal weights" came into vogue with insurance companies. A 30-year-old now is heavier than a 30-year-old 23 years ago. Next, he says, the older you get, the heavier you get. So not only are you heavier than a 50-year-old was in 1959, you are heavier than you were at 30.

"So what?" poses Pozefsky. "Are these people becoming less healthy?" Then he answers his question: If a person weighs 20 percent more (insurance company figures) to 30 percent more (his figures) than ideal weight, then yes, his weight alone can put him at risk. But, if an individual is 10 to 20 percent overweight, he may feel unattractive, but there's "no evidence" to suggest that his weight alone will affect the longevity or quality of his life. Certain died-related diseases can, however.

Doctors know that people with low blood cholesterol levels are less likely to suffer from heart disease than those with elevated levels. But, Pozefsky says, "the studies are in progress" to determine if going from high levels to low levels reduces the chance of heart problems. "We think it might," he says, but there is no final verdict.

That is not the case with hypertension, however. Lowering blood pressure reduces risk of stroke and other problems. A controlled diet--weight and sodium reduction--often lowers blood pressure without need of medication.

Insulin and blood sugar activity change with weight and age. The heavier a body becomes, the more the tissues resist insulin, so more insulin is required to carry the sugar into the tissues. Since weight increases with age, the incidence of "insulin independent" diabetes also increases. "Insulin independent" diabetes is controlled by diet, and often abates when the person returns to normal weight.

If a 30-year-old woman came to him for a physical examination and wanted to know what to expect of middle age, Pozefsky says, he would tell her three things: First, chances are that her weight will increase 10 to 15 pounds in the next 20 years, and that, by itself, would present no "significant health risk."

Second, as a woman, she would "probably be unhappy about that" because her idea of thin is being able to fit into the same size clothes as she did at 30. This will cause the middle-aged woman to "diet like crazy" to maintain what she perceives as an appropriate weight.

Third, he would say that if the minor weight gain is accompanied by elevated cholesterol, blood pressure or blood sugar levels, he would recommend losing 15 pounds, "because you will, in a positive way, affect these factors."

The reasons for the weight gain vary with the people who gain it. Pozefsky attributes it to the "uncertainties, the pace" of life. He says overeating is just one form of "substance abuse" and that the reasons people overeat often resemble the reasons people overdrink.

Dr. Maria Simonson, director of the Johns Hopkins Health, Weight and Stress Clinic, says, "The older you get, the fewer calories you need." Basal metabolism--the calories it takes to run your body while it's resting--declines slightly over the years. Unfortunately, so does exercise, so there are two reasons for weight to come creeping up.

Women at 40 "should be at their peak of life ," says Simonson, but often a woman at that age feels "past her youth." She becomes overwhelmed with family and money problems. Men at that age go through what Simonson calls "psychological menopause," when they need to "reassert their machoism." These factors may lead to overeating, which ends up putting the same person, at 50, at risk of degenerative diseases that lead to heart attacks and strokes.

Men, says Simonson, tend to gain weight when they lose their jobs. They also seem to be more resolute than women about losing weight, but women are more concerned with dieting overall.

The consensus confirms that weight will creep up, and the experts say that a slight weight gain per se isn't necessarily bad, although the person may feel uncomfortable. If other "middle-age" afflictions (hypertension, high blood lipids, diabetes) complicate the issue, weight loss and slight adjustments in diet might alleviate the problem without requiring any medication.

Weight gain that accompanies aging seems particularly difficult to handle because it means changing dietary habits well after they've become established. New recipes help the transition.

BROCCOLI BISQUE (6 servings)

In this recipe, broccoli stems may be substitute for whole broccoli. 1 head fresh broccoli (including stems, trimmed of the very woody part at the bottom) 1 medium onion, sliced 1 1/2 teaspoons dried basil 3 cloves garlic 1/8 teaspoon ground cloves 1/2 teaspoon freshly ground pepper 2 tablespoons sherry 4 cups skim milk 2 tablespoons cornstarch 2 teaspoons lemon Plain yogurt, if desired

Cut broccoli into pieces and place, with onion, in a saucepan with 1/2 cup water. Bring to a boil, cover, and reduce heat. Simmer until the broccoli is tender--15 to 20 minutes (you may want to start stems and add florets after 5 minutes or so). Transfer the vegetables and liquid to a blender. Add basil, garlic, cloves, pepper and sherry. Pure'e and return to saucepan to reheat. In two batches, combine milk and cornstarch in blender and blend briefly. Add to the pure'e and stir over medium heat until thickened. Add lemon juice. Serve hot or cold, topped with plain yogurt.

CHICKEN CACCIATORE (6 servings) 2 small green peppers 1 clove garlic 2 tablespoon chopped pimiento 1 bay leaf 1/4 teaspoon freshly ground pepper 1/4 teaspoon thyme 6 sprigs parsley 1 1/2 cups sliced mushrooms 2 cups canned tomatoes 6 skinned chicken breasts (or equivalent chicken parts)

Combine all ingredients except chicken in a heavy pot or skillet with a tight-fitting lid. Bring to a boil, breaking up the tomatoes with a spoon. Add chicken and reduce heat. Cover and simmer about 20 minutes (30 minutes for dark meat) or until chicken is done. Uncover and remove chicken to platter. Reduce sauce over heat while you remove chicken from bones. Add chicken to sauce and serve over or to the side of spaghetti or noodles.

GREEN BEANS WITH HERBS (6 servings) 1 1/2 pounds green beans 1 tablespoon olive oil 1 tablespoon chopped fresh parsley 1 tablespoon chopped fresh chives 1 shallot, finely chopped 1 clove garlic, minced

Trim beans and cut them on the diagonal. Cook in boiling water until crisp-tender, about 7 minutes. Drain well. Heat olive oil in skillet and add parsley, chives, shallot and garlic. Stir over heat 1 minute, then add beans. Stir to coat the beans with the herb mixture. Serve hot.