"MUSCLE IS MORE important than fat," said Gloria Steinem, at that moment lounging on the beach in the British Virgin Islands during a break from a day of yoga, aerobics and mini-calorie meals. Gloria Steinem, both feminist and feminine, reaching her mid-century this week, had seen the light. Rather, she had seen herself light, down to 117 pounds after a vacation at a health spa, and had liked what she had seen--and felt.

Two years ago she went to an upstate New York spa with a group of friends and returned with only 114 pounds on her 5-foot-7 frame, feeling the best she had physically since her younger days as a dancer. Ever since, she has sought a fitness-oriented vacation twice a year. This February found Steinem and six friends spending a week in Tortola at Prospect Reef resort, a hotel-fitness center where for six weeks Vermont chef and spa-master Jimmy LeSage had transported his New Life Spa program to the Caribbean.

Thus Steinem's group and nine other fitness seekers spent a week breakfasting on wheat berries and bran muffins, easing into yoga-designed stretches, puffing and panting to Michael Jackson tapes, bobbing aerobically in the olympic pool, lunching and dining on zucchini soup and island cioppino and facing yet another iced glass of Red Zinger tea and Cuisinart-shredded salad with yogurt dressing.

At the end of it most had lost only a disappointing couple of pounds ("Well, I guess I gave people too much food," shrugged LeSage), but inches and inhibitions had been shed. And Steinem, a mere three pounds above her 114-pound goal, shrugged and answered for her week with, "I got rid of my cold."

That was the flip side of her summing up the regime of light eating and heavy exercise, though.

Whatever else she firmed, she also firmed her resolve to seek fitness. "I realize," she said, "I've been remiss all these years" about diet and health. However, given her stamina and strength (she made it through the most strenuous aerobics classes that week) the omission would not seem to have been serious. How has she developed such capacity for exercise without keeping at it over the years? "I run through airports a lot," she quipped, insisting that only recently had she broken her sloth, joining Manhattan's Biofitness Institute, where so far she had worked out a total of four times.

The question remains whether for feminists a week at a diet spa is subversive activity. For Steinem it is a matter of emphasis: "We ought to talk about being strong and not about being thin." Diet and exercise should not be a matter of fitting into some psychological corset, of trying to be beautiful at all cost. Said Steinem, "If we address the question of health and strength and where we feel the best, we will be on the track."

Thus the talk nowadays is less of fat farms or diet spas and more of fitness centers. And even at Prospect Reef there was little conversation about food or of losing weight in itself, much about exercise and other spas. As Steinem put it, the best spas deal with health, strength and cardiovascular fitness, "not the scale and tape measure."

By and large the group consisted of women over 40 who had spent most of their adulthood managing busy careers, and were for the first time taking stock of their physical well-being. Koryne Horbal, for instance, had been a political organizer in Minneapolis when she was appointed to the U.N. mission by President Carter. Not only was it her first job outside Minnesota, but it involved considerable travel. It also became the first time she had to face the deficit of lugging around 250 pounds on her frame. Travel for her was far more exhausting than for others.

Several years ago, Steinem encouraged Horbal to go to a spa with her and begin to lose weight; over a year Horbal's loss amounted to 50 pounds. Then Horbal took on a new job--executive director of the Wonder Woman Foundation in New York--and coasted on her weight for a year; at Prospect Reef she was getting started on the next 50 pounds. She also brought a friend from Minnesota who was attempting to pare down for the first time, so the chain was continuing.

"You cannot do it yourself," reiterated Steinem. Women who have a lot of weight to lose need knowledge, reinforcement, support and freedom from sabotage, she said, and the American diet (the amount of fat in the food, the chemicals) makes the battle for fitness even harder. "We are not totally to blame for our struggle," Steinem emphasized.

Nor is the problem strictly American, said Steinem. "By and large, culturally it is more difficult for women--we are not encouraged to be as active." In all countries, she suggested, it is considered admirable for women to be weak; unlike men, women have not been encouraged to participate in sports, to sweat, to get dirty. But that is changing; in her book, "Outrageous Acts and Everyday Rebellions" (Holt, Rinehart & Winston. $14.95), Steinem noted that women "have begun to challenge the equation of beauty with weakness . . . All women need strength--health, muscles, endurance--if we are to literally change the world."

Traditionally, growing fat has been one of the few outlets for rebellion among women, said Steinem, and those rebellious forms tend to be self-punishing ones. "We control so little, we become obsessed with what we can control," she concluded; women thus concentrate on such arenas as weight and the kitchen.

And in their powerlessness women may resent those who control even those, she has suggested in her book. When she returned to her college for a reunion, she "discovered that being famous is not the worst crime a woman can commit . . . No, the worst crime is to be thin."

Despite her litheness, Steinem claims to be just like everybody else--a foodaholic (though certainly one of the thinnest foodaholics). She insists that there are only a few minutes each day when she is not thinking about food, that she can scarf down a whole Sara Lee cake and two quarts of Baskin-Robbins ice cream at a time. She keeps virtually no food at home, but buys for each meal as she needs it, or eats out. As she describes people such as herself: "You are never not a foodaholic, you are only a thin foodaholic."

It is her spa experiences that have "taught me how little food I needed to live on," she said, and that exercise is an important part of fitness. She now considers her normal maintenance level to be 1,100 calories a day, and at Prospect Reef was turning down the desserts, low-calorie as they were, and eating even less than the spa regime allowed.

Still the question remained whether at some level the issue was beauty--fitting a cultural stereotype--rather than fitness. What if at 114 pounds, her goal, Steinem saw herself as unattractive? At that weight, she admitted, "It's true that people tell me universally I look too thin, and I don't care." But if she saw for herself that she was unattractive at that weight? "I don't know how long I could hold out."

Here, from Jimmy LeSage's New Life Spa, are some of the recipes that were helping to pare down Steinem and the rest of the Prospect Reef fitness seekers. BLUEBERRY BRAN MUFFINS (Makes 12 muffins, 96 calories per muffin) 1 cup whole wheat flour 1 1/2 cups bran 1 teaspoon baking soda 2 egg whites 1 cup blueberries 1/3 cup honey 3/4 cup skim milk 1 tablespoon oil (optional

Mix together dry ingredients and blueberries (if they are frozen, add them undefrosted to the batter, or else the batter will turn blue). Moisten with egg, honey, milk and oil. Stir only enough to blend.

Place one scoop of the mixture into muffin tin sprayed lightly with oil (from an atomizer). Bake at 400 degrees for 15-20 minutes. WHEAT BERRIES (Makes 5 cups, about 10 servings, about 110 calories each plus honey and milk) 1 cup wheat berries 2 cups water 1/2 cup raisins 1/3 cup nuts Honey and warm milk for serving

Put water and wheat berries in crockpot on low heat overnight. Serve hot as a breakfast cereal with raisins, nuts, honey and milk. HERB SAUCE (Makes 10 cups, 275 calories per cup) 1/2 cup finely chopped parsley 1/2 cup finely chopped basil or 1 tablespoon dried 1 clove garlic, minced 2 tablespoons chopped scallion 1 1/2 cups low-fat cottage cheese 1/4 cup skim milk 1/4 teaspoon freshly ground black pepper 1 1/2 pounds whole-wheat or spinach pasta

Combine the parsley, basil, garlic, scallion, cottage cheese, milk and pepper in a blender. Blend until smooth. Cook the spaghetti in plenty of boiling water until just tender. Toss with sauce and serve. ZUCCHINI SOUP (Makes 5 1/2 cups, 50 calories per cup) 1 1/2 teaspoons vegetable oil 3 tablespoons tamari, soy sauce or Dr. Bronners Soy Base 2 medium carrots 4 stalks celery 3-4 cups zucchini, cut into small pieces 1 large onion 1/2 cup chicken stock Vegit seasoning to taste 2-3 cups water (water will determine how thick or thin your soup will be)

Add oil, tamari or soy sauce or soy base to a saucepan. Add carrots and celery and cook until fairly tender. Add remaining vegetables, stock, seasoning, water and cook until mushy. Pure'e in blender. Serve hot. PITA BREAD PIZZA (2 servings, 150 calories each) 1/2 cup mushrooms 1 green pepper 1 onion, small Oil, sprayed from an atomizer Dash of tamari 1 whole-wheat pita bread 6 tablespoons tomato sauce (recipe follows) 1/3 cup grated mozzarella cheese

Chop mushrooms, green peppers and onion into medium-size pieces. Mist the bottom of a saucepan with oil from an atomizer. Add a dash of tamari and saute' vegetables until browned. Cut pita two rounds. Place on cookie sheet. Bake pita halves in a 350-degree oven for 5 minutes until crisp. Place sauce, vegetables, and cheese on pita. Bake until cheese melts. ALL-PURPOSE TOMATO SAUCE 4 medium onions 1/2 head of garlic 1 large carrot 3 tablespoons tamari 3 28-ounce cans of whole tomatoes 28-ounce can of tomato pure'e 6-ounce can of tomato paste 2 tablespoons oregano 2 bay leaves

Chop onions and garlic finely. Grate carrots. Place onions, garlic and carrots in large saucepan. Add tamari. Saute' until onions brown and liquid is almost evaporated. Add tomatoes, pure'e, and paste. Break up whole tomatoes while in the pot, bring ingredients to a simmer. Add oregano, bay leaves. Simmer for 20-40 minutes. Serve with whole-wheat spaghetti, pizza, eggplant, parmesan or other tomato sauce dishes. CIOPPINO (8 servings, 270 calories each) 3 tablespoons tamari 2 medium onions, diced fine 2 green peppers, seeded and chopped 3 cloves garlic, minced 1 cup chopped parsley 2 cups whole tomatoes 2 cups tomato sauce 2 cups of water 1 1/2 cups wine--reduced by 1/2 1 tablespoon fresh ground pepper 1 tablespoon rosemary 1 tablespoon thyme 2 bay leaves, broken up 2 pounds fish, cut into 1/2-inch cubes 1-2 pounds large shrimp 1 pound scallops

Heat tamari in two-quart kettle. In it cook the onions, peppers and garlic until the onion is clear but not brown. Add the parsley, tomatoes, tomato sauce, water, wine and pepper. Add the rosemary, thyme and bay leaves. Let simmer one hour. Add the cubed fish, shrimp and scallops. Cook until shrimp turn pink, about 5 minutes. GREEN GODDESS DRESSING (Makes 1 1/2 cups, 10 calories per tablespoon) 1 cup plain low-fat yogurt 1 teaspoon worcestershire sauce 1 tablespoon lemon juice 1 tablespoon hot mustard 1 bunch chopped parsley 1/4 cup chopped chives 2 cloves garlic

Put all ingredients in blender and blend until smooth. MUSTARD DRESSING (Makes 1 1/3 cups, 11 calories per tablespoon) 1 cup yogurt 1/3 cup of your favorite mustard

Mix until smooth. BLUE CHEESE DRESSING (32 calories per tablespoon) Blue cheese dressing Yogurt

Mix one part of your favorite blue cheese dressing to three parts yogurt.