It has been said many times: we are what we eat. The less obvious fact is that our restaurants are what we eat: our tastes and inclinations shape their menus.

A restaurant may want to serve rabbit hash, but if nobody orders it, it can't stay on the menu long. And when Hecht's lunchrooms offered Pritikin diet entrees daily, they lasted only a few months. On the other hand, if diners consistently make known their desire for freshly brewed decaffeinated coffee or whole-grain bread, restaurants are going to start meeting those demands. Where can't you find light beer these days? And show me a first- class restaurant that has no fresh fruit for dessert and I'll show you a restaurant that won't long be considered first class.

Some waiters would have you believe that dining out is an honor bestowed on you by the management and that you are at risk of being denied it if you cause the slightest inconvenience. But times are hard for restaurants, and those that want to stay in business are doing more to make that wish obvious to their customers.

Over the past six months I have issued a dietary challenge to dozens of restaurants. I have called ahead anonymously to order a low- calorie menu. I have prevailed on waiters and ma.itres d'h.otel for an impromptu diet dinner. I even issued challenges to two prominent French chefs to come up with a stellar low-calorie banquet. Most often I simply went to dine at a variety of restaurants, said I was on a low-fat diet (since fat is the most calorically intense food), and asked what they could provide--with no oil, butter, cream or fat of any kind.

A low-starch diet is easy to keep--just skip the potatoes, bread or rice. But a low-fat diet is hard to verify because you can't always spot the butter in the vegetables or the oil in the tomato sauce. Most difficult of all is a low- salt diet. Not only is salt invisible, but it is used so absentmindedly that the chef may not even realize his ingredients are already salted when he composes a specially ordered dish.

It's your stomach, and your money, so if you want it, ask for it--or if you want it left off, insist. A restaurant should always be ready to do less--to keep the butter off the grilled fish and the dressing out of the salad. It is not fair, however, to expect a kitchen to vary its hectic routine with no warning in the middle of lunch or dinner rush. To produce a more imaginative or troublesome variation--pasta cooked without salt, for instance-- advance warning is justified. So call ahead. And ask to talk to somebody with the authority. At best, discuss your menu ahead with the chef; at least get the m.aitre d'h.otel's assurance (and name).

Our restaurants have come a long way. Virtually all can serve an aperitif of club soda or Perrier, perked with lime, bitters or a splash of orange juice; a salad with an oil-free dressing of lemon, vinegar and a little Dijon mustard. Baked potatoes are prevalent. And fresh fish--to be grilled or poached--is ubiquitous.

Vegetarians can regularly find pastas or vegetable plates, and for people allergic to caffeine, herb teas are now commonplace (although not so popular that you wouldn't be safer carrying your own and ordering a cup of hot water). In New York, Tre Scalini offers whole-wheat pasta and the Chase Manhattan executive dining room lists calorie counts on its menu. In Philadelphia the Hunan promises Pritikinized versions of its dishes at dinner. And the Galleon Restaurant in Annapolis now provides dinner entrees that are selected and approved by both the American Heart Association and the Diet Center of Annapolis. In fact, heart associations and vegetarian groups are publishing lists of restaurants that honr their special dietary interests. And Washington workaholics, no strangers to heart disease, are educating their favorite restaurants to the need to serve light, low-fat and low-cholesterol lunches.

Waiters--or even chefs-- may have no understanding of dietary considerations even when they are willing to help. They may leave out salt but add soy sauce or bouillon that is already salted. They have been known to carefully forgo butter as requested but add cream or cheese instead. The "diet plate" often consists of hamburger and a scoop of cottage cheese (garnished with sliced egg) that is even higher in calories than the usual hamburger roll.

Waiters may insist your needs are impossible to meet or they may not pass your message on to the kitchen. I have blotted the grease of skinless chicken supposedly broiled without butter or oil and have fumed over oily vegetables accompanying my low-fat dinner. I have called ahead only to find my requests ignored in person. And I have ordered a specific variation that a restaurant promised in a survey we conducted, only to be denied that variation when--anonymously--I made the request in person.

If variety isn't crucial to you, if you can survive dining out on broiled fish and salad alone, you can dine well in Washington with a minimum of embarrassing tussle. A dieter shunning calories, fat and cholesterol can do well in sushi bars. A vegetarian can dine comfortably in an Italian restaurant. And if calories do count you can often, on standard menus, find the following:

Appetizers: clear soups, tomato or vegetable soups, melon, grapefruit, artichokes, asparagus (hold the dressing), smoked fish, gravlax, raw or steamed clams, oysters, crab cocktail, seviche, salad with a touch of vinegar or lemon.

Main dishes: fish, shellfish, chicken, veal, lamb, beef, venison grilled or baked with no fat; fish poached or cooked in parchment (be sure to hold the butter); pasta with steamed vegetables, steamed clams or mussels. Sauces can frequently be served on the side.

Accompaniments::bread without the butter (but not biscuits or muffins), rice, pasta, baked or boiled potato, salad, vegetables steamed with no butter, and in Latin restaurants rice and beans or tortillas topped with chicken and salad greens.

Desserts: fresh berries or melon, fruit salad (make sure the fruit is fresh and unsugared), sherbets, occasionally a souffl,e made of just egg whites and fruit with minimal sugar.

What you can find only rarely are prepared dishes with no added fat or salt. Lasagna, stew, navarin or baked chicken casserole are usually denied you if you have problems with fat or salt. Whole-grain breads or pastas and brown rice are rare in restaurants, as are skim milk, low-fat cottage cheese or low-fat yogurt. And in Chinese restaurants that use monosodium glutamate, expect to find it in the soup or in sauces made with broth even if you request that it be left out, for the stocks are already laced with it.

For salt-restricted meals, some table-side spices could help. And oil-free dressing-- perhaps yogurt-based-- should be easy for a restaurant to keep on hand.

Though diners don't necessarily ask for what they want, increasingly they are becoming aware of the health implications of what they eat, and wanting to fill their own special needs in restaurants. If the demand were greater, surely the restaurants' response would be, too.

Senior citizens are the pioneers in these arenas. In those communities with an overwhelming proportion of senior citizens--in Florida, for instance--restaurants cater to low-salt diets, offer reduced portion sizes when asked, and willingly serve an entire meal low in fat, cholesterol or whatever.

In testing Washington's restaurants for this diet guide, I chose a cross-section of price ranges and ethnic mixes to discover how a dieter might fare. The restaurants that did best by this test did not fall into an obvious category; they were neither predictably cheap nor expensive, French nor Chinese, highly reputed nor mediocre. It was the restaurants where the chef or manager had personally dealt with a diet, had lost weits ght or convalesced himself (notably La Brasserie and Germaine's) that came up with the most creative and delicious solutions to the dieter's needs. After all, Michel Gu,erard's cuisine minceur followed his own diet.

Several restaurateurs expressed reluctance to become known as dieters' restaurants. All we can say to them is that with the wholesomeness of our diet of increasing concern to Americans, with looking well and being fit becoming ever more fashionable, they'd better not publicize their disdain for the bodies that are filling their restaurants' costly seats. More and more diners are leaving restaurants not only commenting on how good dinner tasted, but on whether they feel good having eaten it.