IF THE people who work for the federal agencies that dispense nutrition information to the public don't know how to eat, why should anyone else?

Last Wednesday the daily special at the Department of Agriculture cafeteria cost $2.17, and it was a bargain. Where else can you get a large piece of fried chicken, a generous scoop of mashed potatoes topped with gravy, a roll and butter with apple pie for dessert for so little money?

Some bargain. By two that afternoon the fat globules must have been having a field day running up and down the arteries of dozens and dozens of employes.

Another special being offered, with one redeeming feature, was a bacon cheeseburger with french fries and a piece of fresh fruit.

The few conscientious lunchers who knew that something was missing from the fried chicken special and bought a vegetable for 30 cents more, were faced with gray-green greens or scalloped tomatoes.

It isn't that the USDA cafeteria has no low calorie, low fat foods; it's just that what little there is doesn't look very appetizing. There's the ubiquitous and dull salad bar, but how many days a week can you eat salad? You also get a scoop of cottage cheese and flavored yogurts. I think there may have been some hard-cooked eggs, too.

I stood at the cash registers for four minutes and watched 30 lunches go by. I counted six fried chicken specials; four burgers or cheeseburgers and french fries, invariably with soft drinks; a pina colada soft yogurt with cottage cheese, jello, a roll and a soft drink; bologna sandwich with jello and cake; toasted cheese sandwich, french fries and potato chips; lasagna; lasagna with a salad and a roll; chicken salad and cheese, mashed potatoes, something that looked like goulash and a soft drink. There were four bag lunches, two salads . . . You get the drift.

A little closer to the Hill I visited the Food and Drug Administration, the other guardian of our dietary guidelines. There is no cafeteria, just a snack bar. I was afraid to even look at what is considered lunch there, so I went where most FDA'ers go, to the House of Representatives Annex Two.

The selection was more limited than at USDA, but no less fattening: baked ham steak with pineapple sauce, to be trimmed with candied sweet potatoes. A few people chose the broccoli. Other "heart's delights" included lasagna, fried fillet of haddock, the standard deli items, plus bean soup and chef's salad.

I watched that line at 18 meals went by: lasagna and cake; ham steak, yams, jello, roll and soft drink; lasagna, cake and a roll; fried fish, yams and cake; fried fish, mashed potatoes and Coke; hamburger, french fries and Coke; tuna salad; chef's salad twice; cottage cheese and bean salad.

By the time I got to the Humphrey Building (Health and Human Services), another lunch spot for FDA personnel, the cafeteria had closed, so I could only read the signs. The special of the day was spaghetti and meat sauce with French bread, butter and jello.

There is something called a Slim Line at this operation, where you can get tuna, chicken, macaroni and potato salads (slim?) as well as hard cooked eggs, fresh fruit, fruit salad, cottage cheese and plain yogurt.

There's even a sign which reads Let The Tasty Lo Calories Tempt You, but a woman who had come in to get some water said, "Everything tastes awful."

Is it any wonder kids want cake and potato chips for lunch instead of baked chicken, broccoli and whole wheat bread