NOT SINCE Dr. Jean Mayer wrote his book on nutrition in 1975, "Diet for Living," has there been another that steers the tricky course between food fads and food companies. Unlike Mayer's book, most of what has come along since then has had some ax to grind. The books usually have been come variation on one of the following themes: O.D. on Megadoses of Vitamin Q; How I Lost My Friends on 16 Tablespoons of Bran a Day; 73 Nutritious Menus Using Purple Cow Soda Pop; Sugar -- Good to the Last Drop Diet.

But now there is an ungimmicky book with an ungimmicky title: "Jane Brody's Nutrition Book . . . A Lifetime Guide to Good Eating for Better Health and Weight Control" (Norton, $17.95).

The book, says its author, "is designed to steer you between to profit-motivated food companies and fadism."

Brody lays it all out, with what evidence is available, along with a plea to follow the rational course: "Whatever you do, don't read this book and go tearing through your kitchen and recipe file tossing out this and that because you now deem it to be 'unhealthy.'

". . . if you make the changes too abruptly, you're likely to resent them and you'll probably build up cravings for various beloved foods you've banished from your menu."

The book deals individually with each class of calorie-contributing nutrient -- protein, fat, carboydrates -- and with sugar. It has chapters on the non-caloric nutrients: fiber, vitamins, minerals and salt. It covers the questions of safety as well as health. It deals with special groups including vegetarians, athletes, pregnant women, children. There isn't much missing.

In addition to the usual discussion that Americans consume too much protein, the author makes it crystal clear why, for instance, a T-bone steak is a poor buy both economically and nutritionally. In a T-bone steak 20 percent of the calories are from protein, 80 percent are from fat.

And speaking of fat, Brody tells us that "you don't need to eat any fat to acquire body fat. Your daily need is for a mere tablespoon of dietary fat to maintain good nutrition. Yet the average consumption in this country is 6 to 8 tablespoons a day."

Particularly in this chapter Brody faces the problem of inconclusive evidence about the relation between diet and health, specifically fat and cholesterol. ". . . There is no direct, incontrovertible proof that the risk of premature death can be lowered by dietary change," Brody writes. "Lacking such proof, some argue that reducing your intake of fat and cholesterol is unwarranted.

"But if you're waiting to change your diet until there's an airtight case against fats and cholesterol, you may die waiting."

Brody puts in more than a few good words for carbohydrates, which she says have gotten a bad press. "Carbohydrate foods -- at least the starches, fruits and vegetables -- fulfill both your physical and your psychological need for food. They make you feel as if you've eaten something. They fill your stomach, stick to your ribs, give you something to chew on.

"A platefull of spaghetti with meat sauce may not sound like a typical dieter's meal, but it can contain fewer calories and be more satisfying than a steak dinner or tuna salad."

In the realm of essential non-caloric nutrients. Brody warns against megadoses of vitamins or minerals. But she adds: "Though self-dosing with vitamins in amounts many times greater thant the Recommended Dietary Allowance is usually worthless and . . . fraught with potential hazards, recent studies have suggested that megadoses of certain vitamins sometimes can have drug-like benefits quite apart from their usual role as vitamins."

Chemicals found naturally in water are a problem, Brody says. Discussing the relationship between organic chemcials in drinking water and cancer, Brody says air-tight proof does not exist but the problem is "worrisome." To remove organic impurities many people have installed water purifiers in their homes, which, for the most part are "worthless." So Brody describes how to construct your own carbon filter for very little money.

The chapter on coffees and teas reminds people that ". . . herbal teas may contain potent chemicals that can disrupt the normal function of the body and mind." Brody describes what different herbs can do and what cautions should be observed if you want to drink herbal teas.

Milk, as many people have discovered the hard way, is not the perfect food for everyone. Two-thirds of the world's population outgrows its need for milk around the age of 2. On the other hand, Brody reports on the evidence that milk "can lower the level of cholesterol in the blood."

And then Brody gets the BIG PROLEM, obesity, a subject with which she is intimately familiar, having been 35 pounds overweight until a few years ago. Brody says the typical American make is 20 to 30 pounds overweight; the typical female is 15 to 30 pounds too heavy.

Brody, who writes on health for The New York Times, offers many suggestions on how to lose weight and keep it off, none of which is new or unusual. At the same time she carefully explains why diets like the Scarsdale, Stillmand and Atkins don't work permanently.

One bit of insight into the problems of overeating will be news to most people. Brody says that people who aren't tempted by food, "though seeming paragons of gustatory virtue are no more endowed with strength of character than you or I.There is no such trait as 'will power' or 'won't power.' What does distinguish them from other is that they eat in response to their body's demands for food and not because someone urges them to eat, or because some delectable goody is waved before them, or because some emotional anguish needs assuaging, or because the clock says it's mealtime. Instead they are said to be 'internally cued' eaters.

"The rest of us march to a different drummer. We are 'externally cued' eaters, eating in response to environmental stimuli."

Brody describes herself as one of those externally cued people, and she finally broke the yo-yo cycle of fasting and binging. ". . . fourteen years ago, desperate, disgusted, and fat, I made a decision: I stopped dieting and started to eat like a 'normal' person. I ate every meal, and I ate all the foods I loved. But instead of becoming even more like a Goodyear blimp I started to lose.

"After two years of not dieting I weighed 35 pounds less than when I had lived from one best-selling weight-loss scheme to the next."

I have a few small quibbles with the book. Brody recommends liver as a food particularly rich in various vitamins and minerals. And it is. But unlike other matters she does not offer the negative along with the plusses. Liver is the final resting place for chemicals that are consumed. If an animal has been injected or fed antibiotics, and if there are residues, that is where they will turn up. Not much, to be sure, but for those who are concerned about such things, liver is not the food of choice.

The other quibble is over the description of fructose as more useful than other sweetners for diabetics. Many diabetologists say fructose is no better, and no worse than other sweetners, and in any case, they say, the evidence is not in yet.

There are small points. For all of those people who have, over the years, looked for a single source of reliable information on nutrition and safety of foods, Jane Brody's book may be the answer.