WHAT'S BEHIND Jane Fonda's hugely successful involvement with health and fitness? On page 196 of her new book, Women Coming of Age, there is this sentence: "Congress received more mail protesting a proposed ban on the artificial sweetener saccharin than it did on the entire Vietnam War!" So there it is. Fonda did not give up her politics; she merely traded the relatively small audience of peaceniks (since transformed into computer salesmen and female MBAs) for a much larger crowd of healthniks.

It was a brilliant move. Healthniks come in all sizes, ages and political parties. Millions of former peaceniks have joined the fitness ranks. Heaven knows, MBAs go on diets. Even computer saleswomen do aerobics. Having thus repositioned herself, Fonda charged once more to the forefront of American life, rather than be left behind with the rest of the post- Vietnam debris. Her first writing effort, the Workout Book, combined with video cassettes, records, calendars, exercise studios and an apparel line, won her the hearts of the premenopausal masses.

Now, with Women Coming of Age, she embraces the growing constituency of older women, while addressing the still- youthful ones as well. Using health as her subject, Fonda bids to do here what Geraldine Ferraro did for women on the political front: fundamentally alter the way women see themselves.

This is not really a sequel to the previous book. Only 130 of the more than 400 pages are devoted to her "prime time workout." Preceding and following the exercises are chapters on "women in midlife" -- the physiology and biomechanics of our bodies, the changes that occur in our plumbing and our brains, what menopause fels like, the nutritional requirements, the onset of old age.

It's easy to make fun of this book and its author. I mean, all you have to do is look at her. Most women I know, whatever their age, would kill for that figure. I'd kill for those teeth. Ever tried to keep up with one of her exercise tapes? I defy you to try it; long after you've quit, Jane's still bouncing.

So a skeptical "Harumphf!" is in order when she begins, in chapter one, to talk about those lines in her face, the touch of arthritis, the sense of feeling squeezed by the brevity of life since the death of her father left her an orphan. Cm'on, Ms. Teeth, how can you show the rest of us how to age gracefully?

THE ANSWER is that she disarms us first with her candor. ("I remember walking through my kitchen early one morning and seeing reflected in the chrome of my toaster the face of a haggard, puffy- eyed, definitely no-longer-youthful person. 'Who's that?' I started to wonder, till with shock I realized it was me.") Then, in a straightforward, occasionally self-deprecating way ("In the course of writing this book there have been times when I've actually hoped I'd have a hot flash") she presents the facts, citing both sides when there is a controversy, as with the issue of estrogen therapy. There's a reasonable bibliography for those who want to read further.

Fonda knows she can get our attention because she is a star. Once she gets it, she delivers. Her message: we're all getting older, so let's age with our eyes open and our leotards on. Briskly, unsentimentally, she leads us through a primer on the changes age will bring. There's plenty of practical advice about how to get strong, take care of your skin, handle periods of depression.

By the end of the book, Fonda has demystified the post-menopausal period of life, a period that for most of us is fraught with anxiety over our bodies and our sexuality. The material has been presented before, but never for such a vast public of eager listeners. Considering the influence Fonda wields, we should be thankful that she is so responsible about disseminating what seems to be carefully researched information.

Oh sure, things get a bit sticky when Jane, masquerading as a hausfrau, babbles about stir-frying tofu and veggies in her wok for Tom and the kids. I also found myself giggling at her special exercises for the "pc" or "love" muscle. (Joan Rivers is gonna have a field day with this one!)

There are a few questionable recommendations. One is taking tryptophan, an amino acid found in health food stores in pill form, to overcome insomnia. Another is doing repeated exercises for the legs that put pressure on untested and possibly fragile knees. Overall, Fonda views exercise as a panacea for everything from osteoporosis to depression, which is further than most medical authorities would go.

But give Fonda credit for doing her homework; she and co- author Mignon McCarthy have gone way past the workout studio to report on the research of gerontologists, endocrinologists and others on the frontiers of aging. Give her brownie points, too, for taking women seriously, something many diet and exercise book authors do not.

Most of all give her a hand for becoming a cheerleader on the side of real health, not merely the facade of health based on moisturizers, tight buttocks and Vitamin E rubs. You can't help but believe that just like the rest of us, Jane Fonda is nervous about getting older. She comes across as a feminist who sincerely wants to help us all break the bonds of a culture that places such a premium on the female package, rather than the contents. She is doing her best to shed light on what had been a nether world of voodoo medicine, not to mention dangerously misleading best-sellers. It's a relief to see women reading Jane Brody and Jane Fonda instead of the reincarnations of Dr. Tarnower.