BOOK REVIEW / NONFICTION : Have Your Cake--Just Don’t Eat Any of It : EAT RIGHT, LIVE LONGER by Neal Barnard; Harmony Books $24, 380 pages

“Eat, drink and be wary” is the message of this new book on nutrition and aging. It is going to be a hard sell in this season of pie crusts and basted birds, but Dr. Neal Barnard offers a tantalizing set of come-ons for the reluctant reader. Forget the transitory pleasures of weight loss, good skin, shiny hair and a healthy sex drive. This diet will give you all of those--but more to the point, it will provide more years in which to enjoy them.

Maybe. The sacrifices Barnard asks us to make are plentiful. To boil it down, the road to wellville is paved with tofu. Nary a piece of chicken, beef or--holy PCBs--fish should pass your lips. Dairy, even a pious cup of nonfat frozen yogurt?

Uh-uh. When Barnard looks at all the literature about food and health (and he has done a prodigious amount of research), he comes up with low-fat vegetarianism as the answer. His is a very steep food pyramid indeed.

His descriptions of what the wrong foods can do to your body are vivid and frightening. In fact, forget food: The very air you breathe, its pollutants aside, turns into dangerous free radicals once you’ve inhaled (always a problem, for the president on down).

Taking vitamin supplements with your chicken breast--even your skinless poached chicken breast--is not the answer. Barnard insists that the best sources are natural ones. Since few of us grew up on the kind of diet Barnard espouses, he employs the services of Jennifer Raymond, a consultant to Dr. Dean Ornish’s program to reduce heart disease, to provide an array of long-life recipes.

Now Barnard is absolutely right that the traditional American high-fat, high-fried, high-protein diet is the road to eventual ruin--heart trouble, diabetes, certain cancers and a host of less ominous troubles. Being overweight can make you sick. We certainly can eat less and better; we can try to find organic produce to hedge our bets on pesticides and get off the slab protein bandwagon.

But the wholesale shift he recommends ignores a whole other aspect of eating. If we did it just to stay alive, we could process the right stuff in a blender three times a day and just chug it down.

Food is more than that--more, even, than a component of our social interactions. Food is culture--if we are what we eat, we are also what our particular ancestors ate. The problem with Barnard’s position is that it is so radical as to be out of most people’s reach.

And then there is the question of that causal guarantee. Eat right, live longer. Maybe yes, maybe no. KCET’s Huell Howser once did a charming segment about an Armenian gentleman in his 90s who attributed his longevity to the quart of yogurt he made and consumed every day of his life. George Burns probably does not eat a lot of tofu. Neither, for that matter, does my 97-year-old grandmother.

If long life were merely a matter of food, we might be predisposed to sign up for this new regimen. It isn’t. It is part genetic crapshoot, part mystery, part luck. Food counts, but it doesn’t compensate for all the rest.

Yes, you ought to toss that doughnut and have a bagel while you read this. But “Eat Right, Live Longer” is just like any other insurance policy: You pay and pay, and then there are all sorts of obstacles when it comes time to collect.