When biochemist Robert Benowixz started out to write a book about vitamins, he figured he'd probably wind up saying that people get them naturally: by way of knife and fork, rather than out of a bottle.

But a funny thing happened on the way to the typewriter.

Benowicz learned some things about politics, agribusiness, economics, the problesm of food delivery to 200 million Americans and the difference between optimum harvest time and optimum market time -- and which takes precedence.

And what happens to the vitamins in, say, an apple, when its harvested in September, but not sold until the following June? Answer: There are hardly any left.

Sure, it looks just like the apple William Tell shot off his son's head. It may even still be crisp: It certainly got enough anti-sprout, anti-rot, anti-fungus, and anti-drying agents (among others) in its warehouse purgatory to keep it that way months after it was plucked from its tree.

But B bitamins? Or C?

Forget it, says Benowicz. The same with other supposedly fresh fruits (oranges, for example) and vegetables which, in their turly fresh state, are more wholesome and vitamin-rich than the apple. It's all a matter of harvest versus market. Was this orange or peach or melon or tomato allowed to ripen naturally on tree or vine, or was it ripening chemically induced -- to the inevitable ditriment of its food value?

Now the following may be considered nutritional sacrilege among members of the health-nutrition-consumer network. The ones who warn about the necessity for interaction of vitamins with minerals and other substances occuring nautrally in foods. And worry about among other things, upsetting the chemical balance of the cells with unbridled consumption of vitamins isolated in pills.

Benowicz worries about that, too, but everything else being equal, his research has convinced him -- and through his new book, "Vitamins and You" (Grosset & Dunlop, 186 pages, $5.95) he would convence you -- that a "sensible" vitamin-supplement regiment is more beneficial than not. That the ills from too-few vitamins far outweight what he considers the negligible of too many.

He is, by and large, not al all at odds with nutritional dogma calling for the classic balanced diet: high in fresh fruits, vegetabless, proteins, wholegrain cereals, low in refined sugars and junk foods, with some fats.

All Benowicz is saying is that a balanced diet is probably not enough.

"We may be getting enough vitamins to prevent vitamin-depletion diseases like scurvy or pellagra." he says, "but it's all a matter of the difference between adequate vitamin intake and optimum intake."

He will tell you that it isn't just a matter of taking a pill. It isn't of course, that easy. Nothing ever is.

"Each of us," he notes, "is biochemically unique."

One person's Vitamin C may be another's scurvy. In other words, one doese of C may be enough for nonsmoking Jones, but too little for tow-pack-a-day Smith, who may get what is known as "smoker's scurvy."

By the same token, one person's Vitamin B may be another's cirrhosis of the liver -- if the second one is a heavy drinker.

So, says Benowicz, seeming almost to savor the knowledge that great bodies of nutritionists and many doctors will find the suggestion scandalous, "The surest way to get every vitamin you want to attempt to reach physical optimum in terms of your body's needs is probably in the form of vitamin supplements.

"Each has got to try to determine by the variables of our lives what is the optimum vitamin intak for us." He adds that the vairables can include age, sex, nutritional history and life syle.

Vitamins, he insists, are foods , not drugs and should be treated accordingly. They are, he writes, "biochemically amiable."

Nevertheless, because specific vitamins and certain drugs may interact with potentially dangerous results, he urges anyone taking any perscription drugs at all to check with a physician before taking regular supplementary vitamins.

He also warns against using vitamins to treat yourself when you are sick. "This is folly," he says. "Go to a doctor."

And don't be hooked by "fads and fetishes," such as taking certain vitamins to enhance sexual prowess and the like. Harmless, perhaps, but not effective. t

Some other odds and ends of vitamin-ology:

If you drink six or eight cups of coffee or tea a day, you are probably flushing away most of your Vitamin C.

Carrots, contrary to popular opinion, are not the best source of Vitamin A, because in that form it is particularly difficult for humans to utilize. A is a fat-soluble vitamin (along with D, E and K) and cannot be absorbed unless dissolved in fat. Benowicz suggests a supplement in which these vitamins come already dissolved in fish oil.

If you're taking an iron supplement, take it at a different time of day from Vitamin E.

If you eat bacon (or other foods with the nitrite perservatives) or smoke, you'll probably need extra Vitamin C.

Long or regular exposure to bright lights or glare may deplete Vitamin A stores in the body.

And finally, writes Benowixz, "For unknown biochemical reasons, even temporary vitamin deprivation or depletion tends to evevate the body's needs for that vitamin on a more or less permanent basis."