Human beings are one of the few species who have to worry about getting enough vitamin C.

Most animals make their own from a simple sugar called glucose. But humans -- along with apes, guinea pigs and the red-vented bulbul bird -- lack the enzyme needed to produce ascorbic acid, or vitamin C.

Long before anyone had heard of vitamin C, sailors knew the ravages of its absence. In what may have been the first controlled clinical trial, James Lind, surgeon's mate on the HMS Salisbury, determined in 1747 that sailors given citrus fruit didn't get scurvy. That prompted the British Navy to ration an ounce of lemon or lime juice a day to every sailor, wiping out scurvy (and giving rise to the nickname "limey").

Today, largely because of the advocacy of Nobel laureate Linus Pauling, Americans are more likely to associate vitamin C with the common cold.

Pauling, the only person to win two unshared Nobel prizes -- for chemistry in 1954 and peace in 1962 -- startled the world in 1970 with a popular book called "Vitamin C and the Common Cold." The book suggested that one gram of vitamin C daily would dramatically reduce the incidence of colds for most people, although some would need up to 10 grams a day -- about 160 times the current recommended dietary allowance -- to ward off a cold.

No controlled study has substantiated Pauling's claim that vitamin C prevents colds. While several studies suggest that vitamin C -- at less than the megadoses Pauling recommends -- may reduce the severity of colds or the duration of their symptoms, there is no conclusive proof.

Pauling, who at 84 still works full-time at the institute that bears his name in Palo Alto, Calif., says he takes 12 grams a day of vitamin C and boosts the dose to 18 grams a day (300 times the RDA) if he feels a cold coming on.

"I always stop the cold," he says. "In fact, I no longer talk much about the common cold. I got bored with it. It's moot."

In 1979, Pauling co-authored another popular book, "Cancer and Vitamin C." The same year he said in an interview in Prevention magazine that "mortality from cancer could be decreased by 75 percent by the proper use of vitamin C alone" -- a claim he has repeatedly affirmed since.

But a study of 123 cancer patients at the Mayo Clinic, reported in The New England Journal of Medicine in 1979, found no differences between those given 10 grams of vitamin C a day and those given a placebo. Another study of 144 cancer patients in seven states and Canada, reported in 1983, found no significant difference in survival time between those given megadoses of vitamin C and those given a placebo (median survival was actually one week longer in the placebo group).

The latest study, reported last January in The New England Journal of Medicine, found that vitamin C did no better than a placebo in 100 untreatable cancer patients at the Mayo Clinic who had not received chemotherapy. (Pauling had dismissed the earlier Mayo study by saying the patients' immune systems were suppressed by chemotherapy, preventing the vitamin C from working.)

Pauling calls this latest study "thoroughly misleading" because the Mayo patients were not given vitamin C long enough.

Pauling's own research on vitamin C has been controversial. Studies at the Pauling Institute of Science and Medicine on the effects of megadoses of vitamin C in mice led to a bitter dispute seven years ago between Pauling and researcher Arthur Robinson, the institute's cofounder and scientific director.

Robinson says the research showed that mice fed high doses of vitamin C were twice as likely as others to get skin cancer, and that the only doses that offered any protection against cancer were so high that they were nearly lethal. After reporting those findings to Pauling in 1978, Robinson says, he was asked to resign from the institute, his research was impounded and some of his data were destroyed.

The dispute led Robinson to file a libel and slander suit against Pauling's institute, which was settled out of court in 1983 with a payment to Robinson of $575,000.

Robinson, who now directs the Oregon Institute of Science and Medicine, says Pauling hurt the cause of vitamin C by making "wild and extravagant claims" about its ability to prevent and cure most cancers. Such claims, he says, are easily disproved, but they divert attention from the more important question of whether vitamin C has less miraculous benefits.

While there is "not a shred of evidence" that megadoses of vitamin C prevent or cure cancer and there is growing evidence that doses above 10 grams a day carry "serious potential for harm," Robinson says it is still possible that smaller doses of vitamin C will be shown to have some beneficial effects against cancer in humans. -- Don Colburn