COULD DRINKING milk be a source of leukemia? Can drinking orange juice help save you from gum disease? Can eating garlic guard against stomach cancer? Medical science marches on, probing deeper into the mysteries of how certain foods can cause or prevent disease. All of the above theories recently received prominent play in medical journals.

Leukemia from milk? Scientists have known for years that a virus causes deadly leukemia in cows. They even surmised that cows could pass that virus on through their milk. But they did not know how extensive the problem was. Researchers at the University of Pennsylvania School of Veterinary Medicine now report that when they injected the milk from infected cows into sheep, all the sheep became contaminated by the virus. That shows, the researchers say, that the leukemia virus can routinely be transmitted through milk.

Moreover, Dr. Jorge Ferrer, one of the researchers, says previous surveys show that numerous cows carry the cancer virus in their blood and milk for life without showing any signs of disease. In fact, he says, from 20 percent to 25 percent of all dairy cows in the country are infected by the leukemia virus. He therefore concludes that the leukemia virus is in our milk supply.

Admittedly, it could add up to an alarming mess: the interspecies transmission of cancer. But before you give up milk, consider these other facts from Dr. Ferrer. One major safeguard: pasteurization kills the leukemia virus, and most of our milk is pasteurized. And Ferrer stresses there is still a big missing link: no evidence that the bovine virus either infects humans or causes human leukemia. He is now looking for the virus in the cells of humans with leukemia and other kinds of cancer. Even so, he notes, not all humans who drink active leukemia virus would develop cancer. Many other factors would have to exist, including probably a genetic susceptibility. And one major safeguard: pasteurization kills the leukemia virus, and most of our milk is pasteurized.

Still Dr. Ferrer does not dismiss the danger. He thinks it is significant enough to warn against drinking raw, unpasteurized milk. That practice, he said, which is now common among some food faddists, is extremely foolish. "I wouldn't let a child or family member of mine drink raw milk that could contain the live leukemia virus," he said.

Garlic against stomach cancer? Garlic is reputed to have all kinds of magical health properties, such as lowering blood pressure. Now, new studies from China offer a plausible explanation of how eating garlic wards off gastric cancer.

Dr. Mei Xing of Sandong Medical College studied two Chinese counties. One had the lowest rate of gastric cancer in the province -- about four cases per 100,000. The other had the highest rate -- 40 cases per 100,000, or 10 times greater. The scientist found that residents in the county with the lowest cancer rate ate about three-fourths of an ounce of garlic every day. Those with the highest rates "rarely ate garlic."

Looking further, the doctor discovered that the heavy garlic eaters had significantly lower levels of nitrite in their stomachs. Nitrite is a chemical that can combine in the stomach with other chemicals, called amines, to form nitrosamines, one of the most powerful cancer-causing agents known.

Dr. Mei Xing also found that nitrite levels in the garlic lovers' stomachs sank immediately after they ate garlic. He speculates that the garlic juice changes the bacterial composition of the gastro-intestinal tract, inhibiting the production of both nitrite and carcinogenic nitrosamines.

Other chemicals can do the same thing. Several studies in the U.S. show that vitamin C retards the formation of nitrosamines. Some researchers attribute the sharp decline of stomach cancer in this country to the year-round availability of fresh fruits and vegetables high in vitamin C. According to Dr. Vincent DeVita, director of the National Cancer Institute, one study shows that gastric cancer is lower in the southeast, mainly Florida, possibly because of the high consumption of orange juice with vitamin C.

Vitamin C against gum disease? And for the first time, there's evidence that a deficiency of vitamin C may predispose you to periodontal disease, the scourge of practically everybody over age 35. It's no secret that severe vitamin C deficiencies can cause bleeding gums, a forerunner of serious gum disease. But the question is: can a deficiency of vitamin C, so slight that it produces no symptoms, also contribute to the breakdown of gum tissue, leading to tooth loss?

The answer, published in the Journal of Periodontal Research, from Dr. Olav Alvares, now at the University of Texas in San Antonio, is yes -- at least in monkeys. Dr. Alvares fed monkeys a diet marginally deficient in vitamin C. Within two weeks, the vitamin C-deficient monkeys showed a 36 percent greater inflammation of the gums and 41 percent deeper pockets around the teeth. Dr. Alvares also found that the white blood cells, called leukocytes, in the vitamin C-deficient monkeys moved more slowly and were less able to engulf and kill bacteria, presumably causing the gum infection.

There's no reason to believe that vitamin C-deficient humans would not react the same way -- but there's also currently no concrete proof that they do. That makes Dr. Alvares reluctant to recommend vitamin C as a preventive until he's done more studies. But he does note that numerous people swear that taking vitamin C helped clear up their gum disease, or at least the inflammation and bleeding. And he points out that vitamin C deficiency is probably much more extensive in this country than suspected, because pharmaceutical drugs, alcohol and smoking all can deplete vitamin C stores in the body. So, if only a little more vitamin C will help keep those gum-infection-killer cells up to par, it makes sense to make the extra effort. There's also no evidence, even from the most conservative scientists, that excessive doses of vitamin C -- in the 1,000 to 10,000 milligram a day range -- are dangerous.

However, a new study in rats does find that taking megadoses of vitamin C might cause slight vitamin E deficiencies, which could make tissue more susceptible to harm. The investigators found that putting more vitamin E in the rats' diet immediately counteracted the deficiencies created by the vitamin C megadoses. The scientists didn't say it, but it seems clear that if you're taking high doses of vitamin C, you probably also should increase your intake of vitamin E.