THE SUN'S RAYS, chock full of ultraviolet light, long have been considered the major cause of most skin cancers, so most preventive efforts have focused on minimizing your time in the sun and wearing protective clothing and sunscreen. But studies in recent years have indicated that nutritional factors also may play a role, according to Dr. Harvey Arbesman, a dermatologist at the University at Buffalo in Buffalo, N.Y. Primary among these factors, Arbesman said, are dietary fat and antioxidant vitamins and minerals. Half of all new cancers diagnosed in the United States are skin cancers.

Melanoma, the most deadly type of skin cancer, will strike over 40,000 Americans in 1998, the number of cases having increased 4 percent annually since the early 1970s, said Dr. Marianne Berwick, an epidemiologist at Memorial Sloan-Kettering Cancer Center in New York. There will be more than 1 million cases of squamous-cell and basal-cell carcinomas this year. Unlike melanomas, these skin cancers, which usually appear as dark bumps on the head, neck, arms or back, are highly curable. Arbesman reviewed the scientific literature dealing with the relationship between nutrition and nonmelanoma cancers Friday at the annual meeting of the American Academy of Dermatology in Orlando, Fla.

Among the findings:
A diet in which no more than 20 percent of calories come from fat appears to prevent against the development of precancers called actinic keratoses, as well as nonmelanoma carcinomas. Antioxidants such as vitamins C and beta carotene may prevent ultraviolet-related skin cancer. The sun's rays can set off chemical reactions that result in the generation of the dangerous free radicals that have been associated with cancer, Arbesman said. Antioxidants sop up these free radicals, preventing them from reaping their damage. Studies indicate that consuming 500 milligrams per day of vitamin C may prevent skin cancer, he said. Foods rich in beta carotene, such as carrots, sweet potatoes and broccoli, also afford protection. Vitamin E also has been shown to reduce the development of ultraviolet light-induced tumors in animals, but because it can interact with other medications, Arbesman cautioned against taking supplements without consulting a physician. Selenium, an antioxidant mineral, also may play a role in reducing the risk of skin cancer. Selenium is found in food sources such as brown rice, whole grains and Brazil nuts. One single Brazil nut provides 120 micrograms of the mineral, for example, more than the 100 microgram recommended daily dose. Again, Arbesman cautioned against taking selenium supplements because of potential adverse side effects, which include blackened or fragile fingernails, irritability, nausea and vomiting. Whole fruits and vegetables offer the best protection against not just nonmelanoma skin cancers, the New York dermatologist said, but other diseases as well.