SAN DIEGO (AP) -- For years, cancer experts have been telling folks to eat fruits and vegetables. Now they are trying to answer the next obvious question: ``Which ones?''

They are still far from having a firm answer. But studies presented at a cancer conference this week suggest tomatoes are looking good. So are oranges and several other kinds of food, including tofu and spaghetti.

Teasing out specifics about the health effects of different kinds of foods is difficult, in part because it may take many years for a particular nutrient to have even a small effect on health.

However, scientists are trying. And tomatoes appear to be the food of the moment.

Last year, Harvard researchers reported that men who get at least 10 servings a week of tomato-based foods are up to 45 percent less likely than usual to develop prostate cancer.

No one knows why, but it could be because tomatoes are the primary source in the diet of a nutrient called lycopene. Now a team from Columbia University has preliminary evidence that this may be especially important for smokers.

Dr. Jean G. Ford and others looked at levels of various vitamins and other nutrients in the blood of 204 people, half of whom had lung cancer. They found concentrations of lycopene were was significantly lower in the lung cancer victims.

After taking smoking into account, they found that those with low levels of lycopene have triple the cancer risk of those with high levels. The association was especially strong in people currently smoking. The lower their lycopene levels, the higher their cancer risk.

Ford cautioned that low lycopene levels might be a result, rather than a cause, of lung cancer. Nevertheless, the findings raise the possibility -- still to be proven -- that this nutrient might somehow help protect smokers from the cancerous effects of cigarettes.

``This is a preliminary report, but it raises questions about whether there are dietary risk factors that we need to take a closer look at for lung cancer,'' Ford said.

Among other reports on the effects of food released Sunday at the American Association for Cancer Research meeting:

--Animal studies suggest that orange juice protects lab animals from cancer. In an effort to find out why this might be, Dr. Najla Guthrie and others from the University of Western Ontario evaluated limonoids, the bitter stuff in limes, lemons, grapefruit and oranges. They found that a particular limonoid called nomilin was an especially powerful inhibitor of cancer in the test tube.

--Dr. Rashmi Sinha of the National Cancer Institute looked for links between meat consumption and lung cancer in 1,216 women in Missouri. Women who ate a lot of red meat were twice as likely as those who had it sparingly to get lung cancer, but fish and chicken had no apparent effect on risk.

--People in Mediterranean countries have traditionally had relatively low risks of colon cancer, and some wonder whether pasta might be the reason. In a study at the University of Florence, Dr. Giovanna Caderni compared the effects of sugar and pasta in rats. She found that pasta-eating rodents had a lower risk of getting precancerous polyps.