People who drink large amounts of orange juice or who eat a lot of grapefruit may be raising their risk of melanoma, the most deadly form of skin cancer, researchers reported Monday.
They say it’s far too soon to suggest that people cut down on these fruits. But the study is worth looking into more deeply, the researchers report in the Journal of Clinical Oncology.
“At this point in time it is not a good idea to avoid citrus fruits,” said Dr. Walter Willett of the Harvard School of Public Health, who worked on the study.
But Willett’s team points out that there is a plausible mechanism for grapefruit and oranges to raise cancer risk. They contain compounds that might explain the link: furocoumarins, which make the skin more sensitive to sunlight, and psoralen. Both interact with ultraviolet light to cause melanoma cells to multiply, and psoralen was used as a tanning activator in suntan lotion until 1996.
For the study, Willett and colleagues looked at more than 100,000 Americans -– more than 40,000 men taking part in the ongoing Health Professionals Follow-Up Study and more than 60,000 women in the Nurses' Health Study. Both are complicated and detailed studies that have been going on for more than 25 years, asking questions about diet, habits and other personal matters and watching for diseases and other health issues.
The risk of melanoma was low overall –- fewer than 2 percent of the people in the study got melanoma in the 25 years. But that risk was 36 percent higher in people who ate or drank at least 1.6 six-ounce servings of citrus fruit or juice daily compared to those who consumed them less than twice per week.
“Just because something is natural doesn’t mean that it can't contain lots of very toxic substances."
There were some puzzling details. The risk was higher for people who drank the most orange juice or ate the most grapefruit, but it wasn’t seen in people who ate whole oranges or drank grapefruit juice. But grapefruit has more psoralens and furocoumarins than does grapefruit juice. As for the orange juice link, it might be seen simply because so many people drink orange juice, the researchers said.
“To test whether the positive association with melanoma was specific to citrus products, we examined the melanoma risk associated with consumption of other fruits and juices and vegetables,” the researchers wrote.
The other fruits and vegetables were not associated with increased melanoma risk.
“We hope to be looking at this further and in more detail,” Willett said. “It just illustrates how complicated this is.”
One lesson is that so-called natural foods are not necessarily completely beneficial, Willett pointed out. “Just because something is natural doesn’t mean that it can't contain lots of very toxic substances,” he told NBC News.
It does support the old recommendation that people eat a varied diet and not too much, or too little, of any one thing.
“Variety is a good thing to have because it means that you are not likely to miss out on something important and it also means you not likely to miss out on something that is good for you,” Willett said.
The researchers checked into all sorts of complicating factors, from whether people smoked, to whether they were drinking fruit juice because they were in the sun more. Nothing else explained the association.
“That was our first thought, was that people who live in Florida and California were out in the sun more and eating more citrus,” Willett said. “But that did not turn out to be the case.”
But the bottom line, experts agree, is that it’s just a very good idea to use sunscreen.
“While the findings are intriguing, it’s far too soon to recommend any broad changes to grapefruit or orange consumption. Until conclusive data are available, we should continue to be cautious about protecting our skin from sun exposure,” said Dr. Gary Schwartz of the American Society of Clinical Oncology, which publishes the journal.
“At this time, we don't advise that people cut back on citrus -- but those who consume a lot of grapefruit and/or orange juice should be particularly careful to avoid prolonged sun exposure,” added Shaowei Wu, a researcher at Brown University who headed up the research.
“At this point in time, a public overreaction leading to avoidance of citrus products is to be avoided," agreed Marianne Berwick, a dermatology professor at the University of New Mexico who wrote a commentary on the study. “The study has much strength. The rationale is clear, the study was large, and data were collected prospectively.”
“At this time, we don't advise that people cut back on citrus."
That means that the researchers didn't ask people to remember what they ate decades later -- a notoriously inaccurate way of collecting information.
It’s far from the first time that eating too much of a good thing has been linked with cancer. One of the biggest studies, done in 1996, showed that smokers who took beta-carotene, which the body turns into vitamin A, in fact had a higher risk of lung cancer than smokers who didn’t take it.
And another study showed that tiny lung tumors grew and spread far more quickly in mice given vitamin E and an antioxidant supplement called acetylcysteine.
But another recent study showed that people who took the vitamin B3 could lower their risk of less-deadly forms of skin cancer.