Unpacking A Citrus-Melanoma Link

Even as the dog days of summer come to an end, it’s important to recognize just how important it is to regularly, and all year round, protect your skin from the sun’s harmful UV rays. How to do that has been well covered here in past issues. It was also reported that incidence of melanoma, the most serious type of skin cancer, alarmingly doubled in the U.S. over the last 30 years.

That statistic has now been projected into the future in a recent Morbidity and Mortality Weekly Report from the CDC. The melanoma incidence rate reached 19.7 per 100,000 people in 2011, while the melanoma mortality rate has stayed fairly constant. The authors estimate that the cost of treating new melanoma cases could nearly triple by 2030 without additional interventions.

Around the same time over the summer that the report surfaced, a study was published in the Journal of Clinical Oncology revealing an association between citrus fruit consumption and melanoma risk. It’s worth exploring this development carefully, to better ensure that its implications are not misinterpreted.

Almost 100,000 Nurses' Health Study participants and those from the Health Professionals Follow-up Study completed questionnaires about consumption of citrus fruits and juices and were followed for about 25 years.

Over 1,800 melanomas occurred. Participants who reported consuming at least 1.6 servings of citrus daily had a 36% increased risk for melanoma, compared with those who ate less than two servings a week. The findings were significant only for orange juice and grapefruit. Specifically, risk increased with five to six servings of orange juice per week as opposed to less than one. Grapefruit consumption increased risk with less than one per week vs. never. There was no relationship between supplemental vitamin C and cancer risk.

The cancer-citrus association could be due to the psoralen found in citrus fruits. Animal studies have found that psoralen has photocarcinogenic properties. A photocarcinogen is a substance that causes cancer upon illumination. Psoralens interact with ultraviolet light to stimulate the proliferation of melanoma cells.

The association appeared to be stronger among those with a higher susceptibility to sunburn as a child or adolescent, those with more blistering sunburns, and those who spent more time in the direct sunlight. The link was also stronger among obese individuals and those who exercised little.

In an editorial accompanying the journal article, the author cautioned against a public overreaction to the citrus-cancer link. For the vast majority of public, citrus is under-consumed, not the other way around. Certainly it’s not good policy at this point to recommend or advise eating less citrus.

The author of the editorial points out that the strength of association is relatively small. Dietary effects on disease incidence are rarely large. It's also worth noting that no other study in another population has found this relationship between citrus consumption and the development of melanoma. One even found a protective effect for dietary vitamin C on melanoma incidence.

The editorialist notes, “This is a potentially important study, given that citrus consumption is widely promulgated as an important dietary constituent and has demonstrated benefit for coronary heart disease, cancer prevention, and overall health effects. At this point in time, a public overreaction leading to avoidance of citrus products is to be avoided. For people who would be considered at high risk, the best course might be to advise individuals to use multiple sources of fruit and juice in the diet and to use sun protection, particularly if one is sun sensitive. There is clearly a need for replication of the study findings in a different population before modifying current dietary advice to the public.”

MMWR, 2015, Vol. 64, No. 21, pp. 591-596, http://www.cdc.gov/mmwr/preview/mmwrhtml/mm64e0602a1.htm?s_cid=mm64e0602a1_x

Journal of Clinical Oncology, June 29, 2015, http://jco.ascopubs.org/content/early/2015/06/24/JCO.2014.57.4111.abstract

Journal of Clinical Oncology Editorial: Dietary Advice for Melanoma: Not Ready for Prime Time by Marianne Berwick, PhD, Department of Internal Medicine, University of New Mexico,
http://jco.ascopubs.org/content/early/2015/06/24/JCO.2015.61.8116.full

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