Fat-Reduction Study Finds No Effect on Skin Cancer Incidence

Kate Johnson

March 23, 2012

March 23, 2012 (San Diego, California) — Contrary to expectations, a reduction in dietary fat was not associated with a decreased risk for skin cancer, researchers reported here at the American Academy of Dermatology 70th Annual Meeting.

"Our findings were unexpected as they contradicted our hypothesis that a low-fat diet would decrease the risk of nonmelanoma skin cancer [NMSC] and melanoma, which we did not see in the overall analysis," presenter Christina Gamba, a fourth-year medical student at Stanford University School of Medicine in Palo Alto, California, told Medscape Medical News.

However, a subanalysis of the findings suggests that an even lower fat intake than was studied might indeed be protective.

"We did see a small signal in the women with a healthier baseline diet who were assigned to a low-fat diet," she said.

The researchers studied 48,835 postmenopausal women 50 to 79 years of age who were enrolled in the Women's Health Initiative (WHI) Randomized Controlled Dietary Modification Trial from 1993 and 2005.

To be eligible, subjects had to have a baseline diet that was at least 32% fat. To put this into context, the American Heart Association and the Institute of Medicine recommend a fat intake of 20% and 35%, respectively, she said. "Your typical American takes in 33% of energy from fat."

Subjects were randomized to a diet with a fat intake of 20% (i.e., a 12% decrease) or to their usual diet for a mean of 8 years.

Age, race, ethnicity, and baseline body mass index (BMI) were similar in both groups, Gamba reported. "Most women were in their 60s, they tended to be white, and they had a BMI generally greater than 30 kg/m²." Skin cancer risk factors such as current ultraviolet exposure and history of melanoma and NMSC were similar in the 2 groups.

Subjects in the low-fat group attended 18 nutritionist-led small group sessions in the first year of the trial, and 4 more sessions over the rest of the trial. They also completed validated food-frequency questionnaires at baseline and once every year during the trial.

At the end of follow-up, there were 4907 NMSCs and 279 melanomas (hazard ratio [HR], 0.98 and 1.04, respectively), which occurred at similar rates in the 2 groups, suggesting no effect of dietary fat intake, said Gamba.

However, there was a trend toward fewer skin cancers in a subset of women who had entered the study with the lowest fat intake and were then randomized to reduce it further.

"Since women were only allowed into the trial if they had a fat intake greater than 32% at baseline, we're really comparing a baseline moderate- to high-fat diet (32% to 36.9%) with a high-fat diet (36.9% to 55% plus)," explained Gamba.

In women with a moderate- to high-fat diet at baseline who were randomized to the low-fat diet, there was an almost significant decrease in melanoma risk (HR, 0.72; 95% confidence interval, 0.50 to 1.02).

"We believe these women may have achieved an even lower-fat diet than their 'high-fat' counterparts, perhaps explaining the decrease in risk," she said, adding that overall, there was a significant interaction between baseline dietary fat and melanoma risk (P = .006).

"These results are specific to the dietary change we saw in this group (i.e., an approximately 11% decrease in percent energy from fat, which lessened as the trial progressed), so we can't rule out that a bigger decrease, like 20%, could show a protective effect.... We still want to look at this question," she said.

Previous research has suggested that a diet restricted to 20% fat is associated with fewer subsequent NMSCs in adults with a history of NMSC, she said (Int J Cancer. 1995;62:165-169).

In that study, after just 2 years of follow-up, subjects assigned to a fat-reduced diet (dietary fat intake of 20%) had significantly fewer actinic keratoses and NMSCs than subjects on a diet that contained at least 37% fat (P < .001).

Asked to comment by Medscape Medical News, the lead author of that study, Homer S. Black, PhD, from the Baylor College of Medicine in Houston, Texas, acknowledged that although the findings from the WHI study by Gamba and colleagues differ from his, they are nevertheless in line with some other observational studies.

He stressed that the population in his study was very different from that in the WHI study.

Although the WHI study assessed "healthy women only, with diverse risk factors," his trial enrolled a more select group of "males and females who were skin cancer [NMSC] victims at the time of enrollment and who had a previous history of skin cancer. In addition, there were other specific exclusion criteria, such as ethnicity."

Dr. Black cautioned that "it is very difficult to modify dietary habits," and finding a link requires "intense effort" beyond simply being randomized to a reduced-fat diet.

His study involved an initial nutritional training period conducted by dieticians, but the follow-up was different than that in the WHI study. "Individual dietary assessments and reinforcement strategies by dieticians were conducted every 3 months."

Monitoring diet changes is also a challenge, and food frequency questionnaires such as the ones used in the WHI trial, "although stated to be validated, have serious weaknesses," he said.

Finally, although his study's 2-year follow-up was adequate to study individuals whose previous history of NMSC put them at high risk for a subsequent NMSC, the WHI study's 8-year follow-up is a short time to study NMSC occurrence in healthy people.

"NMSC has a very long latent period. Exposure occurs from childhood throughout one's life. The long latent period is reflected in the fact that NMSC appears, for the most part, after 40 to 50 years," he said.

Gamba has disclosed no relevant financial relationships.

American Academy of Dermatology (AAD) 70th Annual Meeting: Late-breaking abstract. Presented March 16, 2012.

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