Trimming Fat From the Diet Cuts Down on Ovarian Cancer Risk

SEATTLE, Oct. 10 -- A low-fat diet may modestly reduce the risk of ovarian cancer among postmenopausal women with a larger benefit than for breast cancer.

Women who cut fat intake to 20% of calories had a 40% lower ovarian cancer rate after four to eight years of follow up compared with women who stuck to their normal diet, according to a retrospective analysis of a large dietary chemoprevention trial reported in the Oct. 9 issue of the Journal of the National Cancer Institute.

The same trial had earlier shown a nonsignificant 9% reduction in breast cancer risk with the low-fat diet, said Ross L. Prentice, Ph.D., of the Fred Hutchinson Cancer Research Center here, and colleagues.

"Ongoing nonintervention follow-up of trial participants may provide additional valuable assessment of the effects of a low-fat dietary pattern on these and other cancer incidence rates," they wrote.

The Women's Health Initiative study included a randomized, controlled dietary modification trial of chemoprevention among postmenopausal women ages 50 to 79 at baseline.

In the trial, 19,541 women followed a diet with no more than 20% of energy from fat, at least five servings of fruit and vegetables a day, and at least six servings of grains daily. The other 29,294 women followed their usual diet.

The dieters attended 18 group sessions with nutritionists in the first year and quarterly maintenance sessions thereafter to help them attain dietary goals. Sessions dealt with fat content of food, fat budgeting, high-risk food situations, and other nutritional topics as well as dietary self-monitoring, social influences on eating, and relapse prevention.

Hysterectomy rates were similar between groups (P=0.85).

After an average 8.1 years on the intervention or control, the rate of ovarian cancer was significantly lower at 15,657 cases in the intervention group compared with 23,297 in the control group (P=0.03).

No difference between the groups was seen over the entire intervention period (hazard ratio 0.83, 95% confidence interval 0.60 to 1.14, P=0.24) or over the first four years of the study (HR 1.16, 95% CI 0.73 to 1.84, P=0.53).

However, there was a statistically significant interaction between time on the study and hazard ratio for benefit from the low-fat diet (P=0.01 for trend), and the last 4.1 years of the study showed a significant advantage to the diet for ovarian cancer risk (HR 0.60, 95% CI 0.38 to 0.96, P=0.03).

"Hence, although there was little evidence for an intervention effect on ovarian cancer risk during the first few intervention years," the researchers wrote, "a stronger and nominally statistically significant risk reduction emerged in the later years."

Furthermore, women whose baseline food diaries showed them to be in the top third for percentage of energy from fat had an even greater reduction in ovarian cancer risk with the low-fat diet compared with their usual diet (HR 0.58, 95% CI 0.31 to 1.08), as did those in the top tertile for total fat consumption (HR 0.49, 95% CI 0.25 to 0.93).

The study also looked at the rate of endometrial cancer, but the there was no effect from the diet intervention overall (HR 1.11, 95% CI 0.88 to 1.40, P=0.18) or in the latter half of the study.

Total risk of any invasive cancer tended to be lower with the low-fat diet (HR 0.95, 95% CI 0.89 to 1.01, P=0.10), which "could be of some practical importance," the researchers said.

They cautioned, however, that the reduction in ovarian cancer needs to be interpreted in the light of number of comparisons in the trial. Because ovarian cancer was one of five cancers for per-protocol comparison, there was up to a 15% probability that the finding was by chance, they said.