CORRESPONDENCE

RESPONSE: Re: Meta-analysis: Dietary Fat Intake, Serum Estrogen Levels, and the Risk of Breast Cancer

Anna H. Wu, Daniel O. Stram, Malcolm C. Pike

Correspondence to: Anna H. Wu, Ph.D., University of Southern California/Norris Comprehensive Cancer Center, 1441 Eastlake Ave., MS#44, P.O. Box 33800, Los Angeles, CA 90033-0800 (e-mail: annawu{at}hsc.usc.edu).

We (1) and, in their editorial on our article, Ballard-Barbash et al. (2) have pointed out many of the same limitations now raised by Holmes et al. To summarize, the limitations of the 13 published dietary fat intervention studies are 1) the lack of a control arm in all but one study; 2) concurrent weight loss in six studies; and 3) a substantial increase in dietary fiber intake in five studies. In the discussion section of our paper, we pointed out that, while reductions in serum estradiol levels were found in all six studies that reported significant reductions in body weight, reduction in estradiol levels was also found in four of the seven studies that found no significant changes in body weight. Serum estradiol levels decreased in two of the three studies with low fiber intake (<20 g/day), in two of the three studies with moderate fiber intake (21-29 g/day), and in three of the five studies with high fiber intake (>=30 g/day) during intervention. (Data on fiber intake was not provided in two studies.) It is of note that the only study with a control arm (3) found a 14% reduction in serum estradiol levels when fiber intake remained low (19 g/day) and body weight was unchanged (60.8 kg) during the intervention period. However, we agree that "tighter" controlled and randomized isocaloric dietary fat intervention studies are needed to definitively establish the effects on serum hormone levels of dietary fat reduction to between 10% and 20% of calories from fat. We also agree that measurements of changes in insulin-like growth factor 1 and other possible risk factors in these studies would be very helpful. A reduction in serum estrogens would suggest, but not prove, that breast cancer risk will be reduced. Epidemiologic studies or studies of alterations in breast cancer biomarkers will be needed to definitively conclude that reduced dietary fat intake will lead to reduced breast cancer risk.

The question posed in our paper regarding the measurement error of fat intake determined by dietary history questionnaires was motivated by the observation that subjects reporting low percentages of fat calories on the questionnaires have, on average, a much greater true percentage of fat calorie intake. On the basis of our calibration studies, we estimated that the figure of 20% fat calories given in the Hunter et al. (4) paper represented a true value of 30% fat calories. What we would like to see in future publications are corrected percentage fat calories figures, which would give a more realistic estimate of what the reported low intakes of fat actually represent. In their letter, Holmes et al. again discussed the risk for subjects who report low levels of calories from fat on the questionnaires but did not give the actual corrected fat consumption levels.

REFERENCES

1 Wu AH, Pike MC, Stram DO. Meta-analysis: dietary fat intake, serum estrogen levels, and the risk of breast cancer. J Natl Cancer Inst 1999;91:529-34.[Abstract/Free Full Text]

2 Ballard-Barbash R, Forman MR, Kipnis V. Dietary fat, serum estrogen levels, and breast cancer risk: a multifaceted story. J Natl Cancer Inst 1999;91:492-4.[Free Full Text]

3 Boyd NF, Lockwood GA, Greenberg CV, Martin LJ, Tritchler DL. Effects of a low-fat high-carbohydrate diet on plasma sex hormones in premenopausal women: results from a randomized controlled trial. Canadian Diet and Breast Cancer Prevention Study Group. Br J Cancer 1997;76:127-35.[Medline]

4 Hunter DJ, Spiegelman D, Adami HO, Beeson L, van den Brandt PA, Folsom AR, et al. Cohort studies of fat intake and the risk of breast cancer—a pooled analysis. N Engl J Med 1996;334:356-61.[Abstract/Free Full Text]


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