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Prospective cohort study of soy food intake and colorectal cancer risk in women^1,2,3

¹ From the Department of Medicine, Vanderbilt Epidemiology Center and Vanderbilt-Ingram Cancer Center, Vanderbilt University School of Medicine, Nashville, TN 37203 (GY, X-OS, HC, XZ, and WZ); the Shanghai Cancer Institute, Shanghai, China (HL and Y-TG); and the Division of Cancer Epidemiology and Genetics, National Cancer Institute, NIH, DHHS, Bethesda, MD (W-HC).

² Supported by USPHS grant R01CA70867, the NIH intramural program (N02 CP1101066), and USPHS grant R01CA 122364.

³ Reprints not available. Address correspondence to G Yang, Vanderbilt Epidemiology Center, Vanderbilt University Medical Center, Sixth Floor, Suite 600, 2525 West End Avenue, Nashville, TN 37203-1738. E-mail: gong.yang{at}vanderbilt.edu.

ABSTRACT

Background: Soy and some of its constituents, such as isoflavones, have been shown to have cancer-inhibitory activities in experimental studies. Data from epidemiologic studies linking usual soy food intake with colorectal cancer are limited and inconsistent.

Objective: The objective was to investigate whether soy food intake is associated with colorectal cancer risk.

Design: We prospectively examined 68,412 women aged 40–70 y and free of cancer and diabetes at enrollment. Usual soy food intake was assessed at baseline (1997–2000) and reassessed during the first follow-up (2000–2002) through in-person interviews with a validated food-frequency questionnaire. We excluded the first year of observation to minimize lifestyle changes related to preclinical disease.

Results: During a mean follow-up of 6.4 y, 321 incident colorectal cancer cases were identified. After adjustment for potential confounding factors, total soy food intake was inversely associated with colorectal cancer risk. Each 5-g/d increment in intake of soy foods as assessed by dry weight [equivalent to 1 oz (28.35 g) tofu/d] was associated with an 8% reduction in risk (95% CI: 3%, 14%). Women in the highest tertile of intake had a multivariate relative risk of 0.67 (95% CI: 0.49, 0.90) compared with those in the lowest tertile (P for trend = 0.008). This inverse association was primarily confined to postmenopausal women. Similar results were also found for intakes of soy protein and isoflavones.

Conclusion: This prospective study suggests that consumption of soy foods may reduce the risk of colorectal cancer in postmenopausal women.

Received for publication July 24, 2008. Accepted for publication November 2, 2008.