By Mark Messina, PhD, MS
Soy intake among populations with a long history of soy consumption is often used to guide recommendations for non-Asians and cited as partial evidence of health benefits. There is no dispute that historical rates of breast cancer in Japan and China are remarkably lower than U.S. rates. However, these kinds of ecological observations are valuable only for generating hypotheses, not for reaching conclusions. The question is not whether Japan or any other country has or had a lower rate than the U.S., but rather, whether women who regularly consume soy have lower rates of breast cancer than women who consume soy infrequently (they do, by the way).1
Nevertheless, it is reasonable to use soy consumption by populations for whom soy foods are a traditional food for making intake recommendations for non-Asians, especially with respect to upper intake limits. But such recommendations should be made cautiously because of the possible differences in response to soy among different ethnicities.
With this background in mind, it is important to recognize that as the Japanese diet has become more “Westernized,” soy protein intake has decreased. For example, based on disappearance data, soy protein intake decreased from 9.7 g/day in 1961 to 8.4 g/day in 1977.2 Although that decrease is relatively minor, when expressed as a percentage of total protein, soy protein intake decreased from 13.4% in 1961 to 9.5% in 2002. That decrease occurred primarily due to an increase in animal protein intake. The change in soy consumption is also illustrated by the difference in intake according to age. In a paper published last year, women aged 50 to 59 years consumed “soy” 7.3 times per week versus 5.2 times for those 20-29 (Table A).3 This decreased intake may make it increasingly difficult for observational studies to provide insight into the possible health effects of soy. As a result, recommendations will have to be based entirely on clinical research.Table A: Soy intake among women in Japan according to age
| Age | n |
Frequency (x/week) (mean ± SD) |
| 20-29 | 1091 | 5.2 ± 0.1 |
| 30-39 | 399 | 6.0 ± 0.2 |
| 40-49 | 383 | 6.8 ± 0.2 |
| 50-59 | 306 | 7.3 ± 0.3 |
REFERENCES
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Zhang YF, Kang HB, Li BL, Zhang RM. Positive effects of soy isoflavone food on survival of breast cancer patients in China. Asian Pac J Cancer Prev. 2012;13(2):479-82. doi: 10.7314/apjcp.2012.13.2.479. PMID: 22524810. https://pubmed.ncbi.nlm.nih.gov/22524810/.
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Messina M, Nagata C, Wu AH. Estimated Asian adult soy protein and isoflavone intakes. Nutr Cancer. 2006;55:1-12.
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Iizuka K, Yanagi K, Deguchi K, et al. Sex and age differences in the effects of food frequency on metabolic parameters in Japanese adults. Nutrients. 2024;16.