Soy compounds called isoflavones are among the plant-derived compounds that may significantly reduce the risk of breast cancer recurrence or death, according to a new meta-analysis co-directed by investigators from the Johns Hopkins Kimmel Cancer Center. The results were published in the journal JNCI Cancer Spectrum.
Investigators in Australia, Denmark, England, Norway and the U.S. reviewed 22 published observational studies that examined the impact of dietary intake of soybeans, lignans (compounds found in a variety of plants including seeds and nuts), cruciferous/cabbage-family vegetables, and green tea — and these substances’ phytonutrients (natural compounds derived from plants) — on breast cancer recurrence and mortality, as well as on mortality from all causes. This included 11 studies of soy isoflavones, three of cruciferous vegetables, two of green tea, three of lignans, and three of enterolactone, which is formed in the gut when lignans are digested.
Soy isoflavones were associated with a 26% reduced risk of breast cancer recurrence, according to a meta-analysis that included six of the studies (of 11,837 women) reviewed by investigators.
The results were most notable among post-menopausal survivors. The greatest risk reduction was seen at 60 milligrams per day. This is equivalent to two to three servings per day, where one serving equates to a cup of soy milk, three ounces of tofu or a half-cup of cooked soybeans. However, the effect of soy consumption on risk of mortality was smaller (12%) and not statistically significant, and was seen mostly at 20-40 mg per day, or one to two servings.
Another finding, reported for the first time in a meta-analysis, relates to enterolactone, a compound metabolized from lignans. Lignans are found in a wide variety of plants, such as seeds, nuts, legumes, whole grains, fruit and vegetables.
High levels are found in flaxseeds, cashew nuts, broccoli and brussels sprouts, among other sources. Enterolactone was found to reduce the risk of breast cancer-specific mortality by 28% and death from any cause by 31%, particularly in post-menopausal women (35% reduction in death from any cause). It is not possible to calculate the effective dose of lignans in the diet from these enterolactone findings, because the gut microbiome that plays a role in metabolism of lignans varies among individuals.
Among those who consumed lignans prior to breast cancer diagnosis, there was a non-significant 34% risk reduction in cancer-specific mortality and 19% reduction in all causes of death in post-menopausal women. However, consumption of lignans by pre-menopausal women suggests an increased risk of mortality.
This result indicates that the effects of lignans are dependent on the hormonal environment, although it was likely driven by one large study and needs further investigation. The highest intake was nine or more servings per day in the studies reviewed.
The impact of cruciferous vegetables was inconclusive, possibly influenced by the average intake being quite low (less than a half-cup per day) in the studies reviewed.
Investigators also looked into whether consuming soy, lignans, cruciferous vegetables and green tea, or their phytonutrients in the diet before or after breast cancer diagnosis made a difference. However, the data did not provide a concrete answer. All studies on green tea and lignans measured pre-diagnosis intake, while soy results came from studies that measured intake before and after diagnosis.
M Diana van Die, Kerry M Bone, Kala Visvanathan, Cecile Kyrø, Dagfinn Aune, Carolyn Ee, Channing J Paller. Phytonutrients and outcomes following breast cancer: a systematic review and meta-analysis of observational studies. JNCI Cancer Spectrum, 2024; 8 (1) DOI: 10.1093/jncics/pkad104
Johns Hopkins Medicine. (2024, January 10). Natural compounds derived from soy and other plants reduce breast cancer recurrence and improve survival. ScienceDaily. Retrieved January 10, 2024 from www.sciencedaily.com/releases/2024/01/240110143927.htm
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