Regular dietary intake of soy food and soy isoflavones is associated with reduced sperm count, significantly contributing to male sterility, according to a research article published in the current issue of journal Human Reproduction.
Former studies have suggested the potential anti-cancer and heart-disease preventing properties of isoflavones that constitute 2 to 4 milligrams per gram of raw soybeans. However, few animal studies had reported that increased intake of dietary isoflavones is related to decreased fertility.
In order to investigate a similar association in humans, Jorge E Chavarro, lead scientist at the Department of Nutrition, Harvard School of Public Health, Boston, Massachusetts, and coworkers, conducted a study on 99 male partners of subfertile couples attending a fertility clinic for semen analyses. The investigators assessed the consumption of 15 soy-based foods of the study subjects during the previous three months and the usual size of servings. Linear and quantile regression analyses demonstrated an inverse association between sperm count and soy food consumption, after accounting for various factors such as age, abstinence time, caffeine and alcohol consumption, smoking, and body mass index. The study results showed that subjects belonging to the highest soy food consumption category had 41 million sperms/ml less, compared to those who did not consume soy-based food at all. Although the association was more pronounced among overweight or obese individuals, the food intake was not related to sperm morphology, motility or ejaculated volume.
The scientific rationale behind the association is presumed to be the estrogenic effect exerted by isoflavones, including glycitein, daidzein, and genistein. Isoflavones, which are capable of binding to estrogen receptors with a lesser affinity compared to estradiol, are speculated to interfere with hormonal signaling associated with sperm production.
Perry and coworkers conducted a similar study (Nutrition, 2007) to evaluate the long-term effects of soy isoflavonoid intake on the sperm counts and histomorphology of mammary glands, prostate gland, and testis, of adult male cynomolgus macaques (Macaca fascicularis). During the three-year study, 99 adult male macaques were fed with soy-free, casein-lactalbumin-based diet, low-soy isoflavonoid diet, or a high-soy isoflavonoid diet. The researchers adopted liquid chromatographic-photodiode array electrospray mass spectrometry for measuring the concentration of serum isoflavonoids. They also performed histopathologic and histomorphometric studies of autopsy samples of prostate gland, testes and mammary gland, in addition to epididymal and testicular sperm counting. The study findings demonstrated that dietary soy protein containing isoflavonoids did not have a specific influence on the epididymal and testicular sperm counts or serum estradiol and testosterone concentrations.
Soybean (Glycine max), a principle source of complete protein, containing all the essential amino acids in significant concentration, forms a major part of processed food products in US. The consumption pattern of soyfood varies among people in different parts of the world. In Asia, it is consumed mainly as soymilk, tofu and fermented products, while in Western countries it is consumed primarily as refined soy protein ingredients.
In1999, the US FDA concluded that dietary soy protein helps in reducing the risk of coronary heart disease, and had granted approval for the use of health claim on soy-based food products to tout the health benefits of soyfood consumption. Some nutritionists and physicians believe that the isoflavones present in soyfood help prevent cancer. However, further researches and clinical trials are mandatory to elucidate the possible risks of soyfood intake, including its effects on fertility, before it becomes the staple food of the global population.
References
1. Chavarro JE, Toth TL, Sadio SM, Hauser R. Soy food and isoflavone intake in relation to semen quality parameters among men from an infertility clinic. Hum Reprod. 2008 Jul 23. [Epub ahead of print].
2. Perry DL, Spedick JM, McCoy TP, Adams MR, Franke AA, Cline JM. Dietary soy protein containing isoflavonoids does not adversely affect the reproductive tract of male cynomolgus macaques (Macaca fascicularis). J Nutr. 2007 Jun;137(6):1390-4.
