Someone mentioned to me that you shouldn’t drink soy milk during pregnancy. How can that be true? Surely pregnant women have been drinking soy milk (and eating soy products) for aeons?
Did a Google search – essentially ‘soy wars’ with ‘my friend said this’ and ‘my mother said this’ – so decided instead to have a look at the literature. As the topic is so massive, I’ll try to concentrate mainly on the effect of soy ingestion during pregnancy on the offspring… Coz that’s where I’m at yo.
I’ll also try to summarise the study finding in one easy-to-read sentence (in response to ‘constructive feedback’ that some of my posts are too ‘medicalised’).
Now, as it’s apparently not ethical to perform studies on pregnant women which may potentially affect their offspring, most of these studies are carried out on our furry friends…
Studies have shown that neonatal exposure to soy isoflavones improve bone outcomes in mice at adulthood. Now a recent study has shown that prenatal and early-life exposure to soy isoflavones with folic acid supplementation provides functional benefits to male bone development (eg. higher bone mineral density, and greater resistance to fracture at the femur and lumbar spines). Pregnant mice who ingest soy have male offspring with better bones.
Another study looking at the effect of perinatal exposure to phytoestrogens in rats found that it leads to a higher bone mineral density later in life. It is suggested that these changes may have occurred as a consequence of programming effects, as has been shown for the endocrine and immune systems. Rats exposed to soy early in life have better bones.
Metabolic syndrome is a combination of disorders that combined together increase the risk of cardiovascular diseases and diabetes. Rats that consume a soy-based diet (compared with a casein-based diet) throughout gestation and lactation have increased body weight and food intake and had offspring with fewer characteristics of metabolic syndrome (females more than males). Pregnant rats on a soy-diet have offspring with less metabolic syndrome.
A study from King’s College London found that feeding a soy isoflavone-rich diet during pregnancy, weaning, and post-weaning affords cardiovascular protection in aged male rats. Notably, rats exposed to a soy isoflavone-deficient diet throughout pregnancy and adult life exhibited increased oxidative stress, diminished antioxidant enzyme and eNOS levels, endothelial dysfunction, and elevated blood pressure in vivo. Isoflavones in the fetal circulation may play a role during fetal development in affording protection against CVD in the offspring via their ability to activate eNOS, EDHF, and redox-sensitive gene expression. Rats given a soy-rich diet have offspring with improved protection against cardiovascular disease.
In contrast to popular myth, offspring of rats who consume soy-based diets (as opposed to casein-based diets) have increased resistance to breast cancer later in life. Pregnant rats who consume soy-based diets have offspring with less breast cancer.
One major argument against ingesting soy in pregnancy is the potential effect of phytoestrogens on the reproductive function of male offspring.
Female rabbits were randomly assigned to different types of diets and the weight and morphology of their male offspring’s reproductive organs were evaluated after puberty. It was found that there was no significant differences in litter size and gestation duration between control and treatment groups. Perinatal exposure to soy-containing diet and soy isoflavones did not alter testis, epididymides, proprostate and prostate weight and gross morphology. After puberty, sexual behaviour and semen parameters did not differ significantly from the control group. These results indicate that intrauterine and lactational exposure to soy-containing diet and soy-derived isoflavones may not adversely affect reproductive development and function of male rabbits. Pregnant rabbits on soy diets have male offspring with normal reproductive health.
Another study looking at the effect of maternal soy diet on male rabbit offspring reproductive health found that there was no effect on spermatogenesis, morphology of male genital organs and sexual behavior, and concluded that chronic dietary treatment with soy based diet or soy isoflavones has no adverse effects on the observed reproductive patterns of male rabbits. Pregnant rabbits on soy diets have male offspring with normal reproductive patterns.
Moving on to our closer cousins… On the basis that soy is a good source of protein and also contains isoflavones that may affect plasma lipids, body weight, and insulin action… Monkeys were randomised to one of two diets: one with soy-protein, another with casein-lactalbumin protein. Female monkeys consuming soy-diets had better glycaemic control (as determined by fructosamine concentrations) and offspring born to dams consuming soy-protein diets had similar birth weights, but over the following two years weighed significantly less, had significantly lower triglyceride concentrations, and like adult females, had significantly lower fructosamine concentrations compared to those born to dams consuming casein-lactalbumin protein. Pregnant monkeys on soy-diet had better blood sugar control and their offspring had lower sugar & triglyceride levels.
One human study just to mix things up a bit… the disparity in adult breast cancer incidence between Asian women and Western counterparts is attributed partly to high soy food intake. Studies have found that soy-associated components, by targeting mammary adipocytes, alter paracrine signaling to enhance mammary epithelial differentiation, with important implications for the prevention of breast cancer associated with obesity and obesity-related diseases. In humans, the intake of soy changes development of breast fat cells, thereby preventing breast cancer.
Well so far there’s nothing I can see that makes me fearful of consuming soy in pregnancy. In fact, quite the opposite… it seems there are potential benefits. Before you go having soy latte, tofu, fake soy bacon, soy flour muffins and generally going overboard, please remember (insert mother’s voice) all things in moderation. There can be too much of a good thing… like pavlova.
Kaludjerovic J, Ward WE. Adequate but not supplemental folic acid combined with soy isoflavones during early life improves bone health at adulthood in male mice. J Nutr Biochem. 2013 May 2. pii: S0955-2863(13)00053-3. doi: 10.1016/j.jnutbio.2013.02.008. [Epub ahead of print]
Jahan-mihan A, Smith CE, Hamedani A, Anderson GH. Soy protein-based compared with casein-based diets fed during pregnancy and lactation increase food intake and characteristics of metabolic syndrome less in female than male rat offspring. Nutr Res. 2011 Aug;31(8):644-51. doi: 10.1016/j.nutres.2011.08.003.
Bonacasa B, Siow RC, Mann GE. Impact of dietary soy isoflavones in pregnancy on fetal programming of endothelial function in offspring. Microcirculation. 2011 May;18(4):270-85. doi: 10.1111/j.1549-8719.2011.00088.x.
Wagner JD, Jorgensen MJ, Cline JM, Lees CJ, Franke AA, Zhang L, Ayers MR, Schultz C, Kaplan JR. Effects of soy vs. casein protein on body weight and glycemic control in female monkeys and their offspring. Am J Primatol. 2009 Sep;71(9):802-11. doi: 10.1002/ajp.20716.
Su Y, Shankar K, Simmen RC. Early soy exposure via maternal diet regulates rat mammary epithelial differentiation by paracrine signaling from stromal adipocytes. J Nutr. 2009 May;139(5):945-51. doi: 10.3945/jn.108.103820. Epub 2009 Mar 25.
Cardoso JR, Báo SN. Morphology of reproductive organs, semen quality and sexual behaviour of the male rabbit exposed to a soy-containing diet and soy-derived isoflavones during gestation and lactation. Reprod Domest Anim. 2009 Dec;44(6):937-42. doi: 10.1111/j.1439-0531.2008.01121.x. Epub 2008 Dec 22.
Mardon J, Mathey J, Kati-Coulibaly S, Puel C, Davicco MJ, Lebecque P, Horcajada MN, Coxam V. Influence of lifelong soy isoflavones consumption on bone mass in the rat. Exp Biol Med (Maywood). 2008 Feb;233(2):229-37. doi: 10.3181/0707-RM-202.
Su Y, Eason RR, Geng Y, Till SR, Badger TM, Simmen RC. In utero exposure to maternal diets containing soy protein isolate, but not genistein alone, protects young adult rat offspring from NMU-induced mammary tumorigenesis. Carcinogenesis. 2007 May;28(5):1046-51. Epub 2006 Dec 13.
Cardoso JR, Báo SN. Effects of chronic exposure to soy meal containing diet or soy derived isoflavones supplement on semen production and reproductive system of male rabbits. Anim Reprod Sci. 2007 Feb;97(3-4):237-45. Epub 2006 Mar 13.