Benefits of Soy ProteinSoy protein is very healthy: it is very low in fat, contains no cholesterol and contains phytochemicals. The main phytochemicals in soy protein are isoflavones, saponins and phytic acid. These phytochemicals are strong antioxidants but have many other properties. Soy protein and its associated phytochemicals seem to reduce heart disease, osteoporosis and risk of cancer.
Soy Protein and Heart DiseaseSoy protein reduces the cholesterol and help to reduce the risk of heart disease. The most important scientific study about the relationship of soy protein intake and heart disease is the meta-study by Anderson et al (Meta-analysis of the effects of soy protein intake on serum lipids. N Engl J Med 1995;333:276-282). They concluded that the consumption of soy protein rather than animal protein significantly decreased serum concentrations of total cholesterol, LDL cholesterol, and triglycerides without significantly affecting serum HDL cholesterol concentrations.
Anthony MS et al found that isoflavones in soy protein improve cardiovascular disease risk factors without apparent deleterious effects on the reproductive system of peripubertal rhesus monkeys (Soybean isoflavones improve cardiovascular risk factors without affecting the reproductive system of peripubertal rhesus monkeys. Journal of Nutrition 1996;126:43-50).
A study published in the Journal of Nutrition investigated the effect of soy compared with dairy protein on the gut bacteria of hamsters to determine whether changes in gut bacteria could account for lipid lowering properties of soy protein . Serum total and LDL-cholesterol concentrations were lower in the group fed with soy protein concentrate. The study concluded that the type of protein affected gut bacteria and that soy protein may reduce lipogenesis through alterations of the gut bacteria.
Soy Protein and OsteoporosisResearch suggests that consuming soy protein and the replacement of animal protein with plant protein may protect against osteoporosis. Although Japanese women consume less calcium than most Western women do they have a lower prevalence of fractures. This could be explained by the soy protein consumption of Japanese women. Alekel, D.L. et al found that soy isoflavones attenuated bone loss from the lumbar spine in perimenopausal women.
Source: Isoflavone-rich soy protein isolate attenuates bone loss in the lumbar spine of perimenopausal women. Am J Clin Nutr 2000;72(3):844-52.
Soy Protein and Abdominal Body FatSoy protein contains calories, about 4 kcal per g, but does its consumption increase our body fat? Of course, if you add extra soy protein to your diet you may expect weight increase, but how does soy protein compare with for example dairy protein. One study investigated the effect of a daily supplementation of soy protein or dairy protein on body fat of postmenopausal women . The researchers found that the group who consumed soy protein lost abdominal fat (-12 ml), whereas the group who had the soy protein replaced with dairy protein (casein) showed an increase in abdominal fat (+39 ml). Other parameters, such as total body fat, insulin secretion and lean mass did not differ between the groups. This study concluded that a daily supplementation of soy protein prevents the increase in subcutaneous and abdominal fat, observed with an isocaloric casein placebo in postmenopausal women.
Source: Effect of a daily supplement of soy protein on body composition and insulin secretion in postmenopausal women. Fertil Steril. 2007 Dec;88(6):1609-17.
Soy Protein and Cancer RiskEpidemiological studies suggest that soy protein may reduce the risk of certain cancers, including breast cancer and prostate cancer. Japanese women, who eat a lot of soy protein (mainly from soy tofu and miso), have a much lower risk of breast cancer or prostate cancer than Western people. The anti-cancer effect of soy protein could be explained by the phytochemicals, which are associated with the soy protein. These phytochemicals include the isoflavones (genistein, daidzein and glycitein), phytic acid and saponins. Isoflavones and phytic acid have strong antioxidant properties and can prevent oxidative damage by free radical to our DNA and cells. Shu X.O. et all suggest that high soy intake during adolescence may reduce the risk of breast cancer in later life (Soyfood Intake during Adolescence and Subsequent Risk of Breast Cancer among Chinese Women. Cancer Epid Biomarkers Prev. 10: 483-488). The inverse association between soy protein intake and reduces risk of breast cancer was observed for each group of soyfoods examined.
A study by Mercer et al investigated the effect of soy protein in mice that were treated with diethylnitrosamine and ethanol . As expected the ethanol increased the incidence and multiplicity of basophilic lesions and adenomas. Soy protein significantly reduced adenoma progression and reduced liver injury. The researchers conclude that soy prevents tumorigenesis by reducing pro-inflammatory and oxidative environment resulting from ethanol-induced liver injury.
References Soy Protein Compared with Milk Protein in a Western Diet Increases Gut Microbial Diversity and Reduces Serum Lipids in Golden Syrian Hamsters. J Nutr. 2016 Apr;146(4):697-705.
 Soy protein isolate protects against ethanol-mediated tumor progression in diethylnitrosamine-treated male mice. Cancer Prev Res (Phila). 2016 Mar 22.