Effect of Soybean Milk-Fermented Product on Intestinal Microbiota and Colon Carcinogenesis

A few studies have investigated the effect of fermented product of soybean milk on human health. This study evaluated the effects of soybean milk-fermented product (SFP) on the intestinal flora of humans, and colorectal carcinogenesis in animals. SFP was prepared by culturing soybean milk with lactic acid bacteria and yeast. The effect of SFP (450 mg/day) on human fecal flora was determined by comparing the changes in intestinal flora between human volunteers consuming SFP and those taking a placebo. The occupation rate of Bifidobacterium of more than 25% was greater in the SFP group (n=11) than in the placebo group (n=10, P<0.05). The occupation rate of Clostridium in fecal flora of the volunteers (n=5) increased after shifting from a traditional Japanese diet (TJD) to a Western diet (WD), in which a total daily amount of 300 g of meat (900 kcal) was eaten once at lunch for 3 days (P<0.05). The occupation rate of Clostridium in the WD group was reduced by SFP ingestion (900 mg/day) to a level similar to that in the TJD group. Meanwhile, the occupation rate of Bifidobacterium was higher in the SFP ingestion group than in the WD group. β-Glucuronidase activity in the feces was up to 5 times higher when lunch was changed from TJD to WD, but this was recovered by SFP inclusion in the WD diet (P<0.05). The above findings suggest the possibility that SFP inhibits colon cancer carcinogenesis. To investigate this notion further, a colon cancer-inducing substance, 1,2-dimethylhydrazine (DMH), was given to CF#1 mice. SFP administration significantly decreased the incidence of colon cancer, compared to controls. We also examined the mechanism of SFP tumor suppression. A Winn assay was performed using spleen cells. SFP given for more than 6 days inhibited tumor growth compared to tumor cells inoculated with Meth-A alone (P<0.05). This result indicates the possible induction of anti-tumor immune cells in SFP-treated spleen cells. Furthermore, the number of spleen cells increased more in gnotobiotic BALB/c mice in association with Bifidobacterium than in saline-treated germ-free mice. The number of spleen cells in germ-free mice given SFP or soybean milk (10 mg/day and 0.2 ml/day, respectively) for 4 weeks was not significantly different compared to the saline-treated germ-free animals. Our results suggest that the suppressive effect of SFP on tumorigenesis was mediated by the involvement of the host immune response induced by intestinal bacteria. Further studies are required to clarify the mechanism of this phenomenon.<br>