The immunological consequences of a high intake of soya-bean protein in man

Tempe consumption modulates fecal secondary bile acids, mucins, immunoglobulin A, enzyme activities, and cecal microflora and organic acids in rats

The present study investigated the effect of dietary tempe, a fermented soy product, on the colonic environment of rats fed high-fat (HF, 30 % fat; experiment 1) or low-fat (LF, 6 % fat; experiment 2) diets. Growing male rats were fed the experimental diets with or without 25 % tempe for 21 days. Tempe consumption slightly but significantly increased the growth of rats fed both the HF and LF diets (P < 0.05). With both the HF and LF diets, dietary tempe markedly reduced a harmful fecal secondary bile acid, lithocholic acid (a risk factor of colon cancer) (P < 0.05), and markedly elevated fecal mucins (indices of intestinal barrier function) and immunoglobulin A (IgA, an index of intestinal immune function) (P < 0.05). With the HF diet, dietary tempe increased cecal acetate, butyrate, propionate, and succinate concentrations (P < 0.05). Analysis of the profile of cecal microflora revealed lower Bacteroides and higher Clostridium cluster XIVa levels in the tempe group of rats fed the HF diet (P < 0.05). Compared with the control group, the fecal activity of β-glucosidase was markedly higher in the tempe group (P < 0.05), while that of urease was lower (P < 0.05) with both the HF and LF diets. The present results suggest that tempe consumption modulates the colonic environment in rats.

Implications of antinutritional components in soybean foods

There are a number of components present in soybeans that exert a negative impact on the nutritional quality of the protein. Among those factors that are destroyed by heat treatment are the protease inhibitors and lectins. Protease inhibitors exert their antinutritional effect by causing pancreatic hypertrophy/hyperplasia, which ultimately results in an inhibition of growth. The lectin, by virtue of its ability to bind to glycoprotein receptors on the epithelial cells lining the intestinal mucosa, inhibits growth by interfering with the absorption of nutrients. Of lesser significance are the antinutritional effects produced by relatively heat stable factors, such as goitrogens, tannins, phytoestrogens, flatus-producing oligosaccharides, phytate, and saponins. Other diverse but ill-defined factors appear to increase the requirements for vitamins A, B12, D, and E. The processing of soybeans under severe alkaline conditions leads to the formation of lysinoalanine, which has been shown to damage the kidneys of rats. This is not generally true, however, for edible soy protein that has been produced under milder alkaline conditions. Also meriting consideration is the allergenic response that may sometimes occur in humans, as well as calves and piglets, on dietary exposure to soybeans.

A long-term metabolic study to assess the nutritional value of and immunological tolerance to two soy-protein concentrates in adult humans

Seventeen healthy young adult men participated in a long-term metabolic study (11 wk) to evaluate the tolerance to and protein nutritional value of two commercially produced soy-protein concentrates. Danpro-S (nine subjects) and Danprotex-H 40 (eight subjects) (Aarhus Oliefabrik A/S, Aarhus, Denmark). Each test protein (0.8 g.kg-1.d-1) served as the sole source of dietary nitrogen. Subjects remained healthy, no problems of clinical or metabolic significance appeared, body weight remained constant, and body composition indices (lean body mass by H2(18)O dilution and creatinine excretion) and basal metabolic rate did not change. N balances fluctuated around body N equilibrium. Immunological studies confirmed the absence of any allergic responses in these subjects. It is concluded that these protein concentrates can be consumed as the sole source of dietary protein for protein nutritional maintenance and with excellent tolerance.

Dietary protein and blood pressure

Vegetarians have lower blood pressures than omnivores. Dietary protein may be partly responsible. Human volunteers, whose normal diet contained little soya protein, were given soya based foods to replace some of the meat in their diet. During this period over 20% of the total protein intake was derived from soya, however blood pressures remained unchanged. Rabbits were given diets based on either soya, casein, or fish protein. The animals' diets were then changed to one of the other protein sources. During the subsequent 3 weeks, small increases in blood pressure were seen in the casein and soya groups. When rabbits were given fat enriched diets, blood pressures rose but the increase was independent of the type of protein in the diet. It is concluded that the type of protein consumed is unlikely to account for the blood pressure differences between vegetarians and omnivores. Arguments are presented suggesting that other dietary components, such as fat or fibre may be responsible.