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The increasing availability of palatable soya bean protein materials for use in human foods raises the question of possible effects on the health and well-being of the consumer. On the basis of available evidence, no unfavourable effects are to be expected, but apart from nitrogen balance investigations, systematic human experiments with these novel foods are scarce. Therefore, we carried out a large-scale experiment covering many physiological and health aspects. Two diets were compared in 4 + 4 weeks cross-over design with 92 healthy volunteers. One diet contained a wide variety of soya protein foods (test diet), about 25% of the protein intake being from soya, the other (control) diet contained similar products made from conventional protein sources. The diets were given in two identical menu cycles. Blood, urine and feces were sampled at the end of each period. Health status and subjective reactions were monitored throughout. About 90 out of the more than 100 parameters investigated did not show any difference. Statistically significant reactions to the diet composition were found in the following areas: of the blood serum enzymes, alkaline phosphatase was higher after consumption of the test diet, while serum inorganic phosphate showed a decrease. The higher magnesium content of the soya protein materials was reflected by an increase in fecal excretion, and in serum levels in the fed state. Fasting serum levels, however, were lower on the soya diet. Measurement of the intestinal noise indicated that more intestinal gas was produced on the test diet; this was confirmed by reports from the volunteers. Immunological tests showed that the females had increased IgA and soya specific IgE levels on the test diet, but no indications for allergenicity were found. All the mentioned effects were well within normal physiological ranges and do not indicate unfavourable trends. We conclude, that these results confirm the prevailing view that soya bean protein materials are acceptable ingredients for our daily food.