ARS | Publication request: Selenium Bioavailability from Soy Protein Isolate and Tofu in Rats Fed a Torula Yeast-Based Diet

Submitted to: Journal of Agriculture and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 19, 2009
Publication Date: November 18, 2009
Citation: Yan, L., Graef, G.L., Reeves, P.G., Johnson, L.K. 2009. Selenium Bioavailability from Soy Protein Isolate and Tofu in Rats Fed a Torula Yeast-Based Diet. Journal of Agriculture and Food Chemistry. 57:11575-11580.

Interpretive Summary: Selenium is an essential nutrient, and soy is a major plant source of dietary protein to humans. The United States produces one third of world’s soybeans, and the selenium-rich Northern Plains produce a large share of the nation’s soybeans. The present study examined the bioavailability of selenium from protein isolate and tofu (bean curd) prepared from a soybean cultivar we recently developed specifically for food grade markets. We fed rats a selenium-deficient diet to deplete selenium, and then we replenished them with the same diet supplemented with 20, 30 or 40 µg selenium/kg from protein isolate or tofu. The selenium bioavailability was determined on the ability of selenium from protein isolate or tofu to restore selenium-dependent enzyme activities and tissue selenium contents in these rats. Selenomethionine, a selenium-containing amino acid commonly from plant foods, was used as a reference. Dietary supplementation with protein isolate or tofu resulted in dose-dependent increases in selenium-containing enzyme activities in blood and liver and selenium content of plasma, liver, muscle and kidneys. These responses indicated an overall bioavailability of approximately 97% for selenium from both protein isolate and tofu, relative to selenomethionine. These results demonstrate that selenium from this soybean cultivar is highly bioavailable in this model and that high-selenium soybeans are a good dietary source of selenium.

Technical Abstract: Selenium (Se) is an essential nutrient, and soy is a major plant source of dietary protein to humans. The United States produces one-third of world’s soybeans, and the Se-rich Northern Plains produce a large share of the nation’s soybeans. The present study used a rat model to determine the bioavailability of Se from protein isolate and tofu (bean curd) prepared from a soybean cultivar we recently developed specifically for food grade markets. The soybean seeds contained 2.91 mg Se/kg. Male Sprague-Dawley rats were depleted of Se by feeding them a 30% Torula yeast-based diet containing 5 µg Se/kg; after 56 days they were replenished of Se for an additional 50 days by feeding them the same diet supplemented with 20, 30 or 40 µg Se/kg from soy protein isolate or tofu. L-selenomethionine (SeMet) was used as a reference. Selenium bioavailability was determined on the basis of the responses of Se-dependent enzyme activities and tissue Se contents, comparing those responses for each soy product to those for SeMet using a slope-ratio method. Dietary supplementation with protein isolate or tofu resulted in dose-dependent increases in glutathione peroxidase activities in blood and liver and thioredoxin reductase activity in liver, as well as dose-dependent increases in Se contents of plasma, liver, muscle and kidneys. These responses indicated an overall bioavailability of approximately 97% for Se from both protein isolate and tofu, relative to SeMet. These results demonstrate that Se from this soybean cultivar is highly bioavailable in this model and that high-Se soybeans can be a good dietary source of Se.