Adequate, lifelong intake of selenium—an immune-boosting trace mineral found in soil—may help to slow age-related mental decline, says new research from the Indiana University School of Medicine (American Journal of Epidemiology, 2007, vol. 165, no. 8).

Researchers recruited 2,000 rural Chinese men and women 65 and older, most of whom had lived in the same village since birth. They administered several cognitive tests to establish brain function; analyzed toenail samples for individual selenium content; and collected local water, soil, and food samples to determine environmental selenium levels.

Participants with the lowest levels of selenium scored on cognitive tests as if they were a decade older than those with the highest selenium levels.

Studying such a stable population, scientists noted, helped establish a link between long-term selenium intake and mental function. Villagers generally eat locally grown food and rarely take selenium supplements. (Environmental selenium levels did vary; about half of the study subjects came from "low" selenium areas, half from "normal" selenium areas.) By contrast, the U.S. population has proven difficult to study, partly because individuals are mobile and eat foods grown in different parts of the world, with varying local selenium levels.

"Selenium is a powerful antioxidant and also supports the regulation of thyroid hormones," says Debra Boutin, MS, RD, CD, assistant professor of nutrition at Bastyr University in Kenmore, Washington.

As a trace mineral, we need very little selenium: The RDA is 55 mcg per day; just one Brazil nut, for example, supplies 120 mcg. It's difficult to get too much dietary selenium—best food sources include Brazil nuts, organ meats, and other meats and poultry—but it is potentially toxic in its highly concentrated supplement form.