Selenium is an antioxidant, helping to prevent free-radical damage. It is also essential for proper functioning of the thyroid gland and immune system.

How Much: Recommended dietary allowance (RDA) = 55 mcg daily.

Too Much: More than 400 mcg per day of supplemental selenium may be toxic.

Too Little: Rare; impaired immunity and heart disease.

Foods: High-selenium yeast; nuts, especially Brazil nuts; shrimp; tuna; turkey; chicken; and whole grains.

Research Note: Some research suggests selenium may help prevent rheumatoid arthritis (RA), but it has not been shown to relieve pain or stiffness in people with established disease.