Selenium

What it does

Selenium is needed in only tiny amounts, but it is essential because it forms part of a special group of proteins called selenoproteins. Several of these are powerful antioxidant enzymes — the glutathione peroxidases — that neutralise reactive molecules and protect cells from oxidative damage. In this role selenium works closely with vitamin E, each covering different parts of the body's antioxidant defences.

Selenium is also essential for the thyroid gland. Selenoproteins are needed to convert the storage thyroid hormone (T4) into its active form (T3), and to protect the thyroid from the oxidative stress generated when it makes hormones. Through this, selenium supports normal metabolism alongside iodine. It additionally contributes to a healthy immune response and to normal sperm production and male fertility.

The selenium content of food depends heavily on how much is in the soil where crops are grown, so intakes vary from region to region. Brazil nuts are an exceptionally rich source, with useful amounts also in fish, meat, eggs and wholegrains. The gap between too little and too much is relatively narrow for selenium, so while a shortfall is a problem, routinely taking large supplement doses is not advisable.