Selenium is an essential trace mineral for human beings and animals and is normally supplied only in the diet. The distribution of selenium in soils varies greatly from country to country and even within each country. This is reflected in the selenium contents of the food grown in those areas.
Low soil levels occur in Europe, the USA, New Zealand and Australia. High levels are present in Taiwan, Japan, the Philippines, Puerto Rico, Venezuela and Costa Rica. Within the USA, Wyoming and Dakota have high soil levels. In the UK, Norfolk has the highest soil levels.
Seafood, kidney, liver and, to a lesser extent, other meats are consistently good sources of selenium, whereas grains and other seeds are more variable, depending on the soil content in the areas where they are grown. Fruit and vegetables generally contain little selenium. Drinking water usually makes only a small contribution to selenium intake.
Studies have indicated that the selenium in fish is less bioavailable than other food sources of the mineral. However, little is known about the bioavailability of the mineral from foods in general.
Those who are deficient in selenium retain organically bound forms of the mineral better than inorganic selenium, but all forms cause similar increases in glutathione peridoxidase activity.
The adult body contains about 20 mg selenium, most of it in the kidneys and liver and, in the male, the testes.
There are no specific symptoms of mild selenium deficiency but gross deficiency can cause Keshan disease in man and white muscle disease in animals. Keshan disease is a congestive heart disease that primarily affects children and appears to be restricted to China.
Those who may be at risk of dietary deficiency of selenium include:
- Anyone consuming high quantities of processed and refined foods
- Those living exclusively on foods grown in a selenium deficient area
- Some babies fed on selenium-deficient dried milk rather than breast-milk.
Extensive research examining the link between selenium and the complementary nutrient vitamin E has revealed the following:
Selenium can support vitamin E, i.e. partially replace it – selenium is 50 to 100 times more powerful as an anti-oxidant than vitamin E. The protective enzyme glutathione peroxidase requires both selenium and vitamin E to function.
Combined selenium and vitamin E gives better relief from angina than either nutrient being taken in isolation.
Both nutrients are required for efficient resistance to infection and other immune response functions.
Experimentally-induced cancers in animals were inhibited by both selenium and vitamin E, but this effect was most positive when vitamin C was also taken. Skin cancers induced by excessive exposure to ultraviolet light were inhibited by selenium plus vitamins C and E.
The most efficient ratio is 100 mg vitamin E to 25 mcg selenium.
The beneficial effect of combined selenium and vitamin E is greater than the added effects of each single nutrient, i.e. they are truly synergistic.