Vitamin D deficiency is one of the most common vitamin deficiencies but symptoms are subtle.

It is believed that about 42% of people in the US may be vitamin D deficient. (Vitamin D is delivered to cells via the bloodstream and turns genes on or off. Nearly every cell in the body has a receptor for vitamin D.)

Vitamin D has been coined the sunshine vitamin because it is produced out of cholesterol in the skin when it is exposed to sunlight, which is why people living far from the equator are more likely to be deficient.

Dietary sources are scarce, however, cod liver oil and fatty fish (such as salmon, mackerel, sardines or trout) provide pretty high amounts and mushrooms provide some too.