There are four vitamins that are soluble in fat (A, D, E, K) that affects the way they are absorbed, transported, stored and excreted. In developing nations, deficiencies of vitamin A and D are quite common and even in the United States, deficiencies may occur due to a lack of fruits and vegetables in the diet or limited sun exposure.
VITAMIN A is found as preformed retinoids (retinol, retinal, and retinoic acid.) Retinol can be converted to retinal or retinoic acid. Retinal binds to opsin in the eye to form rhodopsin that participates in the light or visual cycle. Retinoic acid affects cell differentiation by altering gene expression. It is needed for healthy epithelial tissue and normal reproduction and immune function. The other form of vitamin A is carotenoids, primarily beta-carotene that functions as an antioxidant and can be converted to preformed vitamin A (the retinoids). it is the primary form of vitamin A found in dietary supplements.
Mild deficiency causes night blindness and more severe deficiency can lead to blindness or increased infections, a problem in the developing world, primarily children. Golden Rice was genetically engineered to alleviate vitamin A deficiency in some developing countries. See a previous post HERE.
Vitamin A is found in foods as both preformed retinoids and precursor forms as carotenoids. The major food sources of preformed vitamin A include liver, eggs, fish and fortified dairy products. Carotenoids are found in plant foods such as yellow-, orange-, and red-colored fruits and vegetables and leafy greens. Need for the vitamin can be met by both forms. Preformed forms can be toxic at doses only 10 times the RDA and may increase the risk of bone fractures and birth defects. In fact, the RDA does not increase for pregnant women. Plant forms are not toxic, but at high doses can turn the skin orange.
VITAMIN D can be made in the skin from sunlight exposure. it is not found in many foods (unless fortified), but occurs naturally in fish oils and fortified milk. Vitamin D must be altered by the kidneys and liver in order to be used by the body. It then promotes calcium and phosphorus absorption and regulation. Deficiency can result in rickets in children and osteomalacia in adults. Deficiencies are exacerbated by time spent in the sun, the frequent use of sunscreen, living at high latitudes or having dark skin.
The RDA depends on sun exposure and would obviously vary considerably for individuals. Vitamin D research has increased lately and in time may result in additional functions and/or relationships to disease. Vitamin D supplements are sometimes recommended for a number of groups who do not drink milk, older adults and people with limited sun exposure. Too much supplemental vitamin D is toxic, but not from sun exposure.
VITAMIN E is found in nuts, soybean oil, sunflower seeds and almonds It functions primarily as an antioxidant and necessary for reproduction and protects cell membranes from oxidative damage. Deficiency can cause hemolytic anemia and neurological problems. Vitamin E supplements are popular and there is little risk of toxicity. However, there is also very little evidence of any benefits from supplementation.
VITAMIN K is found in soybean oil, green leafy vegetables, brussels sprouts and kiwi and is made by bacteria in the gastrointestinal tract. Vitamin K is a cofactor needed for the synthesis of blood clotting factors and for proper bone formation. Deficiency can cause bleeding and low bone density. Since it takes time for newborn infants to begin to synthesize their own vitamin K from gut bacteria, it is routinely given as injections at birth.
A word of caution: Fat-soluble vitamins are stored in the body for long periods of time, and pose a greater risk for toxicity than water-soluble vitamins, so with the possible exception of vitamin D, do not generally need daily supplementation.