You may have heard the saying, "If a little is good, then more must be better." Some people apply this concept when taking vitamins, but this is not always true, especially in the case of fat-soluble vitamins. Because these vitamins (A, D, E, K) are not readily excreted from the body and are stored in fat tissues, they can accumulate and create toxic effects. With adequate intake, however, fat-soluble vitamins support many of our vital functions.
Vitamin A
Vitamin A has three active forms―retinol, retinal, and retinoic acid―which are used for vision, cellular growth and differentiation and immunity. In vision, vitamin A is needed as retinal in the eye retina, to turn light into nerve signals to the brain, and as retinoic acid, to maintain normal differentiation of the cells in the eye, such as the cornea and rod cells.
In cellular growth, vitamin A is needed for the production, structure, and function of epithelial cells in the lungs, trachea, GI tract, and other organs, such as formation of mucus-forming cells.
Deficiency in vitamin A can lead to blindness, lowered immune system, conjunctival xerosis (dryness of the eyes' conjunctiva), and dry skin. Toxicity in vitamin causes headaches, nausea, liver damage, hair loss, bone/muscle pain, dry skin and coma.
Vitamin D
Vitamin D's role in your body is to maintain adequate calcium and phosphorus concentration in normal range that supports neuromuscular function, bone calcification, and growth. This is also the only fat-soluble vitamin that you body can synthesize by exposing your skin to sunlight.
Like vitamin A, too much or too little vitamin D can have negative health consequences. Vitamin D deficiency causes rickets (children) and osteomalacia (adults) where the bones are not solid enough to support the body, causing fractures and weakness. Too much vitamin D causes overcalcification of bones and kidney problems since it has to work harder to excrete excessive calcium.
Vitamin E
Vitamin E, a major antioxidant, helps prevent accumulation of free radicals by "donating" electrons to them within the fat cell membranes. This process causes the free radicals to stabilize, stopping them from destroying the cell membranes. This is essential to reduce the risk of cancer and many diseases.
Vitamin E deficiency causes free radicals to attach cell membranes, particularly red blood cells. This can lead to hemolysis (destruction of red blood cells). However, the overdose symptoms of vitamin is still unknown.
Vitamin K
Vitamin K is essential to blood clotting by acting as a cofactor in the synthesis of prothrombin, a protein that provides blood clotting.
Deficiency in vitamin causes hemorrhage and fractures, but there are no known symptoms of overdosing of this vitamin.
Summary
Because we have an abundant food supply and more choices in the United States, you should be able to obtain adequate fat-soluble vitamins in your diet without relying on supplements, which often contain megadoses of vitamins. Consult with a registered dietitian if you have any concerns about your current dietary health. Otherwise, eat a variety of fruits, vegetables, and whole grains to get enough vitamins for you.
References
- Persepctives in Nutrition, 5th Edition; Gordon Wardlaw, Margaret Kessel; 2002
- Exercise Physiology: Energy, Nutrition, and Human Performance; William McArdle, Frank Katch, Victor Katch; 2003.
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