Vitamins, Fat-soluble Health Article

Vitamins, Fat-Soluble

Because they dissolve in fat, vitamins A, D, E, and K are called fat-soluble vitamins. They are absorbed from the small intestines, along with dietary fat, which is why fat malabsorption resulting from various diseases (e.g., cystic fibrosis, ulcerative colitis, Crohn's disease) is associated with poor absorption of these vitamins. Fat-soluble vitamins are primarily stored in the liver and adipose tissues. With the exception of vitamin K, fat-soluble vitamins are generally excreted more slowly than water-soluble vitamins, and vitamins A and D can accumulate and cause toxic effects in the body.

Vitamin A

Vitamin A was the first fat-soluble vitamin identified (in 1913). Vitamin A comprises the preformed retinoids, plus the precursor forms, the provitamin A carotenoids. Preformed retinoids is a collective term for retinol, retinal, and retinoic acid, all of which are biologically active. The provitamin A carotenoids include beta-carotene and others, which are converted to retinoids with varying degrees of efficiency. Retinoids are sensitive to heat, light, and oxidation by air. Beta-carotene is relatively more stable. Vitamin E helps protect vitamin A from oxidation. There is some loss of vitamin A with cooking, but only after boiling for a comparatively long period.

Retinoids are converted to retinol in the intestines and transported with dietary fat to the liver, where it is stored. A special transport protein, retinolbinding protein (RBP), transports vitamin A from the liver to other tissues. Carotenoids are absorbed intact at a much lower absorption rate than retinol. Of all the carotenoids, beta-carotene has the highest potential vitamin-A activity. The active forms of vitamin A have three basic functions: vision, growth and development of tissues, and immunity.

• Vision. Vitamin A combines with a protein called opsin to form rhodopsin in the rod cells of the retina. When vitamin A is inadequate, the lack of rhodopsin makes it difficult to see in dim light.
• Growth and development of tissues. Vitamin A is involved in normal cell differentiation—a process through which embryonic cells transform into mature tissue cells with highly specific functions. Vitamin A supports male and female reproductive processes and bone growth.
• Immunity. Vitamin A is essential for immune function and vitamin-A deficiency is associated with decreased resistance to infections. The severity of some infections, such as measles and diarrhea, is reduced by vitamin-A supplementation among those who suffer from vitamin-A deficiency.

It has been suggested that beta-carotene and other carotenoids (also called phytochemicals) may function as antioxidants by neutralizing free radicals. Free radicals are unstable, highly reactive molecules that damage DNA, cause cell injury, and increase the risk of chronic disease. Beta-carotene has also been associated with reducing the risk of lung cancer. Lutein and zeaxanthin, yellow carotenoid pigments in corn and dark green leafy vegetables, may reduce the risk of macular degeneration and agerelated cataracts. Lycopene, a red carotenoid pigment in tomatoes, may help reduce the risk of prostrate cancer, cardiovascular disease, and skin damage from sunlight.