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FUNCTIONS
Vitamin A (retinol) is a fat-soluble vitamin essential for vision, growth, reproduction, cell division, and the integrity of the immune system.* The most well-known function of vitamin A is its role in vision. As part of the membrane-bound visual protein rhodopsin, vitamin A is essential in the eyes for converting light into nerve impulses. Night blindness and xeropthalmia are signs of moderate to severe vitamin A deficiency. Xerophthalmia is a common cause of preventable blindness in many underdeveloped countries around the world.*

Vitamin A is also essential for normal cellular differentiation. Its metabolite, retinoic acid, assumes a central role in gene activation and transcription. As a result, your body's vitamin A status has profound effects on all rapidly dividing tissues in the body (such as immune cells and intestinal cells) and affects fertility, fetal development and growth.*

Cellular differentiation is crucial for normal immune response, and vitamin A deficiency can start a vicious cycle affecting the immune system.* During vitamin A deficiency, immune function is impaired, which puts the body at increased riskfor infections. Acute infections further deplete the body of vitamin A, which leads to an even more impaired immune function and an even lower resistance to infections. While a few studies have found that vitamin A status in developed populations may be adequate, many studies showed that marginal vitamin A deficiency is quite common. Women often have lower vitamin A levels than men, who tend to consume more meats high in vitamin A. The elderly are also at increased risk for marginal vitamin A deficiency. Persons with HIV infection or AIDS often have low serum vitamin A and are at high risk for moderate to severe vitamin A deficiency.*

The intestinal absorption of vitamin A is associated with fat absorption. Therefore, some dietary fat must be present for efficient vitamin A absorption to occur.

Vitamin C (ascorbic acid) has numerous biological functions. Foremost, it is essential for the synthesis of collagen and glycosaminoglycans which are the building materials of all connective tissues, such as skin, blood vessels, tendons, joint cartilage and bone. Vitamin C is the required coenzyme for two groups of enzymes ( and prolyl hydroxylases) that catalyze the crosslinking of collagen fibers. As such, vitamin C is essential for normal wound healing and capillary health. It also participates in the biosynthesis of carnitine, serotonin, and certain neurotransmitters, including norepinephrine.*

Vitamin C is among the most powerful antioxidants in humans and animals. It is a water-soluble, chainbreaking antioxidant that reacts directly with superoxide, hydroxyl radicals, and singlet oxygen. Laboratory studies show that vitamin C completely protects lipids in plasma and low-density lipoprotein (LDL) against atherogenic peroxidative damage. In addition, vitamin C interacts with glutathione and alpha-lipoic acid, and regenerates vitamin E. The antioxidant functions of vitamin C appear to have clinical significance in providing protection from free radical damage to the eyes, lungs, blood and the immune system.*

Vitamin E is one of the body’s most important antioxidant nutrients. Vitamin E is an especially valuable antioxidant in the cell membranes, where it prevents oxidation of unsaturated fatty acids by trapping free radicals.   This helps stabilize and protect cell membranes, especially red blood cells and tissues sensitive to oxidation, such as the lungs, eyes, and arteries. Vitamin E also protects the liver and other tissues from the free-radical damage of toxicants, such as mercury, lead, ozone, nitrous oxide, carbon tetrachloride, benzene, cresol, and various drugs. Related to its antioxidant properties, vitamin E is important for normal immune function, and many studies show that it prevents lipid peroxidation of blood lipoproteins, such as LDL-cholesterol.*

Zinc is an essential trace element required for the activity of over 300 enzymes and is involved in most major metabolic pathways. Zinc participates not only in catalytic processes, but also in the structure and stability of some regulatory proteins.*

General signs of human zinc deficiency indicate that zinc has important functions in maintaining immune function, reproduction, healthy skin, and growth. Numerous studies support the fundamental role of zinc for normal immune response in humans. Immune cells must be able to rapidly divide in order to respond to daily challenges. Like all rapidly dividing cells, immune cells depend on adequate amounts of dietary zinc.*

As a cofactor of the antioxidant enzyme superoxide dismutase (SOD), zinc can be considered an antioxidant nutrient. Zinc supplementation has been shown to increase the antioxidant activity of SOD, and provide increased free radical protection. Zinc deficiency is associated with increased oxidative damage. Absorption of toxic heavy metals, especially cadmium and lead, is lower in individuals with high zinc status compared to those with low zinc status.*