The “Family” of Vitamin
Fat-soluble vitamins have important roles in the synthesis and degradation of nutrients, immune function, homeostasis, and growth. The A vitamins are unsaturated fat-soluble organic compounds, including retinol, retinal, retinoic acid, and some provitamin A carotenoids . They have important roles in vision, reproduction, bone growth, and function of the immune system and skin. Oral carotenoids, which are bioconverted to vitamin A, are often recommended for disease prevention. In particular, epidemiological studies reported that the consumption of carotenoid-rich foods is associated with decreased risk of several cancers, cardiovascular diseases, macular degeneration, and cataracts . β-carotene is the most commonly used carotenoid in functional foods and pharmaceutical products, because of its strong provitamin A and antioxidant activities.
The vitamin B group consists of B1, B2, B3, B5, B6, B7, B9, B12, and related derivatives. Among these, B1, B2, B3, B6, and B12 play important roles in disease prevention. For example, vitamin B1 is a cofactor of dehydrogenase and transketolase; polyneuritis, Alzheimer’s disease, and colon cancer are linked to B1 deficiency. Vitamin B2 is a precursor to flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN), which are cofactors for flavoenzymes, a large group of oxidoreductases. B2 deficiency can cause mucosal disorders, skin disorders, and anemia. Vitamin B3 is the precursor of nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP), and B3 deficiency is associated with pellagra, depression, and dementia. Vitamin B6 is a cofactor for enzymes that function in the metabolism of amino acids, carbohydrates, and lipids. The monitoring of B6 concentration is important for patients with cardiovascular diseases. Vitamin B12 has a complex structure, with a corrin ring and an embedded cobalt ion. This vitamin is important for erythrocyte formation, nerve cell maintenance, and DNA synthesis. B12 deficiency may lead to megaloblastic anemia. Dairy products are the major dietary sources of vitamin B12, although this vitamin has bioavailability of 8 to 12% from milk preparations, but 12 to 33% following the consumption of tofu and cheese
Vitamin C is the only water-soluble vitamin not in the vitamin B group. It is one of the most essential vitamins and it has roles in many physiological processes, including immune response and iron absorption. Vitamin C is abundant in many fruits and vegetables, such as mango, kiwi fruit, papaya, lettuce, tomato, and strawberry. Vitamin C is also a strong antioxidant that can reduce oxidative stress. Vitamin C may improve physiological function by increasing baroreflex sensitivity, ameliorating vascular conductance, decreasing systemic inflammation, reducing cancer cell-specific toxicity, and augmenting the inotropic response to β-adrenergic stimulation.
Vitamin D has two major forms: D2 and D3, each of which the body converts into the bioactive calcitriol (25-dihydroxyvitamin D). Ultraviolet irradiation of ergosterol in plants leads to the formation of vitamin D2, and ultraviolet radiation of 7-dehydrocholesterol in human skin leads to the formation of vitamin D3. Vitamin D is also obtained from foods, including egg yolk, fish, and milk. Vitamin D has important roles in the mineralization of bone and teeth, due to its regulation of calcium and phosphorus homeostasis. There is also evidence that vitamin D supplements can prevent malignancies, cardiovascular diseases, osteoporosis, and diabetes.
Molecules in the vitamin E group, which function as antioxidants and free radical scavengers, include four tocopherols (α, β, γ, and δ) and four corresponding unsaturated tocotrienols. These compounds have anti-inflammatory activities and they are recommended for the treatment of cardiovascular disorders and cancers.
Vitamin K is essential for the activation of specific proteins that function in bone metabolism and blood clotting. Based on its source, vitamin K is classified as plant-derived vitamin K1 (phylloquinone) or animal/bacteria-derived vitamin K2 (menaquinones). Vitamin K1 is a procoagulant that is used in cases of hemorrhage, and vitamin K2 has roles in the regulation of blood clotting factors, namely prothrombin and five other proteins (Factors VII, IX, and X, and proteins C and S). During the process of pancreatic lipolysis, vitamin K is solubilized in mixed micelles with bile salts, and is then absorbed through the proximal intestine.