Vitamin C: Functions and Deficiency

Vitamin C (Ascorbic Acid)

• Sources: citrus fruits, tomatoes, berries
• Function: anti-oxidant nutrient, necessary for hydroxylation of proline and lysine in collagen formation, and reduction of dietary Fe in the stomach for absorption
• Possibly protects vitamins A, E, and K from oxidation
• Deficiency: Scurvy, characterized by impaired wound healing, hemorrhages, and anemia
• Low vitamin C status in the elderly, alcoholics, and adolescents, especially those on 'junk' food
• Smokers need twice the normal intake (80 mg/day) due to increased turnover of ascorbic acid
• Megadoses: uncertain benefits, but may ease symptoms of cold, and risks of kidney stones, diarrhea, and systemic conditioning

Fat-Soluble Vitamins

• Causes of deficiency: primary dietary deficiency, low fat diet, and fat malabsorption

Vitamin A

• Sources: retinol from animal liver and fish liver oils, whole milk, and egg yolk; plant carotenoids (mainly β-carotene) from green/yellow/orange vegetables and fruit
• Absorption: carotenoids are cleaved to 2 retinol molecules, with inefficient conversion
• Active forms: retinoic acid, retinal, and β-carotene
• Functions: control of protein synthesis, vision, and maintenance of epithelial cells and mucopolysaccharide synthesis
• Deficiency: rare in developed countries, but common in developing countries, especially with inadequate protein diets
• Deficiency symptoms: night blindness, progressive keratinization of the cornea, and finally keratomalacia and irreversible blindness
• Toxicity/Teratogenicity: dermatitis, hair loss, mucous membrane defects, hepatic dysfunction, and thinning and fracture of long bones

Vitamin E

• Source: vegetable oils, especially wheat germ oil, nuts, and green vegetables
• Function: prevents oxidation of unsaturated/poly-unsaturated fatty acids in cell membranes and circulating lipoproteins, and acts as a free radical scavenger
• Deficiency: virtually unknown, except in premature, low birth weight infants, and characterized by haemolytic anaemia
• Requirements: difficult to establish, likely to be higher with high PUFA intake
• Possible protective effect against cardiovascular disease and cancer

Vitamin D

• A group of similar compounds, with cholecalciferol (vitamin D3) occurring naturally in animals and ergocalciferol (vitamin D2) derived from ergosterol in plants
• Sources: milk and dairy products, fortified margarine, eggs, and skin formation through UV light
• Function: maintains correct levels of calcium and phosphate in the blood for proper bone mineralization
• Deficiency: rickets in children and osteomalacia in adults, characterized by mineral:matrix ratio decrease in bone, bending of long bones, and delayed tooth eruption
• Recommendations: consider taking 10 microg between October and March for adults and children over 4 years
• Toxicity: excessive consumption is toxic, causing hypercalcaemia, GI tract disturbances, and calcification of soft tissues

Vitamin K

• Sources: green leafy vegetables, milk, meat, eggs, and cereals, as well as the bacterial flora of the jejunum and ileum
• Function: essential for blood clotting, which depends on a cascade system of interacting proteins
• Deficiency: rare, except in long-term antibiotic therapy, and characterized by defective blood clotting
• Limiting intake: necessary for anticoagulation therapy, such as warfarin, and in haemorrhagic disease of the new-born