الكاتب: Multiple References

Vitamins are essential to maintain normal metabolic processes and homeostasis within the body. The amount of a specific vitamin required by an individual varies considerably and it is influenced by such factors as body size, growth rate, physical activity, and pregnancy. Most vitamins are stored minimally in human cells, but some are stored in liver cells to a greater extent. Vitamins A and D, for example, may be stored in sufficient amounts to maintain an individual without any intake for 5 to 10 months and 2 to 4 months, respectively. However, a deficiency of vitamin B compounds (except vitamin B12) may be noted within days, and the lack of vitamin C will manifest within weeks and may result in death in 5 to 6 months. The current recommended dietary allowance (RDA) of vitamin C is 75 mg for woman and 90 mg for men, based on the vitamin's role as an antioxidant as well as protection from deficiency. High intakes of the vitamin are generally well tolerated, however, a Tolerable Upper Level (TUL) was recently set at 2 g based on gastrointestinal upset that sometimes accompanies excessive dosages. Several populations warrant special attention with respect to vitamin C requirements. These include patients with periodontal disease, smokers, pregnant and lactating women, and the elderly