Human Biochemistry, Volume 2 - Murray R. 1993

Special Topics
Nutrition, Digestion, and Absorption
Vitamin Requirements

Vitamins are organic nutrients required in small amounts for normal METABOLISM that cannot be synthesized by the body in adequate quantities. The daily human requirement for each vitamin is expressed in milligrams or micrograms. Vitamins perform specific biochemical Functions. These functions, deficiency syndromes, and dietary sources of vitamins are summarized in Table 53.5 (for Water-Soluble Vitamins) and Table 53.6 (for Fat-soluble vitamins).

Water-soluble vitamins include the B-complex vitamins and ascorbic acid (Vitamin C). Water-soluble vitamins are absorbed into the HEPATIC PORTAL VEIN, and excess amounts are excreted in the urine. Consequently, only a small reserve of free vitamin is maintained, which in most cases must be continuously replenished through diet. A certain reserve of folic acid is stored in the Liver. Depletion of reserves may take a few months for ascorbic acid and several years for Vitamin B12 (which is also stored in the liver). Excess intake of vitamins in this group is generally well tolerated, aside from adverse effects associated with high doses of nicotinic acid, ascorbic acid, or pyridoxine.

Class="center">Table 53.5. Major water-soluble vitamins: Principal properties

¹⁾ The metabolism of most water-soluble vitamins shares common features. They are absorbed in the intestine, stored in enzyme- or transport-protein-bound forms, and excreted in the urine when plasma levels exceed the renal threshold. The only major exception is vitamin B_|2, the absorption of which in the distal ileum requires intrinsic factor (secreted by gastric parietal Cells); this vitamin is stored in the liver in milligram amounts, excreted in Bile (and reabsorbed via enterohepatic Circulation), and excreted in urine.

²⁾ Excess water-soluble vitamins are not always toxic. Exceptions include excess nicotinic acid (but not nicotinamide), which causes cutaneous vasodilation (flushing); high doses of ascorbic acid, which have been reported to cause diarrhea, oxalate Kidney stones, and several other adverse effects; and high doses of pyridoxine (5 g per day), which cause sensory ataxia, sensory nerve dysfunction, and, rarely, axonal degeneration. Deficiency of these vitamins primarily affects Tissues with high metabolic rates, typically manifesting as lesions of the gastrointestinal and nervous systems, Skin, and Blood Cells.

³⁾ With few exceptions, requirements for water-soluble vitamins are met by consuming adequate quantities of the following foods: whole-grain cereals, legumes, leafy green vegetables, meat, and dairy products.

The absorption of vitamin B₁₂ requires the presence of intrinsic factor, a glycoprotein secreted by The Stomach. Patients who have undergone gastrectomy and do not receive parenteral vitamin B₁₂ may develop deficiency symptoms after approximately 5 years. Vitamin B₁₂ is absent from plant-derived foods, but it is synthesized by microorganisms found in the intestinal flora of herbivores, particularly ruminants, whose meat and milk contain the vitamin. Consequently, animal-derived products serve as the primary source of vitamin B₁₂. Vegetarians who do not take appropriate supplements are at risk of developing vitamin B₁₂ deficiency. Because most water-soluble vitamins are found in the same food sources, isolated deficiencies are rare, and most patients present with symptoms of multiple B-vitamin deficiencies.

Fat-soluble vitamins (A, D, E, and K) are present in the Lipids of both animal and plant foods. They are digested along with dietary fats, absorbed in the intestine, and incorporated into chylomicrons. Subsequently, these vitamins are transported (primarily within chylomicron remnants) to the liver, which serves as the main storage site for vitamins A, D, and K. Adipose tissue is the primary storage depot for vitamin E. Fat-soluble vitamins are not excreted in the urine, and excessive accumulation in the body exerts toxic effects (particularly for vitamins A and D).

Deficiencies of fat-soluble vitamins (such as Vitamin D deficiency causing Rickets) are observed predominantly in children. However, they can also occur in adults (Osteomalacia), particularly in conjunction with lipid malabsorption disorders. Several cofactor metabolism disorders responsive to specific vitamin therapies are listed in Table 53.7.