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Vitamins

Vitamins are organic molecules required in microgram-to-milligram amounts for health, growth, and reproduction. Because humans cannot synthesize them in adequate quantities (or at all), they must come from the diet. The one partial exception is vitamin D, which can be synthesized in skin exposed to sunlight. - Henry's Clinical Diagnosis and Management by Laboratory Methods

Figure: Classification of vitamins (Lippincott Illustrated Reviews: Biochemistry, 8th ed.)

Classification

Vitamins are divided into two broad groups based on solubility. This classification has important consequences for absorption, storage, and toxicity.

Property	Water-Soluble	Fat-Soluble
Members	Vitamin C + B-complex (B1, B2, B3, B5, B6, B7, B9, B12)	Vitamins A, D, E, K
Absorption	Direct absorption	Require bile and pancreatic lipase; absorbed with dietary fat; travel in chylomicrons
Storage	Minimal	Stored in liver and adipose tissue
Excretion	Rapidly excreted in urine	Eliminated slowly
Deficiency timeline	Weeks	Months to years
Toxicity risk	Low (pyridoxine is an exception)	Higher - especially vitamins A and D

Lippincott Illustrated Reviews: Biochemistry, 8th ed.

Fat-Soluble Vitamins (A, D, E, K)

Vitamin A (Retinol, beta-carotenes)

Functions: Vision (formation of rhodopsin in retinal rod cells), epithelial differentiation, immune function, embryonic development
Deficiency: Night blindness (earliest sign), xerophthalmia, keratomalacia, increased susceptibility to infections
Toxicity: Teratogenic at >25x RDA; chronic excess causes liver damage, bone pain, pseudotumor cerebri

Vitamin D (Cholecalciferol / D3; Ergocalciferol / D2)

Functions: Raises plasma calcium and phosphate by promoting intestinal absorption and bone resorption; essential for bone mineralization
Deficiency: Rickets (children), osteomalacia (adults), hypocalcemia
Toxicity: Hypercalcemia, nephrocalcinosis, soft-tissue calcification
Note: Unlike other vitamins, the body CAN synthesize vitamin D via skin UV-B exposure

Vitamin E (Tocopherols)

Functions: Major lipid-soluble antioxidant; protects cell membranes from oxidative damage; supports immune function
Deficiency: Rare; seen in fat malabsorption syndromes - causes hemolytic anemia, peripheral neuropathy, ataxia
Toxicity: Can antagonize vitamin K and increase bleeding risk

Vitamin K (Phylloquinones, Menaquinones)

Functions: Essential cofactor for carboxylation of clotting factors (II, VII, IX, X) and proteins C and S; also involved in bone metabolism
Deficiency: Hemorrhagic disease (especially in newborns), prolonged PT/INR
Toxicity: Low; only the synthetic form (menadione) is toxic

Water-Soluble Vitamins

B-Complex Vitamins

Vitamin	Key Function	Deficiency Syndrome
B1 - Thiamine	Cofactor in pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase (energy metabolism)	Beriberi (wet: cardiac failure; dry: peripheral neuropathy); Wernicke-Korsakoff syndrome (in alcoholism)
B2 - Riboflavin	Component of FAD and FMN; redox reactions	Cheilosis, angular stomatitis, corneal vascularization, glossitis
B3 - Niacin (Nicotinic acid)	Incorporated into NAD+ and NADP+; redox reactions	Pellagra: the "3 Ds" - Dermatitis, Diarrhea, Dementia
B5 - Pantothenic acid	Component of Coenzyme A; fatty acid metabolism	Rare; burning feet syndrome
B6 - Pyridoxine	Coenzyme in amino acid transamination and decarboxylation; heme synthesis	Peripheral neuropathy, seborrheic dermatitis, glossitis; sideroblastic anemia. Note: Toxic in megadoses - sensory neuropathy
B7 - Biotin	Cofactor in carboxylation reactions (gluconeogenesis, fatty acid synthesis)	Rare; caused by raw egg white consumption (avidin binds biotin) or absent from TPN - causes dermatitis, hair loss, depression
B9 - Folic acid (Folate)	Transfer of 1-carbon units; DNA synthesis	Megaloblastic anemia; neural tube defects in fetal development (most common vitamin deficiency in the US)
B12 - Cobalamin	Folate metabolism; DNA synthesis; myelin maintenance	Megaloblastic anemia + subacute combined degeneration of spinal cord (posterior and lateral columns); only found in animal products

Vitamin C (Ascorbic acid)

Functions: Cofactor for hydroxylation of collagen (proline and lysine residues); antioxidant; iron absorption enhancement; immune function
Deficiency: Scurvy - perifollicular hemorrhages, gingival disease, poor wound healing, corkscrew hairs
Toxicity: Chronic megadoses (1-15 g) can cause kidney stones, diarrhea; abrupt cessation after megadosing can paradoxically trigger scurvy

Who Is at Risk for Deficiency?

Several groups are particularly vulnerable:

Alcoholism: Thiamine (B1) most commonly; also folate and other B vitamins
Vegans: Vitamin B12 (only in animal products), vitamin D
Elderly: Vitamin D, folate, B12
Fat malabsorption (liver/biliary/small bowel disease): Vitamins A, D, E, and K
Ileal disease or resection: Vitamin B12 (absorbed in terminal ileum)
Kidney disease: Vitamin D (impaired activation to calcitriol)
Infants/newborns: Vitamin K (given prophylactically at birth)
Pregnancy: Folate (neural tube defect prevention), iron, D
Henry's Clinical Diagnosis and Management by Laboratory Methods, p. 511

Key Principle: Sources

No single food contains all vitamins. Notable points:

B12 (cobalamin) is found only in animal products
Vitamin K and biotin are partially produced by gut microflora, but not in amounts sufficient to sustain metabolism
Leafy green vegetables are rich in folate, vitamin K, and riboflavin
Liver is one of the richest sources of vitamins A, B12, and riboflavin

Toxicity Summary

Water-soluble vitamins are generally safe even in excess, since excess is excreted in urine. Pyridoxine (B6) is the main exception - megadoses cause sensory neuropathy.
Fat-soluble vitamins carry real toxicity risk. Vitamin A has the highest toxic potential, with chronic excess as low as 25x the RDA causing harm.
Vitamin D toxicity leads to hypercalcemia and tissue calcification.
Vitamin E in large doses can antagonize vitamin K and increase bleeding.
Henry's Clinical Diagnosis and Management by Laboratory Methods; Lippincott Illustrated Reviews: Biochemistry, 8th ed.

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