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70 SECTION II BIOCHEmISTRY ``BIOCHEMISTRY—NUTRITION BIOCHEmISTRY ``BIOCHEMISTRY—NUTRITION
Vitamin D D 3 (cholecalciferol) from exposure of skin (stratum basale) to sun, ingestion of fish, milk, plants.
D 2 (ergocalciferol) from ingestion of plants, fungi, yeasts.
Both converted to 25-OH D 3 (storage form) in liver and to the active form 1,25-(OH) 2 D 3 (calcitriol)
in kidney.
3–
2+
FUNCTION intestinal absorption of Ca and PO 4 .
bone mineralization at low levels.
bone resorption at higher levels.
2+
3–
REGUlATION PTH, Ca , PO 4 1,25-(OH) 2 D 3 production.
1,25-(OH) 2 D 3 feedback inhibits its own production.
PTH Ca reabsorption and PO 4 reabsorption in the kidney.
2+
3–
DEFICIENCY Rickets in children (deformity, such as genu varum “bowlegs” A ), osteomalacia in adults (bone
pain and muscle weakness), hypocalcemic tetany.
A
Caused by malabsorption, sun exposure, poor diet, chronic kidney disease (CKD), advanced liver
disease.
Give oral vitamin D to breastfed infants.
Deficiency is exacerbated by pigmented skin, premature birth.
EXCESS Hypercalcemia, hypercalciuria, loss of appetite, stupor. Seen in granulomatous diseases
( activation of vitamin D by epithelioid macrophages).
Vitamin E Includes tocopherol, tocotrienol.
FUNCTION Antioxidant (protects RBCs and membranes
from free radical damage).
DEFICIENCY Hemolytic anemia, acanthocytosis, muscle Neurologic presentation may appear similar
weakness, demyelination of posterior columns to vitamin B 12 deficiency, but without
( position and vibration sensation) and megaloblastic anemia, hypersegmented
spinocerebellar tract (ataxia). neutrophils, or serum methylmalonic acid
levels.
EXCESS Risk of enterocolitis in infants. High-dose supplementation may alter metabolism
of vitamin K enhanced anticoagulant effects
of warfarin.
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