Vitamin D
Functions of Vitamin D
(1) Calcium absorption: low calcium levels stimulate parathyroid hormone (PTH). PTH increases the production of calcitriol. Calcitriol stimulates calcium absorption at the intestine (2) Calcium retention/reabsorption: low calcium level stimulate calcitriol (through PTH) which increases renal calcium reabsorption (3) Calcium mobilization: low calcium levels stimulate calcitriol (through PTH) which stimulates osteoclasts which increases serum calcium (4) Cell differentiation: calcitriol may inhibit cancer growth (5) Immunity: autoimmune diseases, may help body eliminate microbes (6) BP regulation: down regulates hypertensive hormone renin, and may help relax vascular smooth muscle cells (7) Decreases risk of falls by providing muscle PRO synthesis and strength (8) Insulin secretion: Decreased vitamin D levels and decreased glucose tolerance in type 2 DM
BP regulation
Down regulates hypertensive hormone renin, and may help relax vascular smooth muscle cells
Vitamin D biosynthesis
Exposure to sunlight converts 7-dehydrocholesterol to previtamin D which is converted into cholecalciferol.
In the kidney
25-OH cholecalciferol is converted via parathyroid hormone into 1,25-(OH)2 cholecalciferol (calcitriol) main active form.
Vitamin D defiency
Blood calcium kept normal at expense of bones: Children will develop rickets (bowed legs, knock-knees) Adults will develop bone pain, osteomalacia, and may play a role in osteoporosis
Decrease risk of falls
By providing muscle PRO synthesis and strength
In the liver
Cholecalciferol is converted into 25-OH cholecalciferol
Insulin secretion
Decreased vitamin D levels and decreased glucose tolerance in type 2 DM.
Those at risk
Fat malabsorption Shut-ins with low milk intake Infants if stay inside/born in winter, Vegans Elderly Renal/liver disease Dark skin Obesity
Sources of Vitamin D
Fish liver oils Fatty fish Eggs Liver Butter Fortified milk Margarine DRI: 600 IU/day
Calcium retention/reabsorption
Low calcium level stimulate calcitriol (through PTH) which increases renal calcium reabsorption
Sunlight
Majority of people get their Vitamin D
Exposure to sunlight
converts 7-dehydrocholesterol to previtamin D
Previtamin D
is converted to cholecalciferol.
Vitamin D
Calcitriol (active Vitamin D3) is also a hormone. Most active form.
Cell differentiation
Calcitriol may inhibit cancer growth
Vitamin D absorption transport metabolism
From micelle with lipids , enter circulation via Chylomicron, in liver, cholecalciferol is converted into 25-OH cholecalciferol. In kidney, 25-Oh cholecalciferol is converted via parathroid hormone into 1-25-(Oh)2 cholecalciferol (calcitriol) main active form.
Vitamin D assessment
Plasma 25-OH cholecalciferol is best to indicate recent intake/sun exposure. Optimal= 30-60 mg.mL
Vitamin toxicity
Elevated serum calcium Soft tissue calcification Anorexia Nausea UL=4000 IU/day
Calcium mobilization
Low calcium levels stimulate calcitriol (through PTH) which stimulates osteoclasts which increases serum calcium
Calcium absorption
Low calcium levels stimulate parathyroid hormone (PTH). PTH increases the production of calcitriol. Calcitriol stimulates calcium absorption at the intestine
Immunity
autoimmune diseases, may help body eliminate microbes
