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