Vit D And Bone Metabolism

Question 1

Sources of vitamin D?

Answer 1

Sun (pro-vit D)
Fish
Supplements

Question 2

Hydroxylation of vitamin D

Answer 2

1. Calciferol hydoxylated in liver to calcidiol
2. Calcidiol (active hormone) = circulating

Question 3

What are the functions of plasma calcium?

Answer 3

2.2-2.6 mol/L
Circulates in plasma
50% protein bound (albumin
50% ionised

Question 4

What are the functions of bone Ca?

Answer 4

Stiffness and structure
~1kg Ca
Hydroxyapatite- 99% Ca10 (PO4)6(OH)2
Reservoir for plasma Ca

Question 5

What are the regulatory functions of Ca?

Answer 5

Neurotransmission
Reproduction
Hormone action
Cellular growth
Enzyme function

Question 6

Clinical symptoms of hypocalcaemia

Answer 6

Albumin adjusted Ca<2.2mmol/L
Problems in neurotransmission
Paraesthesia - numbness, tingling
Muscle spasm and tetany - chvosteks’s and trousseau’s sign, seizures/fits
Cardia abnormalities - arrhythmia, hypertrophy, hypotension, failure
Coma

Question 7

Clinical features of hypercalceamia

Answer 7

Albumin adjusted Ca > 2.6mmol/L
Nausea, peptic ulcers, constipation
Mental disturbances, depression
Retinal stones, renal filariasis, polyuria
Soft tissue calcification - eyes, kidneys

Question 8

What are the symptoms of vit D deficiency in adults and children?

Answer 8

Non specific
Fatigue
Generalised muscle, joint and bone pain
Hyperalgesia (increased sensitivity to pain)
Muscle weakness (myopathy) - espc extremities and pelvic region (difficulty rising from siting/ squatting + waddling gait)
Fragility fractures - with prolonged bone loss

Question 9

Specific vit D deficiency symptoms in children

Answer 9

Bone deformities - rickets
Cardiac proles
Hypocalcaemic fits

Question 10

How does bone mass differ with age?

Answer 10

Reach peak bone mass around puberty
Plateaus from 20s onwards
Loss bone mass around 50s - menopause for women

Question 11

How does parathyroid hormone regulate Vitamin D?

Answer 11

1. PTH increases when [Ca plasma] decreases
2. Inhibits how much Ca is deposited in bone
3. Increases bone resorption if PTH continuously high
4. Simulates reabosrption of Ca in urine
5. Decreases excretion of Phosphate in urine
6. Stimulates 1.25 vit D which in turn stimulates absorption of vit D from diet

Question 12

How does calcitonin regulate vit D?

Answer 12

Stimulates excretion of Ca in urine if [Ca plamsa] is high

Question 13

How does 1.25(OH)2 regulate vit D?

Answer 13

1. Stimulates absorption of vit D from diet
2. Stimulates bone maturation.
3. If PTH is high at the time also stimulates bone resorption
4. Regulates own production and breakdown
5. Also regulates Ca excretion in urine
7. Stimulates production and release of FGF23

Question 14

How does fibroblast growth factor 23 (FGF23) regulate vit D?