SN2 SHORT NOTE 5M

Describe Vitamin D Metabolism in detail.

General Surgery · Final Professional

The Activation Pathway

Vitamin D must be activated in two hydroxylation steps before it can act. Vitamin D₃ (cholecalciferol) is synthesised in the skin from 7-dehydrocholesterol under ultraviolet (sunlight), or is taken in the diet. It is first hydroxylated in the liver to 25-hydroxyvitamin D (calcidiol) — the main storage form and the one measured to assess status — and then in the kidney by 1α-hydroxylase to the active hormone 1,25-dihydroxyvitamin D (calcitriol).

Actions & Regulation

Active calcitriol raises serum calcium and phosphate by increasing their absorption from the gut, aiding bone mineralisation and modulating bone turnover. Renal 1α-hydroxylase is stimulated by PTH and by low phosphate, linking vitamin D to calcium homeostasis.

CLINICAL PEARL

Two clinically useful points: the liver makes the storage form (25-OH-D, measured clinically) and the kidney makes the active form (1,25-(OH)₂-D). Hence renal failure impairs activation and causes bone disease despite adequate intake.

Clinical Relevance

Deficiency causes rickets (children) and osteomalacia (adults). Because activation is renal, chronic kidney disease causes deficiency of the active hormone (treated with active analogues such as calcitriol/alfacalcidol), whereas simple nutritional deficiency is treated with cholecalciferol.

Sources & Deficiency

The main source of vitamin D is cutaneous synthesis under sunlight, with a smaller dietary contribution (oily fish, fortified foods, egg yolk). Deficiency is therefore common where sun exposure is limited — by latitude, skin pigmentation, clothing, indoor lifestyle or institutionalisation — and in malabsorption. Because activation depends on the liver and kidney, liver and kidney disease also impair vitamin D status, which is why the active analogue (calcitriol/alfacalcidol) rather than plain vitamin D is used in renal failure.

CLINICAL PEARL

The two-organ rule captures it: the liver hydroxylates to the stored, measured form (25-OH-D) and the kidney hydroxylates to the active hormone (1,25-(OH)₂-D) — which is why renal failure needs the active analogue while ordinary deficiency needs plain vitamin D.

The renal 1α-hydroxylase step is the rate-limiting, PTH-regulated one.

KEY POINT

Key points TO remember

Skin (UV) or diet → liver (25-OH-D, storage/measured) → kidney (1,25-(OH)₂-D, active).
Renal 1α-hydroxylase is stimulated by PTH; active D raises gut Ca/PO₄ absorption.
Deficiency → rickets/osteomalacia.
CKD impairs activation → give active analogues (calcitriol); nutritional lack → cholecalciferol.

EXAM TIP

Sources: Maheshwari's Essential Orthopaedics; Apley & Solomon's System of Orthopaedics and Trauma; AO Principles of Fracture Management.

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