Vitamin D is a fat-soluble vitamin that plays a major role in calcium and phosphate homeostasis. Its active form, calcitriol, helps maintain adequate mineral availability for bone health. For the MCAT, understanding where vitamin D comes from and how it is activated is especially important.
☀️ Vitamin D Production in the Skin
The body can produce vitamin D in the skin when UVB radiation from sunlight converts 7-dehydrocholesterol into vitamin D₃ through intermediate steps. Vitamin D can also be obtained from dietary sources and supplements. Both sources ultimately feed into the same activation pathway.
🩸 Transport to the Liver
Vitamin D from the skin or gastrointestinal tract enters the circulation and is transported to the liver. Because vitamin D is lipid-soluble, much of it circulates bound to vitamin D–binding protein. The liver performs the first major hydroxylation step.
🧪 The Liver Produces Calcidiol
In the liver, vitamin D is converted into 25-hydroxyvitamin D [25(OH)D], also called calcidiol. This is the major circulating form and the form commonly measured when evaluating vitamin D status.
🫘 The Kidney Produces Calcitriol
The kidney performs another hydroxylation, converting 25(OH)D into 1,25-dihydroxyvitamin D [1,25(OH)₂D], or calcitriol. Calcitriol is the biologically active hormonal form of vitamin D.
🔄 PTH and Vitamin D Activation
When blood calcium falls, parathyroid hormone (PTH) promotes renal production of calcitriol by stimulating 1α-hydroxylase. Calcitriol then contributes to restoring calcium balance, particularly by increasing intestinal calcium absorption.
🥛 Vitamin D and Calcium Absorption
One of calcitriol’s most important effects is increasing the absorption of calcium and phosphate from the intestine. Vitamin D therefore supports the availability of minerals required for normal bone mineralization and other physiological processes.
⚠️ What Happens in Vitamin D Deficiency?
Vitamin D deficiency can reduce intestinal calcium absorption and contribute to impaired bone mineralization. Severe deficiency is associated with rickets in children and osteomalacia in adults. Reduced calcium availability may also trigger secondary increases in PTH.
📊 Vitamin D Synthesis and Function: MCAT Quick Reference
| ☀️ Stage / Concept | 📍 Location | ⚙️ Key Process | 🧠 MCAT Takeaway |
|---|---|---|---|
| Vitamin D3 production | Skin | UVB helps convert 7-dehydrocholesterol into vitamin D3 | Sunlight initiates vitamin D synthesis |
| Dietary vitamin D | Small intestine | Vitamin D is absorbed with dietary fat | Vitamin D is fat-soluble |
| First hydroxylation | Liver | Vitamin D → 25(OH)D (calcidiol) | Major circulating form; commonly measured |
| Second hydroxylation | Kidney | 25(OH)D → 1,25(OH)2D (calcitriol) | Produces biologically active vitamin D |
| PTH effect | Kidney | Stimulates 1α-hydroxylase | Increases calcitriol production |
| Intestinal effect | Small intestine | Increases calcium and phosphate absorption | Supports mineral availability |
| Deficiency in children | Bone | Impaired mineralization | Can cause rickets |
| Deficiency in adults | Bone | Impaired mineralization | Can cause osteomalacia |
🎯 MCAT High-Yield Takeaway
Remember the pathway: Skin/Diet → Liver → Kidney → Active Vitamin D. The liver produces 25(OH)D (calcidiol), while the kidney produces 1,25(OH)₂D (calcitriol). For MCAT questions, connect vitamin D with PTH, calcium/phosphate regulation, intestinal absorption, and bone mineralization.
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