Vitamin D is Activated in the Kidneys to Allow Absorption of Dietary Calcium
Vitamin D is the substance that is activated in the kidneys to allow absorption of dietary calcium. 1, 2
Mechanism of Vitamin D Activation and Calcium Regulation
- Vitamin D undergoes a two-step activation process, first in the liver to 25-hydroxyvitamin D, and then in the kidneys to its active form, calcitriol (1,25-dihydroxyvitamin D) 3
- The active form of vitamin D (calcitriol) controls calcium absorption in the small intestines through a vitamin-D-regulated pathway 2
- Calcitriol is the most active known form of vitamin D3 in stimulating intestinal calcium transport 3
- Without proper kidney function to activate vitamin D, dietary calcium absorption is significantly impaired 3
Physiological Role of Activated Vitamin D
- Activated vitamin D (calcitriol) functions by:
- The kidneys of patients with renal disease cannot adequately synthesize calcitriol, resulting in hypocalcemia and secondary hyperparathyroidism 3
- When vitamin D is inadequate, the body uses calcium from skeletal stores, which can weaken existing bones 2
Comparison with Other Options
- Atrial natriuretic peptide hormone: primarily regulates sodium excretion and blood pressure, not calcium absorption 1
- Erythropoietin: produced in the kidneys but stimulates red blood cell production, not calcium metabolism 1
- Serum creatinine: a waste product filtered by the kidneys, not involved in calcium regulation 1
Clinical Implications
- In chronic kidney disease, the inability to activate vitamin D leads to impaired calcium absorption 3
- Vitamin D supplementation may be necessary when kidney function is compromised 5
- For those at risk of low calcium, consumption of 1500 mg/day of calcium is recommended to optimize bone health 1
- Calcium intake should be spread throughout the day as the gut cannot absorb more than 500 mg at once 2
Molecular Mechanisms
- Calcitriol binds to specific receptors in intestinal cells to stimulate calcium transport 4
- This binding causes transcription of specific genes that code for calcium transport proteins 4
- Calcium binding protein (calbindin) has been identified as being vitamin D dependent 4, 6
- Renal vitamin D receptor expression is required for 1,25-dihydroxyvitamin D3-induced renal reabsorption of calcium 7
The activation of vitamin D in the kidneys is a crucial step in calcium homeostasis, highlighting the important role the kidneys play in maintaining proper calcium balance throughout the body 8.