Vitamin D and Osteoporosis
Vitamin D is inseparable from bone health. Its active hormonal form, 1,25-dihydroxyvitamin D (calcitriol), drives intestinal calcium absorption, and low serum 25(OH)D triggers a compensatory rise in parathyroid hormone (PTH) that accelerates bone resorption — the classic pathway to osteoporosis and fragility fractures.
The mechanism in three steps
- Calcium absorption. Calcitriol upregulates calbindin, TRPV6, and PMCA1b — the intestinal proteins that move dietary calcium into the bloodstream. Without adequate 25(OH)D, only ~10–15% of dietary calcium is absorbed, versus 30–40% in sufficiency.
- PTH regulation. When serum ionised calcium falls, the parathyroid glands secrete PTH, which mobilises calcium from bone. Chronic vitamin D insufficiency causes secondary hyperparathyroidism, a sustained low-grade drain on bone.
- Osteoblast/osteoclast balance. Vitamin D also acts directly on bone cells — supporting osteoblast maturation and modulating osteoclast activity to maintain the mineralised matrix.
What the trials show
- Hip fracture prevention. Meta-analyses of trials in adults ≥65 show ~15% reduction in hip fracture risk with ≥800 IU/day cholecalciferol plus calcium 1,000–1,200 mg/day (Bischoff-Ferrari 2012). Vitamin D alone shows smaller effects; the combination is what carries the evidence.
- Non-vertebral fractures. Same combination: ~14% reduction. Lower doses (400 IU/day) are consistently ineffective — dose matters.
- Bone mineral density (BMD). Effects on DXA are modest but real. Correcting deficiency in older adults typically adds 1–2% BMD at the femoral neck over 2 years.
- Falls prevention. Daily 700–1,000 IU cholecalciferol reduces fall risk by ~19% in adults ≥65, mediated by better lower-limb strength and postural control.
Practical protocol
For osteoporosis prevention or as adjunct to bisphosphonate/denosumab therapy, current consensus is:
- Serum 25(OH)D target: ≥ 30 ng/mL (75 nmol/L).
- Cholecalciferol 800–2,000 IU/day (adjust based on 25(OH)D level and body weight; heavier individuals need more).
- Dietary calcium 1,000 mg/day (women ≤ 50 and men ≤ 70) or 1,200 mg/day (older). Supplements only to fill gaps; get calcium from food when possible.
- Retest 25(OH)D at 3 months to confirm target level is reached.
- Address other bone-loss drivers: smoking, excessive alcohol, low body weight, sarcopenia, chronic glucocorticoid use.
Special situations
Post-bariatric-surgery patients, celiac disease, and inflammatory bowel disease all reduce vitamin D absorption and typically require higher doses (2,000–5,000 IU/day). Glucocorticoid users need supplementation to counter accelerated 25(OH)D catabolism and independent bone-loss effects. Chronic kidney disease impairs the final activation step (1α-hydroxylation) — these patients often need activated vitamin D analogues (calcitriol, paricalcitol) rather than plain cholecalciferol.