I-30. Non-osteoporotic bone diseases; extraskeletal effects of vitamin D deficiency

非骨粗鱬症性骨疾患；ビタミンD欠乏の骨外作用

Vitamin D Reference

Serum vitamin D = 25(OH)D; target 30 ng/mL.
Daily need: adult 1500–2000 IU; child 1000 IU; obese 3000–4000 IU (healthy diet + ~20 min daily sun).
~5% of elderly have extremely low 25(OH)D (<10 ng/mL).

Renal Osteodystrophy

CKD → impaired phosphate excretion:
- Hyperphosphatemia → PO₄³⁻ binds Ca²⁺ (↓free Ca²⁺, ectopic calcification) → non-occluding media-sclerosis = LV hypertrophy + ↑pulse pressure (worse with long-term dialysis).
- Hyperphosphatemia → ↑PTH secretion.
- Suppresses vitamin D + causes parathyroid vitamin D resistance (can’t suppress PTH).
↓vitamin D activation → ↓intestinal Ca²⁺ → hypocalcemia → ↑PTH (secondary hyperparathyroidism) → cardiac hypertrophy.

Tumor-Associated Hypercalcemia

Tumors produce osteolytic hormones: PTH, RANKL, PTHrP.
PTHrP (e.g. breast cancer) → hypercalcemia + generalized osteolysis.
Local osteolytic hypercalcemia: bone metastases with direct osteolysis (e.g. prostate cancer → lumbar vertebrae).

FGF23 in Pathology

From osteocytes; regulates PO₄³⁻ (stimulated by hyperphosphatemia).
CKD: ↑FGF23 → strongly associated with mortality + CVD (LV hypertrophy, vascular calcification).
Other: inhibits PTH secretion, induces cardiac hypertrophy.
Hyperphosphatemia + low calcitriol + low bone density → suspect high FGF23 (genetic FGF23-inactivation failure; tumor-induced osteomalacia releasing FGF23).

Vitamin D Deficiency

Overt deficiency uncommon; subclinical deficiency very common (no specific symptoms, but long-term → ↑osteoporosis/fractures).
Causes: ↓cutaneous production with age/northern latitudes; low dietary intake in elderly; obesity (↑dilution volume).
Symptoms: mild (15–20 ng/mL) asymptomatic (PTH ↑ in 40%, ↑bone loss/fracture risk); severe → ↓intestinal Ca/phosphate absorption → hypocalcemia → osteomalacia + secondary hyperparathyroidism.
Vitamin D toxicity: calcitriol without Ca²⁺ salts shifts toward resorption → hypercalciuria/kidney stones, osteoporosis, hypercalcemia.

Extra-skeletal / Non-endocrine Effects

Malignancy: vitamin D deficiency → ↑breast, colorectal, prostate cancer. Anti-tumor effects: inhibits proliferation/invasion, induces differentiation/apoptosis, enhances chemotherapy.
Non-endocrine deficiency effects: ↑depression, ↑systemic autoimmune disease (RA, SLE, T1DM), ↑infections.
Immune: VDR on activated macrophages/T/B cells; vitamin D inhibits adaptive response (↓IFN-γ, TNF-α), stimulates innate response.
SARS-CoV-2: low vitamin D → more severe COVID-19; modulates immunity → ↓cytokine storm; obese + low vitamin D → higher risk; inverse correlation with complications.

一問一答

▶What is the target serum vitamin D level and how is it measured?

Serum vitamin D is measured as 25(OH)D, with a target of about 30 ng/mL.

▶What is renal osteodystrophy?

Bone disease from CKD: impaired phosphate excretion → hyperphosphatemia and reduced vitamin D activation → hypocalcemia → secondary hyperparathyroidism with disordered bone metabolism.

▶How does hyperphosphatemia in CKD cause cardiovascular damage?

PO₄³⁻ binds Ca²⁺ (↓free Ca²⁺, ectopic calcification) → non-occluding media-sclerosis → LV hypertrophy and ↑pulse pressure (worsened by long-term dialysis).

▶Why is the parathyroid resistant to vitamin D in CKD?

CKD suppresses vitamin D and induces parathyroid vitamin D resistance, so calcitriol can no longer suppress PTH → persistent secondary hyperparathyroidism.

▶What hormones drive tumor-associated hypercalcemia?

Tumor-produced osteolytic hormones: PTH, RANKL, and PTHrP.

▶What is the role of PTHrP in malignancy-associated hypercalcemia?

PTHrP (e.g., in breast cancer) mimics PTH → hypercalcemia with generalized osteolysis.

▶What is local osteolytic hypercalcemia?

Hypercalcemia from bone metastases causing direct local osteolysis (e.g., prostate cancer involving the lumbar vertebrae).

▶Why is FGF23 important in CKD?

↑FGF23 is strongly associated with mortality and cardiovascular disease (LV hypertrophy, vascular calcification).

▶When should high FGF23 be suspected?

With the combination of hyperphosphatemia, low calcitriol, and low bone density (e.g., genetic FGF23-inactivation failure or tumor-induced osteomalacia releasing FGF23).

▶What are common causes of vitamin D deficiency?

Reduced cutaneous production with age/northern latitudes, low dietary intake (especially in the elderly), and obesity (increased dilution volume).

▶What are the consequences of severe vitamin D deficiency?

↓Intestinal calcium/phosphate absorption → hypocalcemia → osteomalacia and secondary hyperparathyroidism.

▶How can vitamin D toxicity paradoxically harm bone?

Calcitriol without adequate Ca²⁺ salts shifts metabolism toward resorption → hypercalciuria/kidney stones, osteoporosis, and hypercalcemia.

▶How is vitamin D linked to malignancy?

Deficiency increases breast, colorectal, and prostate cancer risk; vitamin D inhibits proliferation/invasion, induces differentiation/apoptosis, and enhances chemotherapy.

▶How does vitamin D modulate the immune system?

VDR is on activated macrophages/T/B cells; vitamin D inhibits the adaptive response (↓IFN-γ, TNF-α) and stimulates the innate response.

▶What non-endocrine conditions are associated with vitamin D deficiency?

Increased depression, systemic autoimmune disease (RA, SLE, T1DM), and infections.

▶How is vitamin D related to COVID-19 severity?

Low vitamin D is associated with more severe COVID-19; it modulates immunity to reduce the cytokine storm, and obese patients with low vitamin D are at higher risk.

▶Why does hyperphosphatemia stimulate PTH in CKD?

High phosphate binds calcium (lowering free Ca²⁺) and directly stimulates parathyroid PTH secretion.

▶Why is subclinical vitamin D deficiency clinically important despite few symptoms?

It is very common and, long-term, increases osteoporosis and fracture risk (PTH is elevated in ~40% at mild deficiency).

▶What are the daily vitamin D requirements for adults, children, and obese individuals?

Adults 1500–2000 IU, children 1000 IU, and obese individuals 3000–4000 IU (plus a healthy diet and ~20 min daily sun).

▶How does FGF23 affect PTH and the heart?

It inhibits PTH secretion but also induces cardiac hypertrophy.