Ampath Chat 44.1 – Vitamin D Overview
Authors: Dr Thanusha Reddy & Dr Marita du Plessis
Published: August 2025
Introduction
Overt vitamin D deficiency, such as osteomalacia or rickets, is now rare in developed countries. However, subclinical vitamin D deficiency—manifesting as low bone mass, muscle weakness, and increased risk of falls and fractures—is more common, especially in the elderly. This is due to declining vitamin D stores and reduced skin conversion capacity with age.
The prevalence of deficiency varies depending on the threshold used. In the U.S., 23% of adults have levels below 20 ng/ml. In South Africa, a study at Tygerberg Hospital found 41% of results below 15 ng/ml. Another study in Johannesburg showed 28.6% of Indian adults had levels below 12 ng/ml, compared to just over 5% in Black Africans.
While vitamin D may benefit immune and cardiovascular health, its most consistent benefit is for skeletal health.
Sources of Vitamin D
- Primary source: UVB sunlight exposure, influenced by season, latitude, skin pigmentation, clothing, and sunscreen use.
- Dietary sources: Fatty fish, fortified foods (milk, infant formula, cereals), especially important when sun exposure is limited.
Vitamin D Metabolism
- Synthesized in the skin from 7-dehydrocholesterol via UV exposure.
- Converted to vitamin D3 (cholecalciferol), then to 25-hydroxyvitamin D in the liver, and finally to the active form 1,25-dihydroxyvitamin D (calcitriol) in the kidneys.
- Regulated by parathyroid hormone (PTH), phosphate levels, and fibroblast growth factor 23 (FGF23).
- Degraded by 24-hydroxylation and hepatic metabolism.
Vitamin D Status Definitions
- Deficiency: <12 ng/ml
- Insufficiency: 12–19.9 ng/ml
- Sufficiency: ≥20 ng/ml (adequate for bone health)
- Optimal for older adults: ≥30 ng/ml (to reduce fracture risk)
Upper limits are debated. While 100 ng/ml was once considered safe, recent guidelines suggest 50–60 ng/ml due to risks of mortality and disease. Levels above 88 ng/ml may cause hypercalcaemia and hypercalciuria.
Recommended Intake
Assuming minimal sun exposure and adequate calcium intake:
- Children & adults ≤70 yrs: 600 IU/day
- Adults >70 yrs: 800 IU/day
- Pregnant/lactating women: ≥600 IU/day
- High-risk groups (obese, malabsorption, certain medications): Higher doses required
Causes of Deficiency or Resistance
- Inadequate intake or sunlight
- Liver/kidney dysfunction
- Drug-induced metabolism (e.g., phenytoin, rifampicin)
- Renal loss of vitamin D-binding protein
- Genetic resistance (e.g., hereditary rickets)
Clinical Manifestations
- Mild deficiency: Often asymptomatic
- Severe deficiency (5–8 ng/ml): Hypophosphataemia, hypocalcaemia, secondary hyperparathyroidism, osteomalacia, rickets
- Recommended tests: Calcium, phosphorus, ALP, PTH, electrolytes, renal function
High-Risk Groups for Screening
- Elderly (especially institutionalized)
- Limited sun exposure
- Dark skin, sunscreen use
- Obesity
- Osteoporosis
- Certain medications
- Malabsorptive diseases
- Pregnant women with risk factors
Treatment Guidelines
- Severe deficiency (<12 ng/ml): 25,000–50,000 IU weekly for 6–8 weeks or 6,000 IU daily, then 800 IU daily maintenance
- Insufficiency (12–19.9 ng/ml): 800–1,000 IU daily
- Sufficiency (≥20 ng/ml): 600–800 IU daily
- Pregnancy: 1,000–2,000 IU daily (monitor urinary calcium)
Follow-up testing is recommended 3–4 months after starting therapy. Lack of response may indicate malabsorption or other conditions.
Special Considerations
- Use vitamin D3 (cholecalciferol) when available.
- Avoid high-dose intermittent IM injections due to fracture risk.
- Monitor calcium levels in patients with hyperparathyroidism or granulomatous disease.