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BASICS

  • Vitamin D is a fat-soluble vitamin and hormone essential for calcium homeostasis and bone metabolism.
  • Synthesized in skin (vitamin D3) via UVB exposure; vitamin D2 obtained from diet/supplements.
  • Metabolized in liver to 25-hydroxyvitamin D (calcidiol), then in kidney to active 1,25-dihydroxyvitamin D (calcitriol).
  • Active vitamin D increases intestinal calcium and phosphorus absorption, reduces renal excretion, and stimulates bone resorption.

EPIDEMIOLOGY

  • Prevalence varies geographically, higher in northern latitudes and during winter months.
  • Risk groups include infants, children, adolescents, elderly, individuals with dark skin pigmentation, cultural clothing coverage, obesity, and chronic diseases.
  • Pediatric vitamin D deficiency is common; many children fail to meet recommended levels.

ETIOLOGY AND PATHOPHYSIOLOGY

  • Causes: insufficient sunlight, low dietary intake (fatty fish, fortified foods), malabsorption (celiac, Crohn’s, liver disease).
  • Obesity sequesters vitamin D in fat, reducing bioavailability.
  • Medications affecting metabolism: anticonvulsants, glucocorticoids, weight-loss drugs.
  • Chronic kidney disease impairs conversion to active form.
  • Genetic disorders: Vitamin D-dependent rickets type I (1α-hydroxylase mutation).
  • Increased vitamin D breakdown in liver disease and alcoholism.

RISK FACTORS

  • Limited sun exposure due to lifestyle or cultural practices.
  • Dark skin pigmentation.
  • Obesity.
  • Malabsorption syndromes.
  • Certain medications.
  • Pregnancy and lactation increasing vitamin D demand.

PREVENTION

  • Adequate sun exposure and dietary intake.
  • Recommended daily allowance (RDA):
  • Infants (0-12 months): 400-1000 IU/day
  • Children/adolescents (1-18 years): 600-1000 IU/day
  • Adults (19-70 years): 600-800 IU/day
  • Older adults (>71 years): 800-1000 IU/day
  • Pregnant/lactating women: 600-800 IU/day
  • AAP recommends 400 IU/day vitamin D for all breastfed infants starting within first days of life.

COMMONLY ASSOCIATED CONDITIONS

  • Rickets and osteomalacia.
  • Premenstrual syndrome.
  • Osteoporosis.
  • Chronic renal disease.
  • Hypertension.
  • Increased risk of myocardial infarction and all-cause mortality with deficiency.

DIAGNOSIS

History

  • Assess risk factors: limited sun, dark skin, malabsorption, chronic illness.
  • Symptoms: muscle weakness, bone pain, fractures, tetany, seizures.
  • Consider demographic risk: elderly, pregnant, housebound.

Physical Exam

  • Muscle weakness, gait abnormalities.
  • Signs of hypocalcemia: Chvostek sign, Trousseau phenomenon.
  • Bone deformities (in children): bowed legs.

Tests

  • 25-hydroxyvitamin D (25-OH vitamin D) level: most sensitive test.
  • Deficiency defined as <20 ng/mL (IOM), some guidelines suggest <30 ng/mL.
  • Parathyroid hormone (PTH): elevated in secondary hyperparathyroidism.
  • Serum calcium, phosphorus (low/normal), alkaline phosphatase (elevated in osteomalacia).
  • Radiographs may show pseudofractures or Looser zones in osteomalacia.

TREATMENT

  • Vitamin D supplementation:
  • Adults: 800 to 2000 IU/day vitamin D2 or D3.
  • For deficiency: 50,000 IU weekly for 8-12 weeks followed by maintenance dosing.
  • Calcium intake via diet preferred; supplementation if dietary intake inadequate.
  • Monitor levels and clinical response.
  • Treat underlying causes such as malabsorption.

PEDIATRIC AND PREGNANCY CONSIDERATIONS

  • Breastfed infants require supplementation (400 IU/day).
  • Pregnancy safe supplementation up to 1000-2000 IU/day.
  • Maternal deficiency affects neonatal stores.

ONGOING CARE

  • Monitor clinical symptoms and vitamin D levels as indicated.
  • Encourage lifestyle modifications for sun exposure and diet.
  • Avoid excessive supplementation to prevent toxicity.

PROGNOSIS

  • Vitamin D repletion improves symptoms and biochemical abnormalities.
  • Adequate levels linked to decreased fracture risk, cardiovascular events, and mortality.
  • Ongoing deficiency leads to bone deformities, fractures, and muscle weakness.

COMPLICATIONS

  • Hypocalcemic tetany, seizures.
  • Bone deformities and fractures.
  • Vitamin D toxicity with hypercalcemia if overdosed.

REFERENCES

  1. Bjelakovic G, Gluud LL, Nikolova D, et al. Vitamin D supplementation for prevention of mortality in adults. Cochrane Database Syst Rev. 2011;(7):CD007470.
  2. Zhang Y, Fang F, Tang J, et al. Association between vitamin D supplementation and mortality: systematic review and meta-analysis. BMJ. 2019;366:l4673.
  3. LeBoff MS, Chou SH, Murata EM, et al. Supplemental vitamin D and incident fractures in midlife and older adults. N Engl J Med. 2022;387(4):299-309.

ICD10

  • E55 Vitamin D deficiency
  • E55.0 Rickets, active
  • E55.9 Vitamin D deficiency, unspecified

Clinical Pearls

  • 25-OH vitamin D is the preferred test for vitamin D status.
  • Vitamin D deficiency is common in dark-skinned, obese, and housebound individuals.
  • Supplementation up to 2000 IU/day is generally safe in adults.
  • All breastfed infants should receive 400 IU/day vitamin D starting within days of birth.