What is Vitamin D (Vit D)?

Vitamin D: A Steroid Hormone Precursor Essential for Health

Vitamin D is not a classic vitamin but a steroid hormone precursor that plays critical roles in calcium homeostasis, bone health, immune function, and the regulation of hundreds of genes throughout the body. 1

Structure and Sources

• Vitamin D exists in two main forms: Ergocalciferol (vitamin D2) and Cholecalciferol (vitamin D3) 2
• Primary sources include:
  • Endogenous synthesis in the skin through UV-B exposure (cholecalciferol) 1
  • Dietary sources such as fatty fish, eggs, and some mushrooms, though dietary intake alone is usually insufficient 1
  • Supplements (vitamin D2 or D3) 2

Metabolism and Function

• After production in the skin or ingestion, vitamin D undergoes two hydroxylation steps:
  • First in the liver to 25-hydroxyvitamin D (25-OHD) 1
  • Then in the kidneys to the active form 1,25-dihydroxyvitamin D (calcitriol) 1
• The vitamin D receptor (VDR) is expressed in numerous tissues including:
  • Bone, intestine, and kidneys (classic target organs) 1
  • Muscle (skeletal and cardiac), immune system, skin, and endocrine organs 1
• Once activated, vitamin D functions as a steroid hormone that binds to VDRs, enters the nucleus, and regulates the expression of over 1,200 genes 3

Physiological Roles

• Calcium and Bone Metabolism:
  • Enhances intestinal calcium and phosphorus absorption 4, 5
  • Works with parathyroid hormone to maintain calcium homeostasis 2, 5
  • Essential for proper bone mineralization and skeletal health 5
• Immune System Regulation:
  • Modulates immune response and inflammation 1, 3
  • May help prevent autoimmune diseases 1
• Muscle Function:
  • Supports muscle strength and balance 2, 6
  • Deficiency contributes to neuromuscular dysfunction 6
• Other Systems:
  • Involved in cell growth and differentiation 6
  • Affects cardiovascular, nervous system, and metabolic functions 1, 3

Vitamin D Status Assessment

• Serum 25-hydroxyvitamin D (25-OHD) is the recognized biomarker for vitamin D status 1
• The Institute of Medicine defines vitamin D status as:
  • Adequate: >50 nmol/L (>20 ng/mL) 1, 2
  • Inadequate: 30-50 nmol/L 1
  • Deficient: 12.5-<30 nmol/L 1
  • Very deficient: <12.5 nmol/L 1
• LC-MS/MS is considered the gold standard methodology for measurement 1

Clinical Implications of Deficiency

• Bone Diseases:
  • Rickets in children and osteomalacia in adults (severe deficiency) 1, 7
  • Contributes to osteopenia and osteoporosis 7, 5
  • Increases fracture risk 6, 5
• Neuromuscular Effects:
  • Muscle weakness and pain 7, 6
  • Increased risk of falls, especially in the elderly 6, 8
• Other Health Impacts:
  • May increase susceptibility to infections 3, 8
  • Associated with increased risk of certain chronic diseases 3, 8

Recommended Intake

• Daily oral intake recommendations:
  • 600-800 IU for general adult population 1
  • 1,500-4,000 IU for those at risk for vitamin D deficiency 1
  • 1,000-2,000 IU for female athletes, depending on sun exposure 1, 2
• Upper daily limit is generally 4,000 IU, though the Endocrine Society suggests up to 10,000 IU may be safe for at-risk populations 1

Common Pitfalls and Considerations

• Vitamin D status shows strong seasonal variation due to changes in sun exposure 1
• Factors increasing risk of deficiency include:
  • Living at northern/southern latitudes (>35th parallel) 1
  • Indoor lifestyle or aggressive sun protection 1
  • Dark skin pigmentation 1
  • Obesity (fat sequestration of vitamin D) 3
• Calcium intake should be spread throughout the day as the gut cannot absorb more than 500 mg at once 1, 2
• Vitamin D works synergistically with vitamin A to improve immune function 1