Takeaways

• 15-20 minutes of sun.
• Oily fish, egg yolks.

Summary

• Obtained from diet or synthesized in the skin.
  • After exposure to UV B light.
  • Vitamin D levels fluctuate seasonally.
  • Reliance on dietary sources of vitamin D is greatest during the winter.
• UV B radiation.
  • Wavelengths of 290–315 nm.
  • Affected by latitude, altitude, air quality, cloud cover, time of day, time of year.
  • Affected by sunscreen, clothing.
  • Affected by skin pigmentation, age.
• Diet.
  • Oily fish, egg yolks.
  • Animal products (muscle, liver, fat, and kidney).
• Various forms of vitamin D circulate in the body.
  • Within certain boundaries.
  • Regulate vitamin D activity and signaling.
• Classical actions of vitamin D:
  • Promote calcium homeostasis.
  • Promote bone health.
• Additional benefits.
  • Functioning of immune, cardiovascular, and reproductive systems.
  • Many tissues have vitamin D responsive elements.
• For instance, immunologic effects.
  • Ability to maintain tolerance and to promote protective immunity.
  • Lower levels of vitamin D associated with higher levels of infection.
  • Through a number of pathways, vitamin D:
    • Reduces production of inflammatory cytokines.
    • Increases production of anti-inflammatory cytokines.
    • Avoids over-stimulation (autoimmune diseases).
    • Avoids under-stimulation (spread of infection).
• Vitamin D deficiency linked to various illnesses:
  • Diabetes, multiple sclerosis, and several cancers.
  • Susceptibility to infection and autoimmune diseases.
• Vitamin D supply to tissues important at specific times of life.
  • Older persons (reducing susceptibility to chronic degenerative diseases).
  • Fetus (organogenesis).
  • Childhood and adolescence (bone mineral accrual and somatic growth).
• Intervention trials have rarely shown benefits of vitamin D supplementation as treatments or preventive measures.