Vitamin D is both a vitamin and a hormone precursor. It is unique because the body can synthesize it from sunlight exposure. Deficiency is a global epidemic.

Functions:

• Calcium absorption and bone mineralization
• Immune system regulation
• Muscle function
• Cell growth regulation
• Mood and mental health

Deficiency prevalence: ~1 billion people worldwide are deficient or insufficient. Especially common in northern latitudes, darker skin tones, and people who work indoors.

Sources:

• Sunlight — UV-B converts 7-dehydrocholesterol in skin to vitamin D3
• Fatty fish (salmon, mackerel)
• Fortified foods (milk, cereals)
• Supplements (D3 is preferred over D2)

Testing: 25-hydroxyvitamin D blood test. Optimal level: 40–60 ng/mL.

Reference:

Wikipedia: Vitamin D

https://en.wikipedia.org/wiki/Vitamin_D