Calling vitamin D a vitamin is a scientific misnomer. Unlike true vitamins, your body synthesises vitamin D endogenously — through the action of UVB radiation on 7-dehydrocholesterol in the skin. It then undergoes two hydroxylation steps (in the liver and kidneys) to become the biologically active form, calcitriol. This is the definition of a hormone. And like other hormones, virtually every cell in your body has receptors for it.
What Vitamin D Actually Does
The bone-related actions of vitamin D — regulating calcium and phosphate absorption to prevent rickets and osteoporosis — are well established. But the discovery of vitamin D receptors in immune cells, brain neurons, cardiac muscle, pancreatic beta cells, and hundreds of other tissues has revealed a far broader role:
- Immune modulation — activates macrophages and T cells; critical for fighting respiratory infections
- Mood regulation — required for serotonin synthesis; deficiency consistently associated with depression
- Insulin sensitivity — pancreatic beta cells require vitamin D for normal insulin secretion
- Cardiovascular health — low vitamin D correlates with increased blood pressure and cardiac disease risk
- Cancer prevention — emerging evidence for protective effects in colorectal, breast, and prostate cancers
Why Deficiency Is So Common
The UVB wavelengths that stimulate vitamin D synthesis are only present in sunlight at certain times of day and year — and are completely blocked by sunscreen SPF 15+, glass, and cloud cover. Indoor lifestyles, office work, and appropriate sun protection against skin cancer create conditions in which vitamin D synthesis is severely limited even in sunny climates.
How to Know If You're Deficient
A 25-hydroxyvitamin D blood test is the gold standard. Optimal levels for health are 75–150 nmol/L. Below 50 nmol/L is deficient; below 25 nmol/L is severely deficient. Most people require 2,000–5,000 IU of vitamin D3 daily to maintain optimal levels, with cofactors magnesium and vitamin K2 to ensure proper calcium metabolism.
