Ergocalciferol

Ergocalciferol (vitamin D2) is made by plants and is similar to Cholecalciferol (vitamin D3), but it’s less effective for humans. D3 is better at raising vitamin D levels and has stronger effects in the body, while D2 binds less efficiently to the vitamin D receptors.
Studies have shown that D3 is more effective at improving vitamin D stores and lowering parathyroid hormone. Though some medications use high doses of D2, newer research suggests D3 supplements are a better choice. Plants like mushrooms and algae produce D2 when exposed to UVB light, but D2 levels are much lower than D3 found in fatty fish, for example.
In summary, D2 is similar to D3 but provides less bang for the buck.

Vitamin D

Vitamin D, technically a hormone, is made in the skin when UVB rays convert a cholesterol compound into D3 (Cholecalciferol, or Calciol). Many people don’t make enough due to lack of sun exposure, so we rely on food (like fatty fish) or supplements. Plants provide D2 (Ergocalciferol), but D3 works better in our bodies.

D3 has three key forms:

Calciol: Stored in fat for long-term use; this is the form we produce or ingest.
Calcifediol: Formed in the liver for short-term storage, circulating in blood and commonly tested to check D3 levels.
Calcitriol: The active form, made in the kidneys, with a short lifespan but crucial for biological effects.

We’ll be using the terms Calciol → Calcifediol → Calcitriol throughout future discussions.