Vitamin D overview and misconceptions

Summary

Vitamin D is a group of fat-soluble compounds that help the body absorb calcium and phosphate and support many other biological functions. In humans, the two main forms are vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol). Although commonly called a vitamin, vitamin D acts more like a hormone: skin exposure to UVB light can produce vitamin D3, and dietary intake, fortification, and supplements can also contribute. After intake or skin synthesis, vitamin D is converted in the liver to 25-hydroxyvitamin D [25(OH)D] and then further activated in the kidneys (and some immune cells) to calcitriol, the biologically active form that signals through the vitamin D receptor (VDR).

Key Points

• 1

  Vitamin D is conditionally essential because adequate UVB exposure can allow skin synthesis, but many people rely on diet/fortification/supplements due to limited sunlight.

• 2

  Serum 25(OH)D is commonly used to assess vitamin D status, but there is no universal agreement on what level is “deficient,” “insufficient,” or “optimal.”

• 3

  Common misconceptions include assuming vitamin D supplementation benefits everyone for many diseases; evidence is strongest for preventing/treating deficiency-related bone problems, while benefits for other conditions in already-sufficient people are not well proven.