Vitamin D has two main forms: ergocalciferol, or vitamin D2, and cholecalciferol, or vitamin D3. These are fat-soluble substances, which act as hormones after conversion to functional usage by the body. Most vitamin D3 is obtained through skin exposure to ultraviolet B radiation, typically from sunlight but also from sources such as tanning booths.
Vitamin D3 must be converted to its active form by both the liver and the kidneys. Impairment of either of these organs can cause vitamin D deficiency.
- Vitamin D regulates levels of calcium and phosphorous in the blood by assisting in their absorption from food in the intestines. It is essential for bone health.
Learn more about Vitamin C
Although the main source of Vitamin D3 is sunlight, smaller amounts can be gained from animal sources, particularly beef liver and fatty fish.
Producers of several food products began fortifying them with vitamin D2 and D3 after indoor work became more prevalent, and people often did not obtain enough sunlight. These foods include milk, yogurt, margarine, cereal and breads.
- Vitamin D availability from sunlight is strongest in spring and summer in temperate climates, and it can be difficult for people to get sufficient vitamin D year-round except in tropical climates. In addition, sunscreen prevents absorption.
Read more about Vitamin D at https://en.wikipedia.org/wiki/Vitamin_D