Vitamin D2 and D3 are the two relevant major forms of vitamin D, a group of fat-soluble secosteroids. The term vitamin D when cited alone refers to one or both of these secosteroids. Vitamin D can be absorbed into the body in two ways. Firstly, produced in the skin of vertebrates after exposure to UV-B light from the sun, and secondly through the consumption of vitamin D rich foods. In countries with lower levels of sun exposure, the production of vitamin D enhanced foods such as milk, flour and margarine is commonplace.
Vitamin D: Functions in the Body
Vitamin D is converted to calcidiol – a prohormone, which is then circulated and converted in either the kidneys or the immune – by monocyte-macrophages – into calcitriol, the active bodily form of vitamin D. When synthesis occurs in the immune system by way of monocyte-macrophages, the calcitriol acts as a cytokine which defends the body against microbial intruders. When synthesis occurs in the kidneys, calcitriol circulates as a hormone, whereby it regulates the balance and concentration of phosphate and calcium in the blood, helping the growth and remodeling of bone.
Vitamin D: Recommended Daily Intake
A new RDI table was issued on November 30th 2010 by the Institute of Medicine. The new RDI assumes little to no sun exposure and is based on up to date modern research studies.
New RDI Figures:
- 0-12 months: 400 IU/d
- 1-70 years of age: 600 IU/day
- 71+ years of age: 800 IU/day
- Pregnant/lactating: 600 IU/day
The upper level intakes for vitamin D are:
- 0-6 months of age: 1,000 IU
- 6-12 months of age: 1,500 IU
- 1-3 years of age: 2,500 IU
- 4-8 years of age: 3,000 IU
- 9-71+ years of age: 4,000 IU
- Pregnant/lactating: 4,000 IU
See Vitamin D Rich Foods to reach the adequate level of vitamin D while the winter.
Vitamin D: Risk of Overdose
Sun exposure is highly unlikely to cause a vitamin D overdose because after 20 minutes of sun exposure the concentrations of vitamin D precursors produced in the skin reach a maximum level, and any further vitamin D that is produced is degraded. However, sustained ingestion of vitamin D at 1250mg per day over a period of 7 months can produce overt toxicity. Over ingestion by those with vitamin D sensitive conditions are at risk of developing hypocalcaemia.
Institute of Medicine : Dietary Reference Intakes for Calcium and Vitamin D