How to get vitamin D
There are two ways vitamin D can get into our body: (1) through our GIT (gastrointestinal trac) and (2) by synthesization inside your body.
A major portion of vitamin D (estimated around 80% but varies with sun exposure) is synthesized endogenously in the skin.
The rest of vitamin D is absorbed thorugh food. The best known sources of vitamin D are the liver oil and flesh of fatty fish, such as salmon, tuna, mackerel, sardines, cod. In smaller quantity, some foods that contain vitamin D includes:
- Beef (cow) liver.
- Mushrooms.
- Egg yolks.
- Milk and dairy products (fortified)
- Cereals (fortified)
- Juice (fortified)
Functional levels of vitamin D are estimated to be $30ng/ml$, measured in the plasma. To reach such level, a recommendend daily allowance (RDA) is specified.
The problem is that the bioavailabity of vitamin D seems to be quite delicate. The bioavailability is defined as fraction of ingested amount which actually ends up in the systemic circulation. In our current understanding, this fraction is the product of three fractions: (1) the fraction of vitamin D which is released from the food matrix into the gastric juice; (2) the fraction of vitamine D which is transported through the intestinal epithelium; (3) the fraction of vitamine D that reaches the blood circulation without being metabolized. This is just to show that is it is complex, and we have no fucking idea how to control it.
Treatment of vitamin D deficiency
It has been observed that eating more D-rich foods is not usually sufficient to correct vitamin D deficiency. Therefore, the standard procedure is taking supplements.
The supplements may be given weekly or daily. The adequate dose depends on multiple factors.