Vitamin D helps regulate the immune system
Inflammation occurs as a normal consequence of the innate immune system response to pathogens (e.g., bacteria, viruses, fungi) and molecules that the immune system perceives to be foreign and autoimmune. When the immune system eliminates the invader, inflammation stops. Inflammation that persists is thought to be the cause of many chronic illnesses.
What causes the immune system to continuously create low-grade inflammation? That's the $64,000 question.
The immune system is modulated by a hormone we call vitamin D. Vitamin D was mistakenly named a vitamin when it was discovered in 1928. Vitamins are substances that the body cannot make; they must be ingested. Vitamin D is not simply a nutrient.
Vitamin D3 is produced in the skin when it is exposed to sunlight. This molecule is transported to the liver where it is used to produce calcidiol. This inactive form of vitamin D is transported to the kidneys where it is converted to the active form of vitamin D, called calcitriol. Calcitriol is a hormone that affects almost every cell in the body; it is the key to an effective immune system response to pathogenic invaders.
Doctors assess vitamin D status by measuring calcidiol, the inactive form. This is a simple and inexpensive lab test. Recently, studies have noted that people who are chronically ill often have a low level of calcidiol. This has led to the assumption that low calcidiol makes people ill and that healthy people with low calcidiol will become ill.
