The American Journal of Clinical Nutrition published a paper in 2007 that calls into question this low dose; one short exposure to sunlight produces 20,000 IU of vitamin D within forty-eight hours.
An oral supplement, therefore, should be much higher than 600 IU in order to have any real effect on the immune system. (5)
While it’s possible to overdose on vitamin D, single dosages of up to 50,000 IU have been found safe over short periods. (1)
The appropriate dose of vitamin D for optimal immune system function depends on age, skin type, and season of the year.
One study recommends a baseline of 2200 IU per day for adults during winter, dropping in the summer under the assumption that the balance will be achieved through direct exposure to sunlight. (6)
Other demographic groups (people with dark skin, the elderly, and obese people) may need more per day, up to 5000 IU. In any case—and almost universally—we all need more vitamin D.
Vitamin D deficiency is problematic for many reasons. It’s known that chronic deficiency causes muscle weakness, fatigue, chronic pain, respiratory problems, osteoporosis, hypertension, autoimmune diseases, cognitive decline, cardiovascular disease, depression, and cancer.
The amount of vitamin D you need is what it will take to get you to a blood level of seventy to ninety nanograms per milliliter (ng/ml). The only way to accurately determine vitamin D level is the 25-hydroxy-vitamin D blood test.
How Vitamin D Works In the Immune System
It’s complicated. Basically, vitamin D is involved in the process of gene activation to produce antimicrobial proteins in response to a foreign invader (e.g., virus). In addition, it moderates inflammatory cytokines (cell signaling molecules) and stimulates anti-inflammatory cytokines to keep the immune response in check.
Further, vitamin D balances B- and T-cells (white blood cells) to maintain balance of immune system function in response to normal cell life/decay/death and true foreign invasion. (7)
The pre-cursor to vitamin D lives in our cells. When skin is exposed to sunlight, the pre-D substance is activated and a chemical chain reaction occurs that creates vitamin D3, brings it through the liver and kidneys where it’s converted into other forms, then carried through the body to where it’s needed.
Vitamin D is critical for bone and muscle health as it interacts with calcium and magnesium. Along the way, the immune system is kept working at its best.
People who live at the equator are one thousand times less likely to come down with a cold or the flu than people in temperate and cold climates.
A lot of that has to do with exposure to sunshine, the best source of vitamin D. With at least ten percent of the body directly exposed (without chemical sunscreens), ten to twenty minutes of sunlight per day in spring/summer and sixty to one hundred-twenty minutes in winter is a general rule of thumb for adequate vitamin D. (8)
If you choose to take a natural supplement, make sure it’s of high quality with no added chemical preservatives. You can find a list of food sources for vitamin D here.