Thanks to Aimee McNew at at PaleoHacks for this great article!
Move over, vitamin C—there’s a new flu buster in town
It turns out that vitamin D, also known as the sunshine vitamin, is every bit as important as its antioxidant counterparts whenever the immune system is under duress, like during a cold or flu.
If you’re one of those people who keeps getting sick and you don’t know why, read on. Here’s how this underestimated sunshine vitamin can help boost immunity and fight viral infections.
The Basics of Vitamin D
First, it’s important to understand that vitamin D isn’t a “vitamin” at all, but rather a prohormone that is synthesized in response to interaction between cholesterol, the skin, and sun exposure.
Vitamin D is widely studied for all kinds of health benefits, like bone health, calcium absorption, heart health, and immunity. [1-10] While experts differ on how much vitamin D is needed to provide protection and avoid symptoms of insufficiency or deficiency, nearly everyone agrees that you can’t get enough vitamin D from food alone.  This might explain why even if you’re sipping on that gut-healing bone broth, it might not be enough to keep you fully protected from the flu.
Why You Need Vitamin D to Boost Immunity
When you take vitamin D, you lower your chances of getting sick from viral infections, like colds and the flu, by at least 10 percent. But people who are deficient in vitamin D actually see a greater protective benefit when they supplement, reducing their risk by 50 percent. 
Most Americans get far less than the recommended daily allowance for vitamin D of 600 IU, setting the stage for rampant deficiencies. Certain people, like those with Crohn’s disease, celiac disease, osteoporosis, or women who are pregnant or breastfeeding, have an even higher risk of being deficient because of issues with absorption in the intestines and because of greater demands.
Cold and flu viruses are contagious respiratory infections that affect the nose, throat, and lungs with symptoms like congestion, coughing, wheezing, sore throat, body aches, and fever. The flu, in particular, can have serious consequences in people who are at risk: the elderly, infants, young children, people with suppressed immunity, and those with other chronic disorders. On any given year, approximately 10 percent of the population gets the flu, but during epidemic flu seasons, it can be as high as 20 percent. [12, 13] There are several overlapping factors that are associated to both the severity of the flu and being at risk of vitamin D deficiency, including:
- Age (under age 2 or over age 65)
- Chronic disease (diabetes, heart disease, autoimmunity, etc.)
- Suppressed immunity (HIV, AIDS)
Vitamin D Reduces Risk of the Flu
While flu vaccines are common, they only address a few strains based on the best guesses by experts of which strains will predominate. Sometimes experts guess wrong, rendering the flu vaccine less effective during those seasons. What doesn’t change, however, is the immune system’s reliance on vitamin D levels, making it a more secure way to boost immunity and avoid the flu. Research even shows that optimal vitamin D levels boost the effectiveness of the flu vaccine. 
One study even drew a direct link between vitamin D levels and the risk of getting the flu: Those with the lowest serum concentrations had the highest chances of getting viral infections. 
Receptors for vitamin D are located on cell surfaces in the immune system, allowing vitamin D to bind to them. When this happens, vitamin D reduces inflammatory cytokines that perpetuate illness. It also boosts the proteins that fight bacteria and viruses; which are essentially the natural versions of antibiotics that protect the body from infection.
Vitamin D boosts the innate branch of the immune system – the body’s first line of defense against infection – which mounts an attack against foreign invaders. The response time for this branch of the immune system is strongly associated with our ability to ward off illness or shorten the duration of sickness.  When vitamin D levels are low, immune cells respond slower and the innate immune response is not as rapid as it should be.
Even if you do get sick, increase your vitamin D levels immediately for faster recovery. When your levels are too low, your body will struggle longer to bounce back.
Research shows that for every 4 ng/mL increase in your vitamin D concentrations in the blood, you lower your odds of getting the flu by seven percent. It’s not coincidental that flu season peaks as our natural ability to produce vitamin D falls due to reduced sun exposure in the colder months. 
How to Supplement with Vitamin D
If you cannot get sun exposure regularly—as many Americans do not from September through March each year—then supplementation is essential to maintain steady levels. Vitamin D3 is considered to be the natural form and is most effective at raising serum concentrations. . Dosage recommendations range from 400 IU to upwards of 10,000 IU, depending on health conditions and other factors, like absorption. 
The Vitamin D Council suggests that adults take between 5,000 and 10,000 IU per day, depending on body weight. Children should take 1,000 IU daily per 25 pounds of body weight (up to 125 pounds). The best bet when it comes to supplementing with vitamin D is to have your doctor test your levels and advise you on the proper dosage for your health factors.
Vitamin D supplements absorb best when paired with fish oil or fatty foods to optimize absorption. 
Since vitamin D is a fat-soluble nutrient – meaning your body has limited ability to excrete it – levels can become toxic in the body with over-supplementation, so get your blood levels checked before supplementing. A healthy blood range is above 30 ng/mL, with insufficiency falling between 20 and 29 ng/mL, and deficiency anything below 20 ng/mL. 
Vitamin D is a critical nutrient for immune support. When the body is low in this nutrient, immune system cells will have slower response times to viral invaders, sickness duration will be longer, and overall immunity will be suppressed.
Vitamin D is one of the few nutrients that doesn’t have an abundance of food sources. We were designed to produce most of the vitamin D we need in the skin from sun exposure. But between food and supplements, as well as sun exposure in the warmer months, we can optimize our vitamin D levels for overall health.
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