Updated: Nov 27, 2020
Winter is here and so is the peak of the cold and flu season. Most of us don't get adequate sunlight, which means most of us are low or deficient in vitamin D (1). There's no need to worry with emerging evidence showing just how strong this broad spectrum nutrient is when it comes to strengthening our immune system and protecting us from upper respiratory tract infections and the dreaded flu (2,3).
Vitamin D is technically a vitamin on steroids. I say that because chemically it has a steroid hormone backbone and affects the entire body like a hormone (4), from the immune system, bones, brain, reproductive organs, skin, cardiovascular system, and has even been shown to decrease intestinal inflammation (5). Let's just say it's not your average vitamin, and with all that being said it may come as no surprise that vitamin D has recently been studied for it's ability to fight off colds and the flu (5).
How Does Vitamin D Boost Our Immune System?
Vitamin D plays a key role in activating our immune cells, mainly white blood cells (WBCs) and the cells that communicate with them. Vitamin D receptors have been found on B cells, T cells and antigen presenting cells and these immune system cells are all are capable of synthesizing the active vitamin D metabolite, showing it's vital importance (6).
Our immune system has what's called an innate and adaptive response. The innate response is not specific and is automated as a first line of defense internally when alarms are sounded that tell our immune cells to go into kill or repair mode. The adaptive response is very specific and has the capability of tagging and building memory of the foreign invaders so they can be eliminated quicker if they come back. Vitamin D has the ability to enhance both of these systems and we find an increased risk of infection and higher rates of autoimmunity in people who are vitamin D deficient (6,7).
The beneficial effects of vitamin D on protective immunity are due to its effects on the innate immune system. It is known that macrophages (WBCs that swallow debris and bacteria) recognize lipopolysacharide (LPS - bacterial toxin), a required marker for bacterial infection, through toll-like receptors (TLR - special receptors on macrophages that detect proteins on bacteria). Once TLRs are activated this leads to a cascade of events that produce peptides (small proteins) with potent bacterial killing proteins like cathelocidin and beta-defensins. When the ingested bacteria is engulfed in a shelf called a phagosome these peptides drill holes in the bacterial cell membranes and tear them apart. Because vitamin D acts as a hormone it influences the genes of WBCs when it binds to the cell's vitamin D receptor. This binding sends signals to the cells genetic machinery to start producing more cathelocidin and beta-defensin proteins which allow WBCs to ramp up their chemical defenses (6-8).
Vitamin D Innate Immune Mechanism 1: The above diagram shows how vitamin D moves Helper T cells (Th) in the immune system away from autoimmunity by suppressing Th1 and Th17, promotes Th2 activity which is how our body produces antibodies to tag and destroy infectious bugs, and promotes T Regulatory cells growth and function. Tregs (CD4+ T cells) build immune tolerance, suppress hyperactive immunity, and help suppress excessive inflammatory responses (6).
Vitamin D Innate Immune Mechanism 2: This diagram shows that vitamin D decreases the ability of monocytes (large WBC that kills bacteria and repairs damaged tissue) and dendritic cells (show WBCs proteins of bacteria) to produce inflammatory chemicals called cytokines, and stops these cells from replicating and becoming overactive, which would result in greater inflammation (6).
Vitamin D not only boosts our immunity internally, and also strengthens our body's ability to keep things out. Once again the hormone activity is important in upregulating genes (via the 1a-hydroxylase enzyme) which then codes for proteins (like occludin) that keep cells locked together tightly in our intestines and membranes that line our throat and nose, keeps gap junctions between skin cells and intestine cells strong, and strengthens the glue-like proteins (E-cadherin) that play a key role in barrier strength (9).
Clinical Studies and Reviews Show Vitamin D Reduces Colds and Flu
A high-quality review study that combined 25 randomized controlled trials analyzed nearly 11,000 people between the ages of 0 and 95 years and found that daily doses of vitamin D supported respiratory tract health and reduced the incidence of common seasonal infections. The study reported that this equates to one person being spared a respiratory tract infection (RTI) for every 33 taking vitamin D supplements. In the US this means that just under 10 million Americans could avoid at least one upper respiratory tract infection per year. For those taking vitamin D for a week or a month at a time they saw a reduced risk of upper respiratory infections, such as the flu, by 12%. The individuals who took vitamin D daily or weekly saw the greatest benefits, with a 19% reduction. Those who had low blood levels of vitamin D (< 25nmol/L), and supplemented daily or weekly saw a 70% reduction of infections (10).
Vitamin D Recommendations and Testing
We get vitamin D from our diet or it is synthesized it in the skin after exposure to UVB light, however its synthesis is influenced by latitude, season, diet, body mass index, use of sunblock and skin pigmentation. Melanin absorbs UVB radiation inhibiting the synthesis of vitamin D from 7-dihydrocholesterol, meaning the darker the skin the more vitamin D is required. Even after we get UV B light our body still has to enzymatically transform it to its active form. On a side note the enzymes that transform vitamin D to it's active form require magnesium. This initial vitamin D compound is inactive and it is next hydroxylated in the liver to form 25 OH vitamin D3 (25 D). This is also an inactive compound, but is the most reliable blood measurement of vitamin D status (11).
Vitamin D testing, which requires a simple blood sample, can help determine your specific vitamin D requirements. At Home Test Kit: https://daction.org/start
Here are some general daily dosing guidelines (12):
Generally healthy adults take 2,000 IU (50 μg) of supplemental vitamin D daily. Most multivitamins contain 400 IU (10 μg) of vitamin D, and single-ingredient vitamin D supplements are available for additional supplementation ranging from 2,000-5,000 IU (50-125 μg).
The American Academy of Pediatrics currently suggests that all infants, children, and adolescents receive 400 IU of supplemental vitamin D daily.
The Endocrine Society, recommends daily intakes of 400 to 1,000 IU (10 to 25 μg) of vitamin D in infants and 600 to 1,000 IU (15 to 25 μg) of vitamin D in children and adolescents.
Older adults (>50 years) Daily supplementation with 2,000 IU (50 μg) of vitamin D is especially important for older adults because aging is associated with a reduced capacity to synthesize vitamin D in the skin upon sun exposure.