This article has been medically reviewed by Dr. Charles Penick, MD
Your immune system is your body’s armed force against pathogens, viruses, and bacteria. The lifestyle you lead essentially feeds of harms your immune system, and today we explore how vitamin D plays in supporting your immune system as a whole.
What is Vitamin D?
Vitamin D is a group of hormones (yes, hormones!) that are responsible for increasing intestinal absorption of calcium, magnesium, and phosphate, as well as many other biological effects.  Vitamin D has an impact on health markers like mood, weight, energy, and immune health. [2-5]
The three types of vitamin D are D1, D2, and D3, although D2 and D3 are the most important for human health. [1, 6]. Their role in promoting the bioavailability of minerals facilitates healthy bones, teeth, and muscles.
Vitamin D can be acquired topically through the sun’s synthesis or orally by consuming vitamin D rich foods or supplementation.
The Vitamin D Immune System Link
Vitamin D directly impacts immune modulation because it interacts with the cells that ward off and fight infections.  It plays such a crucial role in generating health for so many of the body’s systems. It has been used as a primary or accompanying treatment for various viral, bacterial, and fungal infections.
Conditions including tuberculosis, psoriasis, eczema, Crohn's disease, chest infections, wound infections, influenza, urinary tract infections, eye infections, and wound healing benefit highly from supplementation of vitamin D. 
Vitamin D supplementation may dramatically reduce the likelihood of catching the flu, as well as undergoing asthma attacks.  Extensive studies also highlight the connection between vitamin D deficiencies and respiratory tract infections like pneumonia, colds, and bronchitis. [9, 10]
Cold and flu season occurs during the colder winter months, unsurprisingly since our natural access to sunshine (free vitamin D) declines during the winter months. This is why topping up your vitamin D stores during the colder months can profoundly impact bolstering your immune system and preventing illness.
How to Get Vitamin D?
The sun is undoubtedly one of the most important ways to get vitamin D because of the wide-spectrum benefits that the sun provides. We synthesize vitamin D from a chemical reaction in the lower layers of the skin when exposed to UVB rays.  which is not always available even if the sun is out. UVB rays are most abundant during the peak hours of the day. Regions closer to the equator will have UVB available year-round, but the more dramatic the seasons, the less UVB will be available during the autumn/ winter (mostly) and spring.
Getting vitamin D from the sun requires exposing bare skin (no sunscreen) to the sun during the ‘UVB window.’ This requires a certain degree of sun safety awareness, knowing that every person has a different capacity to withstand the sun. You want to avoid burning the skin at all costs, which can damage your DNA. 
Certain foods contain vitamin D; they include oily fish (like salmon, sardines, and mackerel), red meat, liver, and egg yolks.  Food quality how much vitamin D is available; notably, you want to ensure animal products you consume are either pasture-raised or wild. The more time the animal spent outdoors consuming a naturally (wild or organic) diet, the healthier, more nutrient-dense it will be.
Supplementation is a great way to ensure your body gets enough vitamin D year-round. Although some foods are fortified with vitamin D, the best option is to get it from a high-quality supplement you trust. Fortified foods rarely contain high-quality vitamins, which are usually synthetic and not high potency or absorbency.
Some of the things to look out for to maximize your benefits when taking a vitamin D supplement include: 
- A vitamin D supplement that combines vitamin D2 with vitamin K2. The combination enhances the benefits and bioavailability of both nutrients.
- The vitamin K2 should be “MK-7”, which is a highly bioavailable type of K2.
- The vitamin D should be naturally derived, which more absorbable than a synthetic.
- The combination should be in an organic oil base (like olive oil) to promote bioavailability.
Quality matter. Since the supplement industry is notoriously unregulated, opting for a high-quality supplement can help ensure that you achieve the intended health-promoting goals instead of wasting your money or generating harm. Not all supplements are created equal!
How Much Vitamin D Do We Need?
Vitamin D is measured in terms of ‘IUs,’ which stands for International Units. They simply measure the potency or biological activity of a product; in this case: vitamin D.
Vitamin D recommendations vary wildly. MayoClinic and government daily recommended intakes advise 600 IUs per day for adults.  Some of the more conservative recommendations suggest taking a daily supplement containing 400 IUs of vitamin D throughout the year to ensure adequate levels. 
On the other end of the spectrum are the studies that highlight the positive impact on taking up to 4,000 IU of vitamin D on a daily basis. This dose can reduce the likelihood of developing upper respiratory tract infections. 
Indeed, suggestions for upper limit dosages vary wildly depending on who you ask. Although vitamin D deficiency is prevalent, getting too much vitamin D is extremely rare, so supplementation is a no brainer. 
Who Should Supplement Vitamin D?
Essentially, everyone. Certain groups are more susceptible to vitamin D deficiency; they include: [6, 15, 16]
- People who live far from the equator (areas with “winter” season)
- People who work indoors or are generally ‘homebodies’
- People who tend to cover up when outdoors
- People in an institution (like a care home)
- Immune-compromised people
- People who are overweight or obese
- People who have darker skin (for example, you have an African, African-Caribbean, or South Asian background) because more melanin synthesizes less vitamin D than lighter skin
- The elderly
Over a billion people are estimated to be vitamin D deficient or insufficient worldwide.  In America alone, 41.6% of adults are deficient.  Remember that these statistics are based on the bare minimum recommendation, not the numbers for optimal health.
Vitamin D is a hormone that can be synthesized topically from exposure to the sun or consumed orally via vitamin D rich foods or supplementation. It plays a crucial role in various bodily processes and is a big player in modulating the immune system. Since vitamin D deficiency is so common, it may be useful to take a vitamin D supplement, especially those in an ‘at-risk’ category.
Supplementation would ideally entail a D3 + K2 combination from a natural source and diluted in an organic oil base. Dosage recommendations vary wildly, but studies suggest that a higher supplementation dosage of 4,000 IUs daily can support the immune system in preventing respiratory infections, among other ailments.
Speaking of vitamin D, did you know that most supplements won’t do a thing to raise your levels —no matter how much you take? That’s because most store-bought pills and tinctures are packed with fillers and low-quality nutrients.
CytoD+K2, on the other hand, is 2X the potency of normal vitamin D and contains the ideal ratio of vitamin D to vitamin K2 for optimal absorption.
With just a dropper full per day, you can strengthen immunity, restore sleep, increase energy, improve skin health, and SO much more.
Medical Disclaimer: This article is based upon the opinions of Revelation Health. The information on this website is not intended to replace a one-on-one relationship with a qualified health care professional and is not intended as medical advice. It is intended as a sharing of knowledge and information from the research and experience of Revelation Health and associates. This article has been medically reviewed by Dr. Charles Penick, MD for accuracy of the information provided, but Revelation Health encourages you to make your own health care decisions based upon your research and in partnership with a qualified health care professional.
 Norman, Anthony W. “From Vitamin D to Hormone D: Fundamentals of the Vitamin D Endocrine System Essential for Good Health.” The American Journal of Clinical Nutrition, vol. 88, no. 2, 2008, doi:10.1093/ajcn/88.2.491s.
 Jorde, R., et al. “Effects of Vitamin D Supplementation on Symptoms of Depression in Overweight and Obese Subjects: Randomized Double-Blind Trial.” Journal of Internal Medicine, vol. 264, no. 6, 2008, pp. 599–609., doi:10.1111/j.1365-2796.2008.02008.x.
 Major, Geneviève C., et al. “Calcium plus Vitamin D Supplementation and Fat Mass Loss in Female Very Low-Calcium Consumers: Potential Link with a Calcium-Specific Appetite Control.” British Journal of Nutrition, vol. 101, no. 5, 2008, pp. 659–663., doi:10.1017/S0007114508030808.
 Aranow, Cynthia. “Vitamin D and the immune system.” Journal of investigative medicine : the official publication of the American Federation for Clinical Research vol. 59,6 (2011): 881-6. doi:10.2310/JIM.0b013e31821b8755
 Johnson, Kevin, and Maryam Sattari. “Vitamin D deficiency and fatigue: an unusual presentation.” SpringerPlus vol. 4 584. 7 Oct. 2015, doi:10.1186/s40064-015-1376-x
 Bikle, Daniel D. “Vitamin D metabolism, mechanism of action, and clinical applications.” Chemistry & biology vol. 21,3 (2014): 319-29. doi:10.1016/j.chembiol.2013.12.016
 Schwalfenberg, Gerry K. “A review of the critical role of vitamin D in the functioning of the immune system and the clinical implications of vitamin D deficiency.” Molecular nutrition & food research vol. 55,1 (2011): 96-108. doi:10.1002/mnfr.201000174
 Urashima, Mitsuyoshi et al. “Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren.” The American journal of clinical nutrition vol. 91,5 (2010): 1255-60. doi:10.3945/ajcn.2009.29094
 Pletz, Mathias W et al. “Vitamin D deficiency in community-acquired pneumonia: low levels of 1,25(OH)2 D are associated with disease severity.” Respiratory research vol. 15,1 53. 27 Apr. 2014, doi:10.1186/1465-9921-15-53
 Jat, Kana Ram. “Vitamin D deficiency and lower respiratory tract infections in children: a systematic review and meta-analysis of observational studies.” Tropical doctor vol. 47,1 (2017): 77-84. doi:10.1177/0049475516644141
 MacDonald, James. “How Does the Body Make Vitamin D from Sunlight?” JSTOR Daily, July 18AD, 2019, daily.jstor.org/how-does-the-body-make-vitamin-d-from-sunlight/.
 “What Happens to Your Skin When You Get Sunburnt?” Cancer Council, www.cancer.org.au/blog/what-happens-to-your-skin-when-you-get-sunburnt.
 Lamberg-Allardt, Christel. “Vitamin D in Foods and as Supplements.” Progress in Biophysics and Molecular Biology, vol. 92, no. 1, 2006, pp. 33–38., doi:10.1016/j.pbiomolbio.2006.02.017.
 van Ballegooijen, Adriana J et al. “The Synergistic Interplay between Vitamins D and K for Bone and Cardiovascular Health: A Narrative Review.” International journal of endocrinology vol. 2017 (2017): 7454376. doi:10.1155/2017/7454376
 “Office of Dietary Supplements - Vitamin D.” NIH Office of Dietary Supplements, U.S. Department of Health and Human Services, ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/.
 “Vitamin D.” NHS Choices, NHS Health,www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/.
 Bergman, Peter et al. “Vitamin D and Respiratory Tract Infections: A Systematic Review and Meta-Analysis of Randomized Controlled Trials.” PloS one vol. 8,6 e65835. 19 Jun. 2013, doi:10.1371/journal.pone.0065835
 “Vitamin D.” You and Your Hormones, Society of Endocrinology ,www.yourhormones.info/hormones/vitamin-d/.
 Sahota, Opinder. “Understanding vitamin D deficiency.” Age and ageing vol. 43,5 (2014): 589-91. doi:10.1093/ageing/afu104
 Forrest, Kimberly Y Z, and Wendy L Stuhldreher. “Prevalence and correlates of vitamin D deficiency in US adults.” Nutrition research (New York, N.Y.) vol. 31,1 (2011): 48-54. doi:10.1016/j.nutres.2010.12.001