Vitamin D, mostly known for its role in helping the body absorb calcium and thus contributing towards bone health, has far more to offer. As more studies stream in and shed new light on various health benefits of vitamin D, we now know that vitamin D plays an incredible role in regulating the immune system. And part of this credit goes to vitamin D’s ability to stimulate the production of natural antibiotics in the body.
Yes, you heard us right. Vitamin D, which is essentially a steroid hormone, works as a natural antibiotic and destroy disease causing micro-organisms such as viruses and bacteria. In fact, those critically ill are found to have seriously depleted levels of vitamin D. It is because your body needs and uses a lot of this natural antibiotic and restorative hormone during illness.
How does Vitamin D work as an antibiotic?
Vitamin D stimulates the production of naturally occurring antibiotics. The magic lies in how vitamin D3 typically works in the body. When your body converts the vitamin D that you get from either sunlight exposure or supplements into the active form, the active vitamin D now binds to vitamin D receptors or VDRs.
These VDRs are present in every cell and tissue – whether it is the immune cells, heart, lungs, muscles or intestines – a fact that underlines the diverse role vitamin D3 plays in the body. So, what happens now is that vitamin D activates VDRs and these receptors now become attached to the DNA and regulate the expression of several genes. The genes, as we know, code for all types of proteins and enzymes, responsible for different processes and functions in the body.
As it turns out, vitamin D activated receptors also influence genes that make antimicrobial peptides such as such as cathelicidin and β defensin 2. These proteins work as antibiotics and help the body fight a range of infections including respiratory infections and tuberculosis.  .
The ability of vitamin D to stimulate the synthesis of these natural antibiotics partly explains why it plays such an important role in boosting the immune system. In fact, this incredible property is so consequential that “it has been conserved through (many millions) of years of evolution and is shared only by primates, including humans - but no other known animal species.” 
More effective than vaccines?
Vitamin D has been found more effective than anti-viral drugs and vaccines when it comes to preventing the flu. The study by Japanese scientists showed that vitamin D3 supplementation at 1,200 IU per day reduced the risk of seasonal influenza A in children during winter. 
A 2017 study found that Vitamin D protects against acute respiratory infections such as colds and flu. According to the researchers involved in this study, this could be the first definitive proof that vitamin D is effective against respiratory infections. This review of 25 randomised controlled trials also found that the supplementation route works best for those who had the lowest levels of vitamin D and also when supplementation is given daily or weekly rather than in doses that are spread apart. 
As reported by Science Daily, “Vitamin D -- the 'sunshine vitamin' -- is thought to protect against respiratory infections by boosting levels of antimicrobial peptides -- natural antibiotic-like substances -- in the lungs. Results of the study fit with the observation that colds and 'flu are commonest in winter and spring, when levels of vitamin D are at their lowest. They may also explain why vitamin D protects against asthma attacks, which are commonly triggered by respiratory viruses.” 
There is an increasing awareness that not all infections can be treated with antibiotics. While these drugs are only effective against ‘some’ bacterial infections and totally unsuccessful in treating viral infections, most doctors are still known to indiscriminately prescribe antibiotics for all sorts of infections.
This not only leads to several health repercussions such as impaired immunity; the trend has contributed to the ‘world-wide’ emergence of superbugs (bacteria that have developed strong resistance against antibiotics). In fact, a 2016 study revealed that a dose of antibiotic put children at an increased risk of developing some kind of drug-resistant infections in the months that follow. 
Maintaining healthy vitamin D levels may be your best bet and provide a far safer and better protection against infections than antibiotics; especially in respiratory infections such as asthma, influenza, colds and wheezing disorders.
- Adrian F Gombart. The vitamin D–antimicrobial peptide pathway and its role in protection against infection. Future Microbiology. 2009 Nov; 4: 1151.doi: 10.2217/fmb.09.87
- Youssef et al. Antimicrobial implications of vitamin D. Dermatoendocrinol. 2011 Oct-Dec; 3(4): 220–229. doi: 10.4161/derm.3.4.15027.
- Key feature of immune system survived in humans, other primates for 60 million years. OREGON STATE UNIVERSITY
- Mitsuyoshi Urashima, Takaaki Segawa, Minoru Okazaki, Mana Kurihara, Yasuyuki Wada, and Hiroyuki Ida. Randomized trial of vitamin D supplementation to prevent seasonal influenza A in schoolchildren. Am J Clin Nutr. 2010 May;91(5):1255-60. doi: 10.3945/ajcn.2009.29094. Epub 2010 Mar 10.
- Martineau et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ, 2017
- University of Queen Mary London. "Vitamin D protects against colds and flu, finds major global study." ScienceDaily. ScienceDaily, 16 February 2017.
- A Bryce et al. Global prevalence of antibiotic resistance in paediatric urinary tract infections caused by Escherichia coli and association with routine use of antibiotics in primary care: systematic review and meta-analysis. BMJ 2016; 352 :i939