The usage of vitamin A in fighting infection has an agelong tradition.(1) Multiple clinical trials conducted in the first half of the 20th in connection with vitamin A observed a decrease in the amount and severity of infections in most cases. However, with the appearance of antibiotics, vitamin A usage fell from popularity. It had a recent comeback by the end of the century, especially in developing countries, where current studies confirmed its efficiency against various infections.(2) Vitamin A is a vital part of the immune system, as it is necessary for the proper function of various immune cells.(2)

An indispensable duo: vitamins A and D make a superb pair 

Sir Edward Mellanby, a British biochemist and nutritionist was the first to recognise vitamin A as a potential weapon in the fight against infections. The widespread use of vitamins A, D and K is due to his trailblazing work on fat-soluble vitamins. In his experiments he used cod liver oil which, in those days contained some vitamin D3 beside vitamin A. Several experiments also used cod liver oil to restore vitamin A. While using vitamin A on its own was often unsuccessful, thanks to the trace amounts of D3 found in cod liver oil he achieved noticeably better results.(1)  

I.G. Spiesman recognized the benefits of the joint use of vitamins A and D3 as well. He published a research paper in 1941 which argued that while using vitamins A and D3 on their own was ineffective against the common cold even in large doses, their joint usage significantly decreased the symptoms of the illness.(3) 

How could this be possible? 

It was much later, only in the 21st century that this synergy came to be fully understood. Both vitamins constitute the foundation of immunity. However, vitamin A is essential to the correct functioning of vitamin D while it can help prevent calcification, a very rare side effect of vitamin D overdose.(4, 5) If vitamin A is not combined with vitamin D, it becomes less effective. Taking vitamins A and D alone can even be counterproductive, as vitamin A can decrease the levels of vitamin D.(6)  

A study published in 2020 further confirmed this. While the joint usage of vitamins A and D3 significantly decreased the risk of stroke and inflammation and increased the levels of vitamins A and D3 found in the body, taking the two vitamins on their own made them less effective and decreased the levels of the other.(7) 

Vitamins A and D have an eventful history. In the 21st century, as one rises in popularity, the other falls. Although we can read several negative reviews about vitamin A, the negative effects are countered if one pays attention to also supplement vitamin D, and consumes Vitamin A in it’snatural form. 

How effective is vitamin A for preventing infections?  

The aforementioned Sir Edward Mellanby was the first to employ vitamin A in the treatment of a postpartum infection, which used to be so severe that fatality was at 90%. A large dose of vitamin A cured the patient in 5 out of 5 cases. This outstanding achievement inspired further research, which almost unanimously concluded that taking vitamin A in the form of cod liver oil decreases the number of infections, the severity of the illness and shortens the period of sickness in the case of upper respiratory diseases.(1) Eventually, millions had been using cod liver oil daily across Europe and the USA to prevent infections before antibiotics gradually thrust it into the background.  

More modern and reliable studies in developing countries have found it efficient against measles, mumps, malaria and childhood diarrhoea.(2) These studies focused on vitamin A in isolation, not combined with vitamin D. As many infectious diseases attack the digestive system and consequently impede the absorption of vitamin A, a large dose of vitamin A is recommended for 6-month-old to 5-year-old children to prevent vitamin A deficiency. One might think that vitamin A deficiency only exists in developing countries today, but people in western countries don’t consume enough vitamin A rich foods either. It is arguable that many people would experience improving health if they increased their vitamin A intake.