If you take 1200 IU vitamin D3 every day, you can halve your chance of getting influenza. Japanese epidemiologists at Jikei University School of Medicine reported this in the American Journal of Clinical Nutrition.
Vitamin D and flu
During the winter people in the northern hemisphere have less vitamin D in their blood than in the summer. We don’t need to explain why this is so. Scientists suspect that lower vitamin D levels are one of the causes of the ‘flu waves’ that start in November and continue until the spring.
If you get flu, it’s usually the work of one of the variants of the influenza A virus. Influenza A is also usually more aggressive than other flu viruses.
The researchers decided to test this theory. If vitamin D is indeed a relevant factor in contracting flu or not, they reasoned, then supplementation with vitamin D during the winter should offer protection against influenza A virus.
The researchers did trials with two groups each of 167 schoolchildren aged between 6 and 15. One group took a placebo and the other 1200 IU vitamin D3 daily from 15 December to 31 March. The dose is equivalent to 30 mcg vitamin D3.
The children who took vitamin D only succumbed half as much to the influenza A virus as the children in the placebo group. In addition, the supplement protected the schoolchildren against asthma attacks. The incidence of asthma attacks was a factor six lower for the children who took vitamin D than for the children in the placebo group.