Extra end-of-summer sunshine might mitigate the flu season that follows, according to a new study (National Bureau of Economic Research, Feb., 2018). Scientists found that when a state gets 10 percent more sunlight in August and September, the severity of the flu season in fall and winter drops by three points. The flu scale index, maintained by the CDC, has 10 points altogether.
What Is the Link Between Summer Sunshine and the Following Flu Season?
The most likely explanation for this correlation is that additional sunlight boosts vitamin D stores before winter begins. Other research suggests that higher levels of vitamin D are associated with a better immune response to infections, such as colds or flu. Humans produce vitamin D in skin exposed to sunlight. When there is little sunshine, they have to get vitamin D from other sources, such as supplements, mushrooms or cod liver oil.
Sunshine Vitamin Helps Fight Flu:
The old wives maintained that a dose of cod liver oil would do a body good. Many children dreaded it because it tasted so awful. But come the dark days of winter, mothers and grandmothers insisted that all family members should hold their noses and swallow a spoonful of cod liver oil.
Over the last couple of decades, this practice has gone the way of the manual typewriter. Few children get cod liver oil these days. Doctors don’t recommend it because it seems like such an unscientific relic of the past. The vitamin D that is abundant in cod liver oil has numerous health benefits though, especially in the winter. That’s because levels of vitamin D frequently drop when people are not exposing their skin to the sun.
Cold, dreary weather and diminished sunlight can create borderline vitamin D deficiency in a surprising number of people. In Boston, 42 percent of people studied had too little vitamin D in winter. In Calgary, Canada, almost no one maintains adequate vitamin D in the winter.
Low Vitamin D Levels and Susceptibility to Influenza:
In 2005, a psychiatrist who treated his patients for vitamin D deficiency noticed something odd. Influenza hit the Atascadero State Hospital, a maximum-security psychiatric hospital, hard. His ward was spared, with not a single person catching the flu even though they had been exposed to the virus just like everyone else. The psychiatrist wondered whether the vitamin D he had prescribed had anything to do with their immunity. This question led to an interesting review of research and a credible hypothesis.
Studies over the last 70 years hint at a connection between vitamin D and overall immunity. The active form of vitamin D greatly increases the body’s production of a natural infection-fighting chemical called cathelicidin. Cathelicidin seems to help fight off illnesses caused by bacteria, fungi and viruses, including influenza.
Colds and Flu Don’t Flourish in Summer Sunshine:
This may help explain why people are more susceptible to colds and flu in the winter. If their vitamin D levels drop, so does their production of cathelicidin and their overall resistance to infection.
Vitamin D also appears to have anti-cancer activity. People who get regular sun exposure are less susceptible to common cancers that affect the colon, breast, prostate, ovaries and lungs. Even conditions like multiple sclerosis, arthritis and type 2 diabetes are less common in people with ample vitamin D levels.