Can sunshine help to keep you healthy during flu season?

Researchers have evidence this might be the case. A new working paper distributed by The National Bureau of Economic Research, found higher sunlight levels are linked to less severe flu seasons.

The authors of the paper used two primary types of data to assess the relationship between relative sunlight levels and incidence of influenza.

They looked at sunlight data from the North American Land Data Assimilation System, which covers the 48 contiguous states and the District of Columbia.

They also looked at the flu index released by the U.S. Centers for Disease Control and Prevention (CDC). The index aggregates data from state health departments and harmonizes the aggregate data to a 10-point scale.

“What we found was that higher relative levels of sunlight lead to lower relative levels of influenza,” David Slusky, PhD, lead author of the paper and assistant professor of economics at the University of Kansas, told Healthline.

For example, when sunlight levels were 10 percent higher than average in September, it corresponded to a three-point reduction in the influenza index during the same month.

The relationship between sunlight and influenza levels was particularly strong during the late summer and early fall, Slusky said. This is the one time of the year when flu season and sunlight levels are both high enough that researchers can look for a link.

“In the late summer, you still have potentially large amounts of sunlight, and therefore a lot of room for variation. You’re also starting to see flu season around the country, and so we have enough flu activity for it to matter,” he explained.

In comparison, during sunny summer months there’s not enough flu activity to see a strong relationship between sunlight exposure and flu infection in the data.

And while there’s a lot of flu activity in December, there’s not enough sunlight to find a link between the two.

Vitamin D may help protect you from the flu

This study contributes to a growing body of research that links vitamin D to influenza risk.

When ultraviolet radiation in sunlight hits bare skin, it triggers the production of vitamin D.

You can also get vitamin D from certain foods, but it’s difficult to get enough of this essential nutrient from dietary sources alone, so researchers have been looking at the benefits of supplements.

In recent decades, several research teams have studied the potential effects of vitamin D supplementation on the risk of influenza and other acute respiratory tract infections.

In an article published last year in BMJ, researchers reported the results of a meta-analysis of individual participant data collected through randomized control trials on this topic.

“In just under 11,000 patients in 25 clinical trials, we showed a modest but highly statistically significant reduction in acute respiratory tract infections with vitamin D supplementation,” Adrian R. Martineau, PhD, lead author of the meta-analysis and a professor at Barts and The London School of Medicine and Dentistry, told Healthline.

“Overall, we saw a modest effect, but this effect became quite large when we drilled down and just looked at the effects of giving a daily or weekly supplement to people who had low vitamin D levels to start with,” he continued.

People who started out with low baseline levels of vitamin D were about half as likely to contract an acute respiratory tract infection when they took daily or weekly vitamin D supplements.

In comparison, the protective effects of vitamin D supplementation were smaller for people who started out with higher baselines levels of vitamin D.

It’s important to find the right balance of sunlight exposure

Too much sunlight exposure can put people at risk of skin damage and raise their chances of developing skin cancer. On the other hand, too little sunlight exposure can increase their chances of developing vitamin D deficiency.

Further studies on sunlight exposure and vitamin D supplementation can help guide public health efforts to reduce flu and other infections without increasing the risk of skin cancer, Slusky told Healthline.

“Many physicians are arguing that people need to get their vitamin D levels up, both through supplementation and through sunlight,” Slusky said.

“There’s some point where the benefit of an extra bit of sunlight is greater than the cost,” he said. “I think one of the roles of public health is to find that balance.”

Remember, the amount of time you need to spend outside to get enough vitamin D varies from person to person. It depends on a variety of factors including where you live, the time of year, the weather, and your skin tone.

People who live farther from the equator tend to get less sunlight exposure than those who live in tropical areas, which puts them at higher risk of vitamin D deficiency.

Black Americans and other people with darker skin are at heightened risk because their skin pigment melanin blocks ultraviolet radiation.

If you suspect that you might not be getting enough vitamin D, speak with your doctor.

In some cases, they might encourage you to spend more time outside or take a vitamin D supplement.

But aside from vitamin D levels, there are clear ways to protect yourself from flu infection.

The CDC recommends getting vaccinated, washing your hands regularly, and avoiding contact with people who are sick.

“Flu vaccination, hand washing, not being in contact with people when they’re most contagious — all of those things work together,” Slusky said. “Now, we are just adding one more piece about vitamin D levels and sunlight exposure.”