Photo/Illutration Sunlight, which comes from behind the “Tower of the Sun,” left, at the Expo '70 Commemorative Park in Suita, Osaka Prefecture, is important for good health. (Kenji Tamura)

Sunbathing in Japan at this time of year might sound a little extreme, so simply spending more time outdoors exposed to ultraviolet rays on a daily basis could help as protection against the novel coronavirus, health experts say.

Just taking a walk with areas of the skin exposed offers immense health benefits because vitamin D, naturally produced in the body with the help of ultraviolet rays, improves a person's immune system.

Experts recommend spending at least 30 minutes, and as much as two and a half hours in sunshine, depending on where one lives.

Of course, the normal caveat applies: Prolonged exposure to strong rays, aside from resulting in a tan, can cause skin blemishes and skin cancer.

Dietary supplements appear to be less helpful in combatting the pandemic, the experts note.

“Even more serious cases (of COVID-19) may occur once winter sets in properly,” said Tetsuya Mizoue, head of the Department of Epidemiology and Prevention of the National Center for Global Health and Medicine, which has been treating numerous COVID-19 patients.

Research shows that viruses tend to spread more easily at lower temperatures and in a dry climate. Under those conditions, smaller amounts of vitamin D are generated during the coldest winter months.

A succession of reports indicates that a lack of vitamin D could result in a higher risk of infection and progression to flu-like COVID-19.

A survey of 489 patients published in the online September edition of the sister magazine of the Journal of American Medical Association showed that individuals with lower vitamin D levels are 1.77 times more likely to contract the virus than those with sufficient vitamin D levels.

A separate research team in Spain in October found that 82 percent of 197 hospitalized COVID-19 patients displayed insufficient healthy vitamin D readings. Such low levels were recorded in only about 47 percent of healthy individuals.

Scientists in Ireland and elsewhere in May published their findings on the relationship between blood vitamin D levels in Europeans and the fatality rate from COVID-19.

The results showed that patients in Italy and Spain with lower blood levels of vitamin D had higher mortality ratios than Norway and Finland, where larger amounts of vitamin D were detected among those tested.

Vitamin D deficiency raises the risk of serious health issues developing because the substance helps to keep the immune functioning normally to protect the body against viruses and other problems.

In severe COVID-19 cases, an excessive immune response known as a cytokine storm emerges, triggering an assault on the patient’s natural defense system.

Although British research suggests no link between vitamin D and COVID-19 infection, Mizoue warns against jumping to quick conclusions based on his extensive studies of the issue and the significance of the substance.

“A final conclusion has yet to be reached, but one should at least be careful not to suffer from a lack (of vitamin D),” he said.


Given that ultraviolet rays can cause skin cancer, cataracts, skin blemishes and wrinkles, it is important to judge the appropriate time spent outdoors in wintry sunshine for better health.

The strength of ultraviolet rays can change depending on area, so time spent outdoors should vary according to parts of the country where a person lives.

“Information on Vitamin D Synthesis / Erythemal UV” is available on the website of the National Institute for Environmental Studies at (

A range of studies suggest than adults require 15 micrograms of vitamin D daily.

Fish and mushrooms are known to be rich in vitamin D. Given that 5.5 micrograms of the vitamin are estimated to ingested daily through food, ultraviolet rays from the sun are required to provide the other necessary 10 micrograms of vitamin D.

The website explains how long people should expose themselves to sunlight daily in different zones of Japan.

For example, in Sapporo, the capital of the northernmost main island of Hokkaido, 250 minutes is sufficient if one’s face and backs of both hands are not covered with clothing in mid-December. For Yokohama, Nagoya, Higashi-Osaka in Osaka Prefecture and Miyazaki, the ideal times are 40 minutes, 60 minutes, 60 minutes and 30 minutes, respectively.

The data was calculated based on the assumption that people do not wear face masks outdoors. But if they do and observe social distancing, there should be no problem if they remove their coverings.

The Web page also shows upper limits: 350 minutes for Sapporo, 100 minutes for Yokoyama, 150 minutes for Nagoya, 130 minutes for Higashi-Osaka and 70 minutes for Miyazaki.

“People should go out for a stroll or do other kinds of activities during daytime so they catch the sun's rays in winter,” said Hideaki Nakajima, a chief researcher at the NIES’ Center for Global Environmental Research.

Aggressive attention is also recommended to ensure a sufficient daily intake of vitamin D via foodstuffs. The substance, detected in large amounts in seafood, totals 32 micrograms in 100 grams of chum salmon, according to the science ministry’s standard tables of food composition in Japan.

Raw Pacific saury and yellowtail contain 14.9 micrograms and 8 micrograms, respectively. Animal meat features little or no vitamin D.

As salted salmon slices are a mainstay of supermarket offerings across Japan, care needs to be taken against consuming too much salt. Doing so increases blood pressure and raises the risk of a cerebral stroke and other disorders, especially during winter.

“Soaking salmon in sake before cooking will reduce its salt level,” said Yukie Yanagisawa, a food study professor at Wayo Women's University in Ichikawa, Chiba Prefecture, close to Tokyo. “Do not add salt before grilling.”


Consumers may prefer dietary supplements to taking a stroll in the sun or relying on food, but no data exists that suggests doing so reduces the risk of infection or more serious complications from COVID-19.

Few surveys support the notion that supplements are effective in controlling other diseases.

For these reasons, Kazutoshi Nakamura, a professor of environmental and preventive medicine at Niigata University, who researches correlations between health and vitamin D, said other methods are better.

“Taking supplements is basically not recommended, except in cases where one has to shy away from exposure to ultraviolet rays,” Nakamura said.