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The connection between flu and vitamin D is complex, and scientists are still studying the issue. Vitamin D is reputed to be useful in preventing influenza as well as reducing the severity and duration of flu in those who become ill. This is thought to be due to some of the ways that vitamin D is used by the body. Studies suggest that people who have low, or deficient, levels of vitamin D are more likely to develop flu and other illnesses than those with adequate levels from sun exposure or supplementation. The research suggests that the lack of sun exposure needed for the body to manufacture vitamin D in the winter compared the summer may be the reason colds and flu are much more common in the colder months.
Research shows that there is a multifaceted connection between flu and vitamin D. Studies have shown that vitamin D supplementation in the winter months and by people who avoid sun exposure may be useful for the prevention of several illnesses including the flu. Extremely high doses of vitamin D for a period of three days at the onset of flu symptoms may also help to make the symptoms less severe and shorten the duration of the illness.
The flu and vitamin D connection is also thought, in part, to be from some of the its action in the body. In addition to helping the body absorb calcium which is vital for bone health, it also supports immune system functions. It stimulates the body to produce proteins called antimicrobial peptides. These proteins are one of the body's defenses against illness, and they are able to rapidly destroy the cell walls of many bacteria, fungi, and viruses including the ones responsible for causing flu. Research has shown that people with deficiencies of vitamin D are much more likely to become ill when exposed to cold and flu viruses than those with adequate amounts in their systems from supplementation or sunlight exposure.
Some scientists suggest that the connection between flu and vitamin D goes even further. It is difficult to acquire the recommended levels of vitamin D from diet alone, but the body is able to manufacture it when exposed to direct sunlight. The flu season is typically during the winter when there are less daylight hours available, the sun's rays are weaker, and cold weather drives many people indoors. It has been suggested that the flu season may actually be caused by diminishing vitamin D levels in large percentages of the population at this time. Research is ongoing on this interesting theory.