A growing body of international research shows that vitamin D deficiency significantly increases the risk of colds, flu, and other seasonal respiratory infections.

An undeniable link between low levels of the vitamin and susceptibility to seasonal upper respiratory infections like colds and flu has been repeatedly demonstrated, and recent research suggests that maintaining optimum levels of the vitamin is one of the best and most reliable means of reducing risk.

Moreover, new Japanese studies suggest that high blood levels of D vitamin may reduce the risk of upper respiratory infections even more effectively than flu vaccines and anti-virals.

The Japanese study, which was conducted by Tokyo's Jikei School of Medicine, involved several hundred school children. Half of the children were given vitamin D supplements of 1200 IU per day and half were given placebos. After just two months it was shown that the children who were not taking the supplements came down with colds and flu more than twice as often as those who were getting extra vitamin D.

The study findings noted that the supplemental vitamin D was at least as effective as specific-strain flu vaccines and anti-virals in reducing the risk of infection.

These findings echo those of many previous studies, including a research project reported in the February 2009 issue of the Archives of Internal Medicine. That project, which studied more than 19,000 US residents of various ages, found that those with the lowest levels of Vitamin D were at roughly 40% higher risk for contracting respiratory infections including flu.

The link between low vitamin D levels and severe seasonal flu is also the premise of an ongoing study by the Public Health Agency of Canada. The PHAC study, which began in 2008 as a survey of vitamin's effect on all types of flu, was adapted in 2009 to include a particular focus on the H1N1 or Swine Flu virus. Researchers said the study, which is expected to continue through three full flu seasons, is looking at both genetic factors and blood levels of the vitamin in hopes of determining a means of preventing serious outbreaks of influenza in the future.

Vitamin D's effectiveness as a flu fighter is largely attributed to its role in the natural production of cathelicidin, a specific antibacterial peptide that is crucially important to protecting the body against respiratory disease. In addition to being a potent bacteria killer, cathelicidin is also known to play an important role in all immune system functions.

D vitamin is also an effective anti-inflammatory that inhibits the excessive production of TFNa, an inflammatory cytokine associated with flu and other respiratory infections.