The researchers added that fewer respiratory tract infections mean lower antibiotic requirements for these patients.
Dr. Peter Bergman said:
"Our research can have important implications for patients with recurrent infections or a compromised immune defense, such as a lack of antibodies, and can also help to prevent the emerging resistance to antibiotics that come from overuse. On the other hand, there doesn't seem to be anything to support the idea that vitamin D would help otherwise healthy people with normal, temporary respiratory tract infections."
Vitamin D is a steroid vitamin which facilitates the metabolism and absorption of phosphorous and calcium. It is synthesized in the skin through sunlight exposure, and is also obtained through some foods.
People who are exposed to "normal" quantities of sunlight do not require vitamin D supplements, because their skin will synthesize enough for their needs.
Sweden, being very far from the equator, has abundant sunlight during the summer months and very little during the winter - there is a sharp seasonal variation of vitamin D in Swedish people's blood. Vitamin D deficiency is a serious problem in Scandinavia during the darker half of the year.
Prior studies had demonstrated that low blood levels of the vitamin are linked to a higher risk of infection. Experts say that vitamin D can activate a human's immune system. Vitamin D has been shown to help tuberculosis patients recover more quickly.
In this study, Bergman and team set out to determine whether treating patients with vitamin D might prevent and relieve respiratory tract infections - they were particularly interested in what the effects might be on patients who are prone to infections. They also wanted to find out whether the participants' use of antibiotics might be less.
Their study - a double-blind randomized controlled trial - involved 140 volunteers who had symptoms of disease in their respiratory tracts for at least 6 weeks before the study began. They were randomly selected into one of two groups:
- The high-dose vitamin D group - they were given Vitamin D3 (4000 IU) daily
- The placebo group
At the end of the study they found that:
- Those in the vitamin D group had a 25% drop in respiratory tract infection symptoms
- The people in the vitamin D group reduced their use of antibiotics by nearly half
This study is different, the researchers explained. While the New Zealand study worked with healthy volunteers whose blood vitamin D levels at the start of the study were normal, those in this study were not "healthy" volunteers. In the New Zealand study participants were given larger doses less often, which is known to be less effective than taking the vitamin every day.
Dr Anna-Carin Norlin, who was involved in this study, said:
"However, the most important difference is probably due to the fact that our participants had much lower initial levels of vitamin D than those in the New Zealand study. There is evidence from previous studies that vitamin D supplements are only effective in patients who fall well below the recommended level, which also suggests that it would be wise to check the vitamin D levels of patients with recurrent infections."
They concluded that patients with frequent respiratory tract infections who were given vitamin D3 supplements experienced reduced disease burdens.
A study published in Pediatrics (August 2012 issue) showed that the incidence of respiratory tract infections among Mongolian children dropped when they were given vitamin D supplements.