WEDNESDAY, Oct. 12 (HealthDay News) -- Vitamin D plays a critical role in the body's ability to fight off infections like tuberculosis (TB) -- a potentially fatal lung disease, according to a new study.

An international team of researchers found that vitamin D, which is a natural hormone, is linked to human immune reactions and might also help protect against cancer and autoimmune diseases.

In conducting the study, the investigators examined how the body manages to kill or stop the growth of pathogens such as M. tuberculosis, the bacteria that causes TB. They found the white blood cells, or T-cells, that are critical to the body's ability to fight off infections cannot function properly without adequate levels of vitamin D.

On the other hand, the study, published in the Oct. 12 online edition of the journal Science Translational Medicine, found there was an 85 percent drop in the TB bacteria when there was no vitamin D deficiency.

"Over the centuries, vitamin D has intrinsically been used to treat tuberculosis. Sanatoriums dedicated to tuberculosis patients were traditionally placed in sunny locations that seemed to help patients -- but no one knew why this worked," said study first author Dr. Mario Fabri, who conducted the research at University of California, Los Angeles and is currently at the department of dermatology at the University of Cologne, Germany. Sunlight is one source of vitamin D in humans, along with food sources and supplements.

"Our findings suggest that increasing vitamin D levels through supplementation may improve the immune response to infections such as tuberculosis," Fabri explained in a university news release.

Vitamin D plays an important role in supporting both the innate immune system (the immunity humans are born with) as well as adaptive immunity (immunity acquired over time following exposure to various pathogens), the study showed.

The researchers noted that this is the first study to show the protein interferon activates cells to kill the TB bacteria. They revealed that T-cells release interferon, which activates infected cells (macrophages) to generate cathelicidin and other proteins to kill TB and ensure these proteins are delivered directly to the part of the cell where the bacteria lives.

"The role of interferon has been speculated for years in numerous studies, but previous research didn't take into account that sufficient vitamin D was needed to help [interferon-gamma] trigger an effective immune response," study author Dr. John Adams, a professor of orthopaedic surgery at the UCLA David Geffen School of Medicine, pointed out in the news release. "Now we understand better how this chain reaction works."

"These current findings provide the first credible mechanistic explanation for how vitamin D critically contributes to acquired T-cell immunity that protects us from infections, particularly tuberculosis," added study senior investigator Dr. Robert Modlin, Klein Professor of Dermatology and Distinguished Professor of Medicine and Microbiology, Immunology and Molecular Genetics, and chief of dermatology, at the David Geffen School of Medicine, said in the news release.

The study authors pointed out that most people with TB do not have symptoms, possibly due to successful immune response and sufficient levels of vitamin D to keep the infection from progressing into active disease.

They also noted that people with darker skin are more susceptible to TB, which could be partly due to the fact that the skin pigment melanin, which is more abundant in darker skin, reduces vitamin D production.

"At a time when drug-resistant forms of tuberculosis are emerging, understanding how to enhance natural innate and acquired immunity through vitamin D may be very helpful," study co-author Barry Bloom, former dean of the faculty at the Harvard School of Public Health, Distinguished University Service Professor, Jack and Joan Jacobson Professor of Public Health, and department of immunology and infectious diseases and department of global health and population at Harvard, said in the news release.

The researchers added that more research is needed to determine if vitamin D supplements boost human resistance to TB and other infections.

More information

The U.S. National Institutes of Health provides more information on vitamin D.