Vitamin D is needed to activate the immune system's response to tuberculosis, said a US study on Wednesday that could lead to new treatments for the lung disease that kills 1.8 million people per year.

Researchers have long known that vitamin D plays a role in the body's response to TB, but the study in the journal Science Translational Medicine shows it must be present in adequate levels to trigger the immune response.

This finding could be crucial to efforts to treat the disease in parts of the world like Africa, because people with dark skin tend to be more susceptible to TB and also are more likely to have vitamin D deficiencies.

Even though people can get vitamin D through sun exposure, dark skin contains more melanin which shields the body from ultraviolet rays and also reduces vitamin D production.

"Over the centuries, vitamin D has intrinsically been used to treat tuberculosis," said first study author Mario Fabri, who did the research for the study while at the University of California Los Angeles and is currently at the Department of Dermatology at the University of Cologne, Germany.

"Sanatoriums dedicated to tuberculosis patients were traditionally placed in sunny locations that seemed to help patients -- but no one knew why this worked," he said.

"Our findings suggest that increasing vitamin D levels through supplementation may improve the immune response to infections such as tuberculosis."

Previous studies by the same research team found that vitamin D played a key role producing a molecule called cathelicidin, which helps the innate immune system kill the tuberculosis bacteria.

The current findings show that vitamin D is necessary for the T-cells, which respond to threats as part of the body's adaptive immune system, to produce a protein called interferon which directs cells to attack the bacteria.

"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," said co-author Barry Bloom, former dean of the faculty at the Harvard School of Public Health.

The World Health Organization reported this week that 8.8 million people had TB last year, with about one quarter of those cases occurring in Africa and 40 percent in India and China.