Study sheds light on TB resistance.

Study sheds light on TB resistance

Vitamin D, the only vitamin that is photosynthesized in humans, may be important in boosting resistance to tuberculosis (TB), new research suggests. The findings might explain the age-old observation that exposure to sunlight seems to help cure the disease.

Alfred J. Crowle and his colleagues performed the research at the University of Colorado Health Sciences Center in Denver -- not far from the mountains where tuberculosis patients once sought the benefits of high altitude, fresh air and bright, sunny days. What those consumptive patients didn't know was that, thus exposed to the sun, they were actively photosynthesizing vitamin D, which was then being converted by their livers into 1,25-dihydroxy-vitamin D.sub.3 (1,25D).

To test the antituberculosis potency of 1,25D, Crowle cultured human macrophages, the white blood cells most active in fighting tuberculosis, and exposed them to tuberculosis-causing bacteria in the presence of varying amounts of the vitamin D metabolite. As reported in the December INFECTION AND IMMUNITY, he found that higher concentrations of 1,25D enabled the macrophages to slow or stop bacterial replication.

"It's very thought-provoking research," says Michael Iseman, chief of the mycobacteriology disease service at the National Jewish Hospital in Denver. "For centuries people went up into the mountains to cure tuberculosis. It would certainly be fascinating in retrospect to prove that, like a lot of folk medicine, there may have been a germinal center of truth in what they were doing."

The amount of 1,25D needed to enhance macrophage antibacterial activity is higher than that normally found in circulating blood. However, Crowle says, white cells can themselves synthesize 1,25D from a precursor that is 1,000 times more concentrated in the blood than is vitamin D. "When properly stimulated, perhaps by a disease-causing organism," he says, "they can make as much as 500 times the normal amounts of 1,25D."

In addition, he says, there is an intriguing possibility that blacks and Asians, who have a higher susceptibility to TB, may prove to have naturally lower levels of circulating vitamin D. If that correlation is found to be true, he says, then the incidence of tuberculosis in much of the developing world may be decreased by improving diets or by providing vitamin D supplements. In the United States, vitamin D is added routinely to milk.

But other factors may be responsible for the higher rate of infection in dark-skinned people. "Poverty, crowded housing, poor nutrition, high levels of stress all could explain it," says Iseman. "And even if you correct for those, I think other genetic differences in susceptibility may be critical."

Worldwide, 8 million to 10 million people contract TB each year, according to the World Health Organization. In the United States, nearly 23,000 new cases of tuberculosis were reported in 1986, a 2.6 percent increase over the previous year. Much of that increase, according to the Centers for Disease Control in Atlanta, is related to the immune-suppressing AIDS epidemic.