A new study of Chinese people has linked obesity with a lower risk of tuberculosis in the country's older population.
Led by Chi C. Leung at the Tuberculosis and Chest Service, Hong Kong, the researchers examined 42,116 individuals aged 65 years or above enrolled in 18 health centres for elderly patients in Hong Kong.
The researchers measured the body mass index (BMI) of each patient at the beginning of the study.
People with a BMI of less than 18.5 were grouped as underweight, 18.5 to less than 23 as normal, 23 to less than 25 as at risk (for obesity), 25 to less than 30 as overweight, and 30 or higher as obese.
The researchers noted that individuals who had developed active tuberculosis were taller on average, but had a lower body weight and BMI (22.5 vs. 24.3) at the beginning of the study than those who did not.
"BMI outside the range of 18.5 to 23 decreased the active tuberculosis risk by 23.5 percent of the observed level. Baseline BMI obesity at 25 or above was associated with a 30.1 percent decrease in risk, whereas BMI lower than 18.5 increased the risk by 6.6 percent," the authors said.
Also higher average initial BMI was found in pulmonary-only cases than in extrapulmonary-only cases.
"Obesity is associated with a lower risk of active pulmonary tuberculosis in the older population of Hong Kong," the authors said.
"The presence of such a strong but selective association across the whole spectrum of BMI could have major biological, clinical and/or epidemiological implications. Further studies are indicated to explore the underlying mechanisms, potential clinical utilities and possible epidemiological consequences," the authors said.
The study has been published in the Archives of Internal Medicine, one of the JAMA/Archives journals.