Obesity associated with lower risk of active pulmonary tuberculosis in older population of Hong Kong

Obese or overweight Chinese individuals age 65 and older have a lower risk of developing tuberculosis than those at a normal weight, according to a study in the June 25 issue of Archives of Internal Medicine.

Tuberculosis is commonly associated with poverty and undernutrition in both developed and developing countries. In addition, obesity is an increasing problem that is associated with a wide range of chronic degenerative conditions, notably, diabetes mellitus, a well-reported predisposing factor for active tuberculosis, according to background information in the article.Few studies have systematically examined the effect of obesity and overweight on tuberculosis, especially in Asian populations.

Chi C. Leung, M.B.B.S., Tuberculosis and Chest Service, Hong Kong, and colleagues, studied 42,116 individuals 65 years or older enrolled in 18 health centers for elderly patients in Hong Kong. The patients were followed up from three months after enrollment in 2000 until December 31, 2005. The body mass index (BMI) of each patient was measured at the beginning of the study. Those 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.

During the follow-up period, 477 cases of active tuberculosis were reported, 326 (68.3 percent) of which were confirmed using cultures of the bacteria involved. The average time between enrollment and notification of tuberculosis was 881 days.There were 395 new cases (82.8 percent) and 82 retreatment cases (17.2 percent). Pulmonary [lung] involvement was found in 426 cases (89.3 percent) and extrapulmonary [outside the lung] involvement in 87 (18.2 percent), including 36 cases (7.5 percent) with both, the authors write.

Individuals who 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, according to the authors. A higher average initial BMI was found in pulmonary-only cases than in extrapulmonary-only cases (22.3 vs. 24.1).

Obesity is associated with a lower risk of active pulmonary tuberculosis in the older population of Hong Kong, the authors conclude.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.

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