Low BMI, diabetes may up TB risk

Patients with type 2 diabetes (T2D) who are also underweight have more than eightfold increased risk of active tuberculosis (TB), according to the Singapore Chinese Health Study, which highlights low BMI and diabetes as two independent risk factors for active TB disease.

“A clear message should be sent out to diabetes clinics that ‘the lean diabetic with a persistent cough’ needs prompt investigation for TB,” wrote Professor Anthony Harries from the International Union Against Tuberculosis and Lung Disease, headquartered in Paris, France, in a linked editorial. [Int J Tuberc Lung Dis 2019;23:1237-1238]

Unlike the Western populations in which T2D cases are commonly overweight, a huge proportion of T2D patients in Asia have low BMI.  

“[This study] has important public health implications in Asia where prevalence of TB infection is high, and T2D occurs at lower levels of BMI,” the researchers stated.   

During a mean follow-up period of 16.9 years, the researchers found that individuals with T2D had more than twofold increase in TB risk compared with those without (hazard ratio [HR], 2.31, 95 percent confidence interval [CI], 1.93–2.78). This was after adjusting for multiple variables such as age, sex, education level, smoking status, alcohol and tea intake, and dietary patterns. [Int J Tuberc Lung Dis 2019;23:1277-1282]

On the other hand, individuals who were underweight were almost threefold as likely to have TB than those who were obese (HR, 2.87, 95 percent CI, 2.15–3.82). The inverse relationship between BMI and TB risk was dose-dependent, ie, the risk of TB increases with decreasing levels of BMI (p-trend=0.0001): HRs were 1.03, 1.50, and 2.87 for decreasing intervals of BMI of 23 to <27.5 kg/m², 18.5 to <23 kg/m², and <18.5 kg/m², respectively, vs the obese reference group (BMI ≥27.5 kg/m²).

When analysing the two risk factors of T2D and BMI together, obese patients with diabetes had almost twofold increased risk of TB (HR, 1.87) while underweight patients without diabetes had more than twofold risk of TB (HR, 2.68), compared with the reference group who were obese without diabetes (ie, both risk factors were absent).

However, when both risk factors of being underweight and having diabetes were present, TB risk jumped to more than eightfold increase compared with the reference group in which both risk factors were absent (HR, 8.30, 95 percent CI, 4.43–15.54).

The prospective study involved 63,257 adults aged 45–74 years (mean age ~56 years) in Singapore. Active TB cases were identified from the National TB Registry. As the study was designed prospectively, the authors believed that the association between diabetes/underweight and TB risk was unlikely due to reverse causality.

“Singapore’s TB incidence rate of 35 to 40 per 100,000 population is 5–10 times that of the US, Australia and the UK,” said co-author Dr Cynthia Chee of Tan Tock Seng Hospital, Singapore. “In order to bring down our TB rate, it is important for all physicians to have a heightened awareness of the link between diabetes and TB, so as to facilitate the early diagnosis of active TB in this important risk group.”

“Good clinical management to better control glycaemic level in diabetic patients may be an efficient way to reduce the risk of active TB and improve treatment outcomes,” said Chee and co-authors.

Harries also called for diabetes clinics to conduct targeted screening on diabetes patients who are underweight, and more trials on targeted TB preventive therapy in these patients.