Is higher body mass index protective against tuberculosis?
Individuals who are obese or overweight are at lower risk of developing tuberculosis (TB), according to a study that reports a strong linear dose–response association between body mass index (BMI) and active TB risk.
In a cohort of 46,028 adult participants (mean age 42.5 years; 50.1 percent male) from three rounds of the Taiwan National Health Interview Survey (2001, 2005 and 2009), 241 incident cases of active TB were recorded. [Int J Obesity 2017;41:971–975]
Compared with normal weight individuals (BMI, 18.5 to 23.9 kg/m2), those who were obese (BMI, ≥27 kg/m2) and overweight (BMI, 24 to 26.9 kg/m2) were 57 and 33 percent less likely to develop active TB (adjusted odds ratio [aOR], 0.43; 95 percent CI, 0.28 to 0.67 and aOR, 0.67; 0.49 to 0.91, respectively).
Analysis of the dose–response association between TB and BMI showed a strong linear dose–response effect per unit change in BMI for active TB (aOR, 0.92; 0.88 to 0.95; p<0.001).
“The lower risk of active TB in obese and overweight individuals raises questions about TB prevention in a population. Because obesity and overweight were associated with lower TB risk, these conditions might reduce TB incidence in a population if the overall BMI distribution shifts to higher values,” the investigators said.
However, they noted that having the overall BMI distribution in a population shift to higher values would have adverse effects on other important public health conditions (eg, increasing prevalence of chronic comorbidities). [Lancet 2005;366:1197–1209]
“Therefore, it would not be advisable at this point to promote raising BMI to ≥24 kg m−2 as a means to reduce risk of TB. However, more studies are needed to identify healthy weight-for-height bands, to achieve optimal and balanced prevention of TB and other chronic comorbidities,” they said.
It is said that the lower virulence of mycobacteria in lipid-enriched environments and immune-modulating effects of leptin could explain the protective association of higher BMI with TB. One study reported that the replication of mycobacteria with lipid bodies is not as rapid as that of mycobacteria without such lipid bodies. Furthermore, mycobacteria can persist without replication in fatty tissues. [PLoS Med 2008;5:e75; PLoS One 2006;1:e43]
“The immune-modulating effects of leptin may also account for the lower risk of TB incidence in obese/overweight persons. Leptin is an adipocyte-derived hormone and there have been some evidence suggesting that leptin could enhance the T-cell-mediated immune functions in humans, which might reduce the risk of active TB infection,” the investigators said. [Mol Aspects Med 2012;33:35–45]
They, however, pointed out that the results should be interpreted while keeping several limitations in mind. These include the missing information on deaths, lack of data on the results of bacteriological studies for TB diagnosis, and the collection of BMI data only at baseline.
“[Nevertheless, the] study has potential biological, clinical and epidemiological implications; therefore, future basic research and animal studies should investigate the underlying mechanisms and potential clinical and epidemiological consequences of the relation between obesity/overweight and active TB,” they said.