HIV-positive South African patients with higher BMIs have reduced risk of death and TB

HIV-positive individuals who are obese or overweight are less likely to die or develop tuberculosis than people with HIV who are of normal weight, South African investigators report in the online edition of AIDS.

“Our findings show a clear protective effect…of increasing BMI [body mass index] on both all-cause mortality and incident TB [tuberculosis] in a South African cohort”, comment the investigators, “person with obese and overweight BMI have a significantly decreased risk of both mortality and TB.”

Earlier research from industrialised countries showed that BMI is strongly associated with the risk of disease progression and death for patients with HIV. Although being overweight or obese are usually associated with illness, for patients with HIV being in these BMI categories seem to be protective against the risk of death.

However, the relationship between BMI and the risk of HIV-related death or TB has not been investigated in resource-limited settings.

Therefore investigators from the South African Perinatal HIV Research Unit (PHRU) conducted a prospective study involving 3456 HIV-positive adults in Soweto.

They were monitored between 2003 and 2008. At baseline and at each study visit they were weighed, and their BMI was calculated. Patients with a BMI below 18.5 m² were categorised as underweight; those with a BMI of 18.6 - 25 m² were considered to be of normal weight; a BMI of 25.1 - 30 m² was classified as overweight; and a BMI of above 30 m² was regarded as obese.

On entry to the study, patients were asked to report if they had a history of TB, and at each study visit individuals were asked if they had any symptoms of the infection.

The patients contributed a total of 8038 person-years of follow-up. During this, there were 280 deaths, providing an overall mortality rate of 3.5 per 100 person years.

Mortality rates differed according to the patients’ BMI. It was highest amongst patients who were underweight (10.4 per 100 person years). The rate for patients of normal weight was 3.6 per 100 person years. Even lower mortality rates were observed amongst patients who were overweight (1.7 per 100 person years) and obese (1.6 per 100 person years).

The investigators calculated that patients who were overweight (adjusted hazard ratio [aHR] = 0.55; 95% CI, 0.37-0.82) or obese (aHR = 0.60; 95% CI, 0.38-0.95) at baseline had a significantly reduced risk of death compared to those who were of normal weight at this time.

In addition, the most recent BMI measurement was also associated with mortality risk. Once again, compared to individuals of normal weight, patients who were overweight (aHR = 0.59; 95% CI, 0.40-0.87) and obese (aHR = 0.48; 95% CI, 0.29-0.80) had a lower risk of death.

Next the investigators explored the relationship between BMI and the risk of TB.

Prevalence of the infection at the baseline visit was 8%, and during follow-up the incidence of TB was 4.5 per 100 person years.

However, this differed according to BMI. It was highest amongst underweight patients (7.3 per 100 person years). Incidence then decreased with increasing BMI (normal weight: 6 per 100 person years; overweight: 3.2 per 100 person years; obese: 1.9 per 100 person years).

Statistical analysis showed that baseline BMI was significantly associated with the risk of TB. Compared to patients of normal weight, it was lower for those who were overweight (aHR = 0.61; 95% CI, 0.42-0.88), and obese (aHR = 0.36; 95% CI, 0.22-0.60).

Current BMI was also significantly associated with the risk of TB, and yet again, when compared to patients of normal weight, the risk was significantly reduced for patients who were overweight or obese (aHR = 0.56; 95% CI, 0.38-0.83 and aHR = 0.33; 95% CI, 0.19-0.55).

None of these findings were significantly affected by adjustment for factors such as CD4 cell count, use of HIV treatment, previous history of TB, age, sex, income, or use of isoniazid preventative therapy.

“BMI may be a useful surrogate marker of risk of TB or death among HIV-positive individuals”, conclude the investigators, “but urgent studies are required to pinpoint the protective factor and to address detrimental health issues that may result from elevated BMI.”