In a first-of-its-kind study in India, researchers have found strong evidence linking air pollution to increased risk of diabetes in the country. The findings from the study indicate that long-term exposure to fine particulate matter (PM2.5) is associated with higher blood sugar levels and increased risk of developing type 2 diabetes mellitus (T2DM).
The study analyzed data from over 12,000 adults participating in the Center for cArdiometabolic Risk Reduction in South Asia (CARRS) cohort study based in Chennai and Delhi.
The study assumes significance in the context of the rising number of diabetes patients in the country. The South Asian nation is estimated to be home to 7.7 crore adults with diabetes as of 2019, and air pollution could be fueling this crisis.
Although diabetes is a complex metabolic disorder caused by multiple reasons, the study shows that air pollution could be a key contributing factor in urban areas.
The study was conducted over a follow-up period of 7 years between 2010 and 2017, when researchers tracked the study participants’ exposure to PM2.5 using high-resolution satellite-based models. They also measured blood sugar levels and incidence of T2DM in this cohort over time.
The findings show that people living in areas with higher PM2.5 levels had higher fasting plasma glucose (FPG) and glycated hemoglobin (HbA1c) on average compared to those living in less polluted areas. HbA1c is considered an important indicator of long-term blood sugar management.
The study also found that long-term exposure to PM2.5 significantly increased the risk of developing T2DM over the 7-year period, even after adjusting for various risk factors like diet, physical activity, and smoking.
Overall, the results indicate that a 10 μg/m3 increase in annual PM2.5 exposure was associated with a 0.4 mg/dL rise in FPG levels, 0.02% increase in HbA1c, and 23% higher risk of developing diabetes. This risk was found to be higher at lower PM2.5 concentrations indicating that even relatively clean air may not be safe when it comes to diabetes.
The study was conducted by researchers from the Madras Diabetes Research Foundation in Chennai, the Public Health Foundation of India in Gurugram, and several institutes in the US including the University of California, San Francisco and the University of Washington, Seattle, and was published in British Medical Journal.
This is the first study from India establishing a quantitative link between PM2.5 and diabetes risk in a highly polluted environment. With annual average PM2.5 levels routinely exceeding WHO guidelines by multiple times in Indian cities, these findings underline the urgent need to tackle air pollution to curb the growing burden of diabetes. I
Previous Studies
This is not the first study implicating air pollution in diabetes risk. Previous studies, mostly from Europe and North America, have reported similar associations.
A 2010 meta-analysis found 10 μg/m3 higher PM2.5 exposure to be linked with a 1% increase in diabetes prevalence worldwide. Studies since then, including in China and Taiwan, have reinforced the possible connection.
The mechanisms theorized to underlie this link include inflammation, oxidative stress, lipid deposition in tissues, endothelial dysfunction, autonomic imbalance, among others caused by air pollutants entering the body. All these effects can interfere with glucose and insulin homeostasis thereby promoting diabetes.
However, most earlier evidence was from developed country settings with lower pollution levels compared to India. The present study is the first to firmly establish this risk in the context of Indian cities with very high PM2.5 exposures.
Study Design and Key Findings
The study analyzed data from the CARRS cohort study, an ongoing population-based study across Chennai and Delhi. Around 12,600 participants aged 20 years or above were recruited randomly between 2010 and 2012 and followed up until 2017.
Daily PM2.5 levels were estimated across the two cities using satellite observations, meteorology, land use patterns and emission inventories. These exposure levels were linked to the residential locations of the CARRS participants.
Blood samples were collected at baseline and follow-ups to measure fasting blood glucose and HbA1c. Diabetes status was defined using standard cut-offs. The researchers then analyzed the associations between PM2.5 exposure over different time periods and changes in blood sugar parameters and diabetes incidence.
In Delhi, a 10 μg/m3 increase in 1-month average and 6-month average PM2.5 was associated with 0.4 mg/dL and 0.7 mg/dL rise in blood glucose levels respectively. The corresponding figures were 0.5 mg/dL and 0.6 mg/dL in Chennai.
PM2.5 increases also led to small but statistically significant elevations in HbA1c in both cities. When pooled together, the effects were remarkably consistent across the two cities with differing pollution levels.
For diabetes incidence, the risk was maximized at 1-1.5 years of exposure. In Delhi, the hazard ratio (HR) peaked at 1.24 for a 10 μg/m3 annual PM2.5 increase over 1.5 years. The pooled estimate for the two cities indicated a HR of 1.23 linking long-term pollution and diabetes risk.
These results were independent of other risk factors like diet, smoking, exercise and alcohol use which were adjusted for in the analysis.
Interestingly, the exposure-response relationship was nonlinear with a leveling off of risk at very high exposures in Delhi. The researchers suggest that “additional risk is small with increasing long-term exposure” beyond a point.
Policy Implications
The findings sound the alarm on the spiraling levels of air pollution across Indian cities and the multitude of linked health risks apart from respiratory disease.
While individual-level interventions on diet and physical activity are no doubt important, the study highlights the overarching need for policy action on air pollution to create an enabling environment for public health.
The good news is that diabetes risk declines with lower PM2.5 exposures underscoring the significance of pollution control measures.
As India revises its National Clean Air Programme, experts believe that much more urgency is needed to cut PM2.5 levels, especially in metropolitan cities. This requires action across sectors ranging from transport, industry and waste management to energy, construction and agriculture.
Besides national level efforts, cities too need localised plans to address pollution hotspots and spikes. Active participation of municipal bodies is key to drive action on the ground.
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