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Air pollution linked to diabetes in landmark Indian study

A seven-year study of 12 000 residents of Delhi and Chennai – the first of its kind in the country – has found links between air pollution and diabetes.

The researchers found that inhaling air with high amounts of PM2.5 particles led to high blood sugar levels and increased type 2 diabetes incidence.

When inhaled, PM2.5 particles – which are 30 times thinner than a strand of hair – can enter the bloodstream and cause several respiratory and cardiovascular diseases.

The Guardian reports that the study is part of ongoing research into chronic diseases in India that began in 2010. It is the first to focus on the link between exposure to ambient PM2.5 and type 2 diabetes in India, one of the worst countries in the world for air pollution.

The study found the average annual PM2.5 levels in Delhi was 82-100μg/m3 and in Chennai, 30-40μg/m3, many times the WHO limits of 5μg/m3. India’s national air quality standards are 40μg/m3.

There is also a high burden of non-communicable diseases, including diabetes, hypertension and heart disease in India; 11.4% of the population – 101m people – have diabetes, and about 136m are pre-diabetic, according to a paper published in The Lancet in June.

The average diabetes prevalence in the European Union was 6.2% in 2019, and 8.6% in the UK in 2016.

The Lancet study found India’s diabetes prevalence to be higher than previous estimations and showed a higher number of diabetics in urban than rural India.

In the latest study, published in The BMJ, the researchers followed a cohort of 12 000 men and women in Delhi and Chennai from 2010 to 2017 and measured their blood sugar levels periodically. Using satellite data and air pollution exposure models, they determined the air pollution in the locality of each participant in that timeframe.

They found that one month of exposure to PM2.5 led to elevated levels of blood sugar, and prolonged exposure of one year or more led to increased risk of diabetes.

They also found for every 10μg/m3 increase in annual average PM2.5 level in the two cities, the risk for diabetes increased by 22%.

“Given the pathophysiology of Indians – low BMI with a high proportion of fat – we are more prone to diabetes than the western population,” said Siddhartha Mandal, lead investigator of the study and a researcher at Centre for Chronic Disease Control, Delhi.

The addition of air pollution – an environmental factor – with lifestyle changes in the past 20 to 30 years, is fuelling the increasing burden of diabetes, he said.

“Until now, we had assumed that diet, obesity and physical exercise were some of the factors explaining why urban Indians had higher prevalence of diabetes than rural Indians,” said Dr V Mohan, chairman of the Madras Diabetes Research Foundation and one of the authors of the paper.

“This study is an eye-opener because now we have found a new cause for diabetes that is pollution.”

Another study on the same cohort in Delhi found average annual exposure to PM2.5 in Delhi (92μg/m3) led to increase in blood pressure levels and higher likelihood of developing hypertension.

Together, the studies show that the higher than safe levels of PM2.5 in the air in Indian cities cause diabetes and hypertension that could lead to atherosclerosis (the build up of fatty deposits in the arteries), heart attacks and heart failures, said Mandal.

PM2.5 contains sulphates, nitrates, heavy metals and black carbon that can damage the lining of blood vessels and increase blood pressure by stiffening the arteries.

The particles can get deposited in the fat cells and cause inflammation and can also attack the heart muscle directly, said Dr Dorairaj Prabhakaran, cardiologist and executive director of the Centre for Chronic Disease Control and one of the authors of the paper.

Acting as an endocrine disruptor, PM2.5 hampers insulin production in the body as well as its effect.

In urban India there has been a rise of hypothyroidism, polycystic ovarian syndrome (PCOS) and gestational diabetes. This study shows that pollution may play a part in causing all of these as it disrupts the endocrine system that produces all hormones in the body, said Mohan.

The researchers are now working to understand the impact of pollution on cholesterol and vitamin D levels in the body, and its impact on the life cycle of individuals, including birth weight, pregnant women’s health, insulin resistance in adolescents, and the risk for Parkinson’s and Alzheimer’s disease, among others.

While its findings are alarming, the study gives scientists hope that bringing down pollution can decrease the burden of diabetes, as well as other non-communicable diseases, said Prabhakaran.

Some public policy initiatives have shown results. Since a public outcry about air pollution in 2016, the central and Delhi Government have banned older diesel vehicles, limited construction, built highways that bypass the city, and banned the burning of crops.

Reports suggest there was a 22% reduction in PM2.5 levels between 2016 and 2021.

“This is a modest but welcome reduction. Similar measures adapted to local conditions are urgently needed across the country,” said Prabhakaran.

Study details

PM2.5 exposure, glycaemic markers and incidence of type 2 diabetes in two large Indian cities

Siddhartha Mandal, Suganthi Jaganathan, Dimple Kondal, Joel Schwartz, Nikhil Tandon, Viswanathan Mohan, Dorairaj Prabhakaran, K M Venkat Narayan.

Published in The BMJ on 5 October 2023

Abstract

Introduction
Exposure to fine particulate matter has been associated with several cardiovascular and cardiometabolic diseases. However, such evidence mostly originates from low-pollution settings or cross-sectional studies, thus necessitating evidence from regions with high air pollution levels, such as India, where the burden of non-communicable diseases is high.

Research design and methods
We studied the associations between ambient PM2.5 levels and fasting plasma glucose (FPG), glycosylated haemoglobin (HbA1c) and incident type 2 diabetes mellitus (T2DM) among 12 064 participants in an adult cohort from urban Chennai and Delhi, India. A meta-analytic approach was used to combine estimates, obtained from mixed-effects models and proportional hazards models, from the two cities.

Results
We observed that 10 μg/m3 differences in monthly average exposure to PM2.5 was associated with a 0.40 mg/dL increase in FPG (95% CI 0.22 to 0.58) and 0.021 unit increase in HbA1c (95% CI 0.009 to 0.032). Further, 10 μg/m3 differences in annual average PM2.5 was associated with 1.22 (95% CI 1.09 to 1.36) times increased risk of incident T2DM, with non-linear exposure response.

Conclusions
We observed evidence of temporal association between PM2.5 exposure, and higher FPG and incident T2DM in two urban environments in India, thus highlighting the potential for population-based mitigation policies to reduce the growing burden of diabetes.

 

The BMJ article – PM2.5 exposure, glycaemic markers and incidence of type 2 diabetes in two large Indian cities (Creative Commons Licence)

 

The Lancet article – Metabolic non-communicable disease health report of India: the ICMR-INDIAB national cross-sectional study (ICMR-INDIAB-17) (Open access)

 

Hypertension article – Exposure to Particulate Matter Is Associated With Elevated Blood Pressure and Incident Hypertension in Urban India (Open access)

 

The Guardian article – Air pollution raises risk of type 2 diabetes, says landmark Indian study (Open access)

 

See more from MedicalBrief archives:

 

Study finds link between air pollution and bone health in India

 

Evidence grows of air pollution link to neurodegenerative disease

 

Large global study finds air pollution link to 1m annual stillbirths

 

Study finds air-pollution's impact on health may be worse than thought

 

 

 

 

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