After Indians migrate from rural to urban areas, the longer they live in a city the worse they score on measures of cardiac health and diabetes risk compared to those who remained in rural areas, according to an Indian study.
Body fat, blood pressure and fasting insulin levels - a measure of diabetes risk - all increased within a decade of moving to a city, while for decades after, blood pressure and insulin kept rising above the levels in rural counterparts, says the study in the American Journal of Epidemiology.
The results raise broad global public health concerns, given the rising urban population.
"The findings suggest that body fat increases rapidly when one first moves to an urban environment, whereas other cardiometabolic risk factors evolve gradually," wrote the study leader Sanjay Kinra, of the London School of Hygiene and Tropical Medicine.
According to the United Nations, the growth change in India's urban population is 1.1 per cent each year, while the change in the proportion of people in rural areas is declining by 0.37 per cent.
But the proportion of urban residents in the United States is much higher. Just 30 per cent of Indians live in urban areas, against 82 per cent of US residents.
For the study, Mr Kinra and his team compared rural Indians to their siblings who moved to one of four cities in India: Lucknow, Nagpur, Hyderabad and Bangalore.
Siblings who lived in a city the longest had the highest average blood pressures.
For instance, men who lived in a city for more than 30 years had an average systolic blood pressure of 126, while men who lived in a city for 10 to 20 years had an average of 124. Those who stayed in rural areas had an average of 123.
The change in body fat was most evident in the first 10 years after moving to a city, but then it levelled off.
Men who stayed in rural areas had 21 per cent body fat on average, while those who moved within the past 10 years had 24 per cent on average.
"Age, gender, marital status, household structure and occupation did not influence the patterns appreciably," Mr Kinra wrote.
"However, stronger gradients for adiposity were noted in migrants from lower socioeconomic positions."
The study did not pinpoint the cause of the difference between siblings, nor did it mention whether the increased levels of body fat, blood pressure and insulin resulted in more disease.
Though other studies have found negative health effects related to moving to cities, city dwellers in the United States tend to be healthier than those who live in rural areas and even the suburbs.
Other international studies have found that people who lived in cities were more likely to be active and to walk places. They also tend to be closer to doctors and hospitals.
Mr Kinra and his colleagues wrote that the changes among Indian city-dwellers might be explained by rapid weight gain once people move to a city, spurred by a less healthy diet and less active lifestyle.
"Programmes focused on preventing obesity in new migrants to urban areas and tailored to the needs of those in lower socioeconomic positions could deliver long-term health benefits," they added.