City folk may not think much of rural living - but they are healthier.

A new study finds that diabetes, once rather uncommon, is now affecting 387 million people worldwide - and 77 percent of it is in developed nations.

The reason is stress, write the authors of a paper in the Journal of Clinical Endocrinology&Metabolism. City life - noise, crime and traffic all lead to higher stress and the body producing more of the stress hormone cortisol. Cortisol can counteract insulin, the hormone that regulates blood sugar, and slow the body's production of it and that makes people more susceptible to diabetes. 

To test the hypothesis, researchers examined people from one ethnic group - the Ovahimba people of Namibia in southwestern Africa. Namibia is the second least-densely populated country in the world, with 38.6 percent of residents living in urban environments. In their prospective, cross-sectional, diagnostic study, the researchers measured cortisol, blood sugar and cholesterol levels in 60 Ovahimba people living in the regional capital, Opuwo. Opuwo has a population of around 21,000. The researchers then conducted the same tests on 63 Ovahimba people living at least 50 kilometers from the nearest town or village. 

Among the urban residents, 28 percent of the people had diabetes or other glucose metabolism disorders. The rate was less than half that for rural residents. The urban dwellers also had significantly higher cortisol levels than their rural counterparts.

While the city residents reported that they exercised less and ate more fast food and desserts than the rural residents, lifestyle changes aren't the only factor at work, said  Peter Herbert Kann, MD, PhD, MA of Philipp's University in Marburg, Germany. The difference in cortisol levels indicates that the hormone is the key part of the equation.

"The results suggest sociocultural instability caused by urbanization contributes to an increased risk of developing diabetes or another metabolic disorder," Kann said. "This is the first prospective study to systematically show the body's regulation of the hormone cortisol plays a part in the metabolic changes brought on by the shift to an urban lifestyle."