The 'First world' health problems such as obesity and heart disease may be gaining ground in developing nations, but they are mostly afflicting the rich and middle class while poor people remain undernourished and underweight, a study said.

Researchers who looked at more than 500,000 women from 37 mid- and low-income nations in Asia, Africa and South America found that there was a clear divide between the better-off and the poor, according to findings published in the American Journal of Clinical Nutrition.

'Yes, on average, people are getting heavier in these countries. But who is getting heavier?' said senior researcher SV Subramanian, a professor of population health and geography at Harvard University.

'A huge amount of the population is still undernourished,' said the expert.

Recent studies have found that rates of abdominal obesity, high blood pressure and diabetes rose steadily in a group of young urban adults in India who were followed for seven years.

Another study found that people in India and other South Asian nations suffer their first heart attack at age 53, six years earlier than the rest of the world.

But there has also been a large social divide noted within developing nations, with wealthier, better-educated people becoming heavier while the poor stay thin, sometimes dangerously so.

To see whether that gap might be narrowing, researchers looked at body mass index (BMI) - a measure of weight against height - from over 547,000 women.

Just over 200,000 were interviewed and weighed between 1991 and 2003, while the rest were studied between 1998 and 2008.

Across countries, the wealthier the women were, the higher their average BMI, a pattern that held steady over time.

While even poor women showed an increase in the number who were overweight, they lagged their wealthier counterparts.

Among the poorest women, the bottom 20 per cent by income, 18 per cent were overweight during the second survey period. That compared with 45 per cent of the wealthiest women.

'The poor stay thinner,' the researchers wrote.

Subramanian said that he didn't want to minimize the problem of obesity, which means that many countries in the world need two opposite public health efforts: encouraging wealthier people to curb their caloric intake, while trying to get more food to the people who need it.

The pattern is different from that seen in wealthy nations, such as the US, where lower incomes and less education often correlate with higher weight.

But food remains expensive in poorer nations, with the most impoverished citizens unable to afford boosting their calorie intake, Subramanian and his colleagues wrote, meaning that a balance of policies is needed.-Reuters