The global rise in body-mass index (BMI) seen over the last 30 years is largely due to increases in the BMI of rural populations, a Nature paper suggests.

The study challenges the predominant paradigm, which links obesity with urban lifestyles, and could have profound implications for public health policies.

While global obesity rates rise, more and more people are living in cities.

This has led to the view that the urban lifestyle is a major driver of obesity, but the studies supporting this view tend to be small and over short periods of time.

Majid Ezzati and colleagues analysed 2009 studies of more than 112 million adults, which enabled them to assess the changes in BMI that occurred in 200 countries from 1985 to 2017.

They found that more than 55 per cent of the global rise in BMI since the 1980s came from rural populations.

In some low and middle income regions, this increased to more than 80 per cent.

The findings reflect the fact that, with the exception of women in sub-Saharan Africa, BMI is rising in rural areas at the same rate as or faster than in cities.

That's except for Australia, where it is city citizens who are rising faster than their rural counterpaths.

General Manager of Heart Health Bill Stavresk says that rising rates have been a problem for three decades. 

"The average Aussie male and female have had quite a significant increase in their weight over that period. What we know about overweight and obesity, it not only leads to chronic illness, but it is a significant cost to our health system." 

The current study is the most comprehensive analysis to date of how BMI is changing in rural and urban areas.

The authors argue that poor, rural communities may be swapping an undernutrition disadvantage for a more general malnutrition disadvantage.

They call for an integrated approach to narrow the focus of international aid on undernutrition, and broaden it to enhance access to healthier foods in both poor rural and urban communities.