More than 640 million people globally now weigh in as obese and the world has more overweight than underweight people, an analysis of global trends in body mass index (BMI) shows.

A startling rise in rates of obesity in the past 40 years meant the number of people with a BMI of more than 30 had risen from 105 million in 1975 to 641 million in 2014, the study found. More than one in 10 men and one in seven women were obese.

BMI is calculated by dividing a person's weight in kilograms by their height in metres squared, and is an indication of whether a person is a healthy weight. A BMI score over 25 is overweight, over 30 is obese and over 40 is morbidly obese.

"The number of people across the globe whose weight poses a serious threat to their health is greater than ever before," said Majid Ezzati, a professor at the school of public health at Imperial College London.

"And this epidemic of severe obesity is too extensive to be tackled with medications such as blood pressure-lowering drugs or diabetes treatments alone, or with a few extra bike lanes."

To try to make a real difference Ezzati said co-ordinated global steps were needed, including addressing the pricing of healthy foods versus unhealthy foods, or taxing high sugar and highly processed foods.

Yet excessively low body weight remained a serious public health issue in the world's poorest regions, the study's authors said, and rising global trends in obesity should not overshadow the problem of many people not getting enough to eat.

In South Asia, for example, almost a quarter of the population was underweight. In Central and East Africa, about 12 per cent of women and 15 per cent of men were underweight.

The study, published on Thursday in The Lancet medical journal, involved the World Health Organisation and more then 700 researchers worldwide. It analysed data on weight and height from nearly 20 million adults from 186 countries.

It found that during the past four decades the average age-corrected male BMI rose to 24.2 from 21.7, and in women rose to 24.4 from 22.1.

That was equivalent to the world's population becoming on average 1.5 kg heavier each decade, the researchers said.

They predicted that if the global trends continued, 18 per cent of men and 21 per cent of women would be obese by 2025.

OTHER KEY FINDINGS OF THE STUDY:

* Japanese adults had the lowest BMIs of all high-income countries, while American adults had the highest BMIs.

* More obese men and women now live in China and the United States than in any other country.

* The lowest BMIs in Europe were among Swiss women and Bosnian men. Men in Britain had the 10th-highest BMI in Europe and women the third-highest in Europe.

* Morbid obesity, where a person's weight interferes with basic physical functions such as breathing and walking, now affects around 1 per cent of men and 2 per cent of women. In total 55 million adults are morbidly obese.