China displaced the United States as the world's most obese country in 2014, while India tops the list of countries with the largest underweight population, says a study on Global Trends in Body Mass Index (BMI), published in the medical journal, Lancet.

The world's obese population rose to 641 million in 2014 from 105 million in1975. More than 19.2 million adults from 186 countries participated in the study. The survey found that 266 million men and 375 million women worldwide were obese.

Low body weight, according to the researchers, persists as a problem in developing countries. India had 101.8 million underweight men and 100.5 million underweight women in 2014. The countries in Asia and sub-Saharan Africa had the highest number of underweight people, with an increasing share of underweight people living in South Asia over time.

On the other hand, countries with the largest number of obese and severely obese people changed over these four decades, with more middle-income countries joining the U.S. in terms of obese population, especially of women. In 2014, China had more obese and severely obese men and women than the U.S. The figures for China stood at 43.2 million obese men and 46.4 million obese women while the U.S. had 41.7 million obese men and 46.1 million obese women. However, more than one in four severely obese men, and almost one in five severely obese women in the world still live in the U.S, says the study.

It also observed that the problem of obesity is more extreme than underweightedness worldwide. In 2014, more people were obese than underweight in 136 out of 200 countries. Around 118 million obese adults (18 percent of the world's obese population) lived in developed countries.

The lowest prevalence of obesity was found in Japan, North Korea and Bangladesh while the highest prevalence was found in the U.S. In the Caribbean and middle-east countries, 40-50 percent of women were obese.

"The number of people across the globe whose weight poses a serious threat to their health is greater than ever before," Majid Ezzati, a co-author of the study and a professor at the school of public health at Imperial College London was quoted by Reuters as saying. He added that the epidemic is too extensive to be solved by severe medications such as blood pressure lowering drugs or diabetes treatments alone, or with a few extra bike lanes. Ezzati suggested a coordinated effort by the countries aimed at tackling the problem by introducing policies which regulate the price of healthy and unhealthy foods.

BMI is the standard used to determine if a person's weight poses a serious health threat. It is calculated by dividing a person's body weight in kilogram by their height in meters squared. A BMI score more than 25 is overweight, over 30 is obese and 40 is fatally obese. The High BMI is an important risk factor for serious non-communicable diseases, such as cardiovascular and kidney diseases, diabetes, cancers, and musculoskeletal disorders.