The number of obese adults globally has grown by over 600 percent in the last 40 years, and if trends since 2000 continue, about 20 percent of the world’s adult population will be obese by 2025, according to research published in the Lancet medical journal. The world’s obese population already outnumbers people who are underweight, the study pointed out.

The world had 640 million obese adults in 2014, up more than sixfold from 1975, when the number stood at 105 million. Global prevalence of obesity was 10.8 percent among men and 14.9 percent among women in 2014, and given the trends of the last 15 years, 18 percent of men and 21 percent of women are projected to be obese by 2025.

Obesity is clinically defined as a body mass index of over 30. BMI is calculated by dividing a person’s height (measured in meters) by the square of the person’s weight (measured in kilograms). For example, if you weigh 80 kg and stand 1.8 m tall, your BMI would be 80 ÷ (1.8 × 1.8), or 24.69, which is at the high end of the normal range.

The Lancet research also estimated that about 6 percent of men and 9 percent of women worldwide will be severely obese (BMI higher than 35) by 2025.

Men and women in high-income English-speaking countries had substantially higher BMIs than their counterparts elsewhere. Six countries — Australia, Canada, New Zealand, Republic of Ireland, the United Kingdom and the United States — accounted for 118 million, or more than a fifth, of the world’s obese population in 2014.

However, China tops the list, with the highest number of obese people — 43.2 million men and 46.4 million women. The United States, with 41.7 million obese men and 46.1 million obese women, came in second. As a percentage of population, the U.S. has a far higher number of obese adults than China.

The highest percentage of obesity among populations was found in the islands of Polynesia and Micronesia, where over 38 percent of men and more than half the women are obese.

The study also found that the number of underweight people in the world fell by over a third in the same time period. About 8.8 percent of men and 9.7 percent of women were underweight in 2014. While warning for the need to tackle the epidemic of obesity, the study also said that underweight concerns remain significant in some parts of the world, especially in South Asia.

Data was pooled from 186 countries for the study, which involved more than 19.2 million adult participants.