Global obesity has tripled among men over the last four decades and more than doubled among women, the biggest obesity study ever undertaken has shown.

According to the findings, at least one in 10 men and one in seven women are now obese - totaling 640 million people worldwide.

The study involved over 700 researchers globally, who used measurements of weight and height from almost 20 million adults in most of the world's countries. Using this information, they calculated and compared the body mass index (BMI) of men and women from 1975 to 2014.

A BMI of 20-24.9 is considered a healthy weight, 25-29.9 is considered overweight, while 30 or more is obese. A BMI of 35 or more is classed as severely obese.

Severely obese people have a significantly increased risk of developing a range of conditions, including type 2 diabetes and heart disease.

The study revealed that in Ireland, the average BMI of both men and women has been steadily increasing since 1975. In men, the average BMI was:
-24 in 1975
-25 in 1985
-26.1 in 1995
-27.3 in 2005
-27.8 in 2014.

In women, the average BMI was:
-22.4 in 1975
-23.8 in 1985
-25.1 in 1995
-26.4 in 2005
-27 in 2014.

Globally, obesity has tripled among men, from 3.2% in 1975 to 10.8% in 2014. Among women, it has more than doubled, from 6.4% in 1975 to 14.9% in 2014.

Furthermore, 2.3% of men globally are now classed as severely obese, along with 5% of women.

Among high-income countries, Japanese adults have the lowest BMI, while Americans have the highest. The country with the highest BMI overall is American Samoa, where the average BMI among men is 32 and among women is 35.

Meanwhile, morbid obesity, which is when a person's weight interferes with basic bodily functions such as walking and breathing, now affects 1% of men globally and 2% of women. A total of 55 millions adults worldwide are morbidly obese.

The researchers predicted that if these trends continue, by 2025, 18% of men globally and 21% of women will be obese.

"The number of people across the globe whose weight poses a serious threat to their health is greater than ever before. 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.

"We need coordinated global initiatives - such as looking at the price of healthy food compared to unhealthy food, or taxing high sugar and highly processed foods - to tackle this crisis," commented Prof Majid Ezzati of Imperial College London, which led the research.

Details of this study are published in the journal, The Lancet.