Globally, over two billion children and adults suffer from health problems related to being overweight or obese, and an increasing percentage of people die from these health conditions, researchers said.
They are dying even though they are not technically considered obese, they said.
Of the four million deaths attributed to excess body weight in 2015, nearly 40 per cent occurred among people whose body mass index (BMI) fell below the threshold considered "obese."
The findings represent "a growing and disturbing global public health crisis," according to a study published in The New England Journal of Medicine.
Among the 20 most populous countries, the highest level of obesity among children and young adults was in the US at nearly 13 per cent; Egypt topped the list for adult obesity at about 35 per cent. Lowest rates were in Bangladesh and Vietnam, respectively, at 1 per cent.
China with 15.3 million and India with 14.4 million had the highest numbers of obese children; the US with 79.4 million and China with 57.3 million had the highest numbers of obese adults in 2015.
"People who shrug off weight gain do so at their own risk - risk of cardiovascular disease, diabetes, cancer, and other life-threatening conditions," said Christopher Murray, from the University of Washington.
"Those half-serious New Year's resolutions to lose weight should become year-round commitments to lose weight and prevent future weight gain," Murray said.
The study, which spans 195 countries and territories from 1980 through 2015, includes analyses of other studies on the effects of excess weight and potential links between high BMI and cancers of the oesophagus, colon and rectum, liver, gallbladder and biliary tract, pancreas, breast, uterus, ovary, kidney, and thyroid, as well as leukaemia.
In 2015, excess weight affected 2.2 billion children and adults worldwide, or 30 per cent of all people.
This includes nearly 108 million children and more than 600 million adults with BMI exceeding 30, the threshold for obesity, according to the study.
The prevalence of obesity has doubled since 1980 in more than 70 countries and has continuously increased in most other nations.
Although the prevalence of obesity among children has been lower than among adults, the rate of increase in childhood obesity in many countries was greater than that of adults.
"Excess body weight is one of the most challenging public health problems of our time, affecting nearly one in every three people," said Ashkan Afshin, from University of Washington's Institute for Health Metrics and Evaluation (IHME).
"Over the past decade, numerous interventions have been evaluated, but very little evidence exists about their longterm effectiveness. Over the next 10 years, we will closely with the FAO in monitoring and evaluating the progress of countries in controlling overweight and obesity," said Afshin.
"Moreover, we will share data and findings with scientists, policymakers, and other stakeholders seeking evidence-based strategies to address this problem," he said.