Although the harmful effects of excess body weight on chronic diseases have been well researched over the past decades, recent studies about what is a healthy weight confused the public by suggesting otherwise. The so-called “obesity paradox” suggest that there is a survival advantage to being overweight. According to the authors of a large international collaborative study, led by researchers at the University of Cambridge, UK and the Harvard T.H. Chan School of Public Health, the “obesity paradox” studies had serious methodological limitations, thus invalidating their findings.

Reverse causation is cited as one of the methodological problems. This occurs where a low body weight is seen as the cause of an underlying or preclinical illness, rather than the result. A similar problem in research results is experienced when including smokers in a weight related study. Although smokers in general weigh less than nonsmokers do, they have much higher mortality rates.

According to Frank Hu, a co-leader of the collaboration and professor of nutrition and epidemiology at Harvard Chan School, the only way to find an unbiased relationship between BMI and mortality, is to exclude smokers as well as anyone with existing chronic diseases at the start of the study. Hu also feels it is critical that doctors should continue to counsel patients regarding the damaging effects of excess body weight. Cancer, cardiovascular disease and diabetes are all diseases that people with excess body weight have a higher risk of contracting.

The World Health Organization (WHO) estimates that 600 million adults worldwide are obese, while 1.3 billion are classified as overweight. As a basis for its classification, the organization body-mass index (BMI, in kg/m2), which relates weight to height. A BMI of 18.5-25 is seen as normal; 25-30 is overweight, 30-35 is moderately obese, while over 40 is severely obese.

Examples for overweight:

Height of 5’3”; 1.6m. Weight of 60-80 kg; 140-170 lb.; 10-12 stone.

Height of 5’11”; 1.8m. Weight of 80-100 kg; 180-210 pounds; 13-15 stone.

For the same height with a lighter weight, the classification is normal, while moderate obesity is seen as the same height but a higher weight.

The occurrence of adult obesity is 31% in North America and 20% in Europe.

In 2013, 500 investigators from over 300 global institutions joined forces to establish the Global BMI Mortality Collaboration. The collaboration’s purpose is to provide definitive evidence of the association of excess body weight with premature mortality.

This BIG problem is taking HUGE efforts to solve.

Shilpa Bhupathiraju, research scientist in the Department of Nutrition at Harvard Chan School notes that this is by far the most rigorous and largest effort ever undertaken resolve the controversy regarding mortality and BMI.

Between 1970 and 2015, 239 large studies have been conducted in 32 countries. 10.6 million Participants in these studies were followed for an average of 14 years and overall 1.6 million deaths were recorded. The collaboration’s new study consisted of looking at all this data from a different perspective. The actual group they ended up analyzing consisted of 4 million adults. Those eliminated from the study included any who died in the first five years of follow-up, were current or former smokers or had chronic diseases at the beginning of the original study.

Considering that a BMI of between 22.5 and 25 kg/m2 is seen a healthy weight range, the researchers found that for each increase in BMI by 5 units, the matching increases in risk were 19% for cancer mortality, 38% for respiratory disease mortality and a massive 49% for cardiovascular mortality. The study also found that the dangers of excess body weight were greater in men than in women and, interestingly enough, in younger than in older people.

As the BMI rose, these percentages increased significantly as shown in the table below:

BMI Range (kg/m2) % higher risk of mortality 25 to 27.5 7 27.5 to 30 20 30.0 to 35.0 45 35.0 to 40.0 94 40.0 to 60.0 300

Surprisingly, participants who were underweight also had a higher mortality risk. The results did however confirm that participants with a BMI that is considered normal (between 22.5 and 25 kg/m2), had the lowest mortality risk.

The full study has been published online in The Lancet journal.