The risks of coronary heart disease, stroke, respiratory disease and cancer are all increased. Overall, the excess risk of premature death (before age 70) among those who are overweight or obese is about three times as great in men as in women.

“On average, overweight people lose about one year of life expectancy, and moderately obese people lose about three years of life expectancy,” says Dr. Emanuele Di Angelantonio from the University of Cambridge, Cambridge, UK, the lead author.

“We also found that men who were obese were at much higher risk of premature death than obese women. This is consistent with previous observations that obese men have greater insulin resistance, liver fat levels, and diabetes risk than women.”

The study found an increased risk of premature death for people who were underweight, as well as for people classed as overweight. The risk increased steadily and steeply as BMI increased. A similar trend was seen in many parts of the world and for all four main causes of death.

Where the risk of death before age 70 would be 19% and 11% for men and women with a normal BMI, the study found that it would be 29.5% and 14.6% for moderately obese men and women (BMI 30-35). This corresponds to an absolute increase of 10.5% for men, and 3.6% for women – three times as big. The authors defined premature deaths as those at ages 35-69 years.

The new study brings together information on the causes of any deaths in 3.9 million adults from 189 previous studies in Europe, the US and elsewhere.

At entry to the study all were aged between 20 and 90 years old, and were non-smokers who were not known to have any chronic disease when their BMI was recorded. The analysis is of those who then survived at least another five years. Of 3951455 participants, 385879 died.

The study also estimated the population-attributable fraction for mortality due to overweight and obesity: the reduction in deaths in a population that would occur if a risk factor were eliminated. The authors say that assuming that the associations between high BMI and mortality are largely causal, if those who were overweight or obese had WHO-defined normal levels of BMI, then the proportion of premature deaths that would be avoided would be about one in 7 in Europe and one in 5 in US.

“Obesity is second only to smoking as a cause of premature death in Europe and North America,” says co-author Professor Sir Richard Peto, University of Oxford, Oxford, UK. “Smoking causes about a quarter of all premature deaths in Europe and in North America, and smokers can halve their risk of premature death by stopping.  But, overweight and obesity now cause about one in seven of all premature deaths in Europe and one in five of all premature deaths in North America.”

The researchers also broke down the normal BMI range and found a slightly increased risk at the lower end of it (at 18.5-20 kg/m2).

The authors note that one important limitation is that their only measure of obesity was BMI, which does not assess fat distribution in different parts of the body, muscle mass, or obesity-related metabolic factors such as blood sugar or cholesterol.

“This is an important study using the largest sample so far of healthy, never smokers from around the world to demonstrate the high risks of mortality for those with obesity.

It demonstrates that for Australia and New Zealand there is an increase in the risk of dying of around one third for every increase in BMI unit after the overweight range.

Professor Anna Peeters, Professor of Epidemiology and Equity in Public Health at the World Health Organization Collaborating Centre for Obesity Prevention at Deakin University, says that with two-thirds of Australian adults overweight or obese this underscores the seriousness of current obesity rates for future life expectancy in Australia.

“Obesity may lead to the first decrease in life expectancy seen in decades,” she says. “Equally important is the demonstration of the impact of overweight and obesity on premature mortality.  

“The study estimates that if we are able to prevent overweight and obesity in Australia we would prevent one in six premature deaths.

“If we needed yet another reason to step up our efforts to prevent obesity, this is it. We must see comprehensive obesity prevention policies from our government, including restrictions on unhealthy food and drink marketing, a sugary drinks tax and a national physical activity and nutrition strategy.”

But Professor Tim Olds is from the Alliance for Research in Exercise Nutrition and Activity (ARENA) at the Sansom Institute, University of South Australia, says that the study has a significant weakness: the exclusion of about 60% of participants for various reasons, including the “rather odd” exclusion of people who have ever smoked.

“In other studies, this has been shown to shift the BMI associated with the lowest mortality (longest life expectancy) towards the lower end of the spectrum,” Prof Olds says.

“In other words, if the study included people who had ever smoked, the ‘ideal’ BMI would likely be higher. Exclusion of those who have ever smoked might bias the sample towards higher socio-economic status participants, and people with generally healthier habits.

“The study does cast some light on, and brings into question, the so-called “obesity paradox” — the finding that overweight people live longer. You see this even in these data in the Australasian and South Asian subsets.”

The study was published today in The Lancet.