The research, by scientists from Montreal's McGill University, suggests extreme obesity can shorten a life by more than eight years and rob the person of 19 years of healthy life — in other words, time spent without diabetes and-or heart disease.
The work is published in the journal Lancet Diabetes and Endocrinology and was funded by the Canadian Institutes of Health Research.
Lead author Dr. Steven Grover says the biggest impact is seen on people who are obese early in life.
The toll is quite a bit more modest on people who become overweight or obese in their 60s or 70s.
Other studies have estimated what obesity does to life expectancy, but Grover says this study is the first to estimate the loss of healthy life resulting from obesity.
The research is what's known as a modelling study. It uses formulas to calculate the effect of being overweight, obese and extremely obese on length of life and healthy life years lost.
People who have a body mass index (BMI) of 25 to 30 are considered overweight. Those 30 to 35 are considered obese and extremely obese means having a BMI of more than 35.
Grover and his colleagues used data on nearly 4,000 participants in U.S. National Health and Nutrition Examination surveys conducted from 2003 to 2010. They drew on measurements related to obesity, blood pressure, glucose concentrations, lipid concentrations, and other risk factors to calculate an individual's risks of developing diabetes and cardiovascular disease.
As they only focused on those two chronic conditions, the model results might be conservative, acknowledges Grover, a clinical epidemiologist and general internist at McGill University Health Centre. But he says the decision was deliberate.
They focused on conditions known to be influenced by obesity where the risk could be changed with weight loss or behaviour modification. And while it is known that being obese raises the risk of developing some forms of cancer, there is no evidence that losing excess weight lowers that cancer risk, he says.
The data pointed to obesity in younger life as being more dangerous than obesity that develops late in life.
"The earlier in life that you're overweight, the bigger the impact is going to be, as you'd imagine," says Grover.
"If you're going to live obese for 60 years, there's a big difference between that and gaining weight in your 70s."
Because the study is a modelling exercise, the authors are not reporting on their observations of individual patients. Therefore the model cannot predict what would happen to a person who was obese in his or her 20s, but who lost the weight later and kept it off.
According to the model, very obese women and men aged 60 to 79 lose less than a year on life expectancy, but spend either 7.3 years more (women) or four years more (men) dealing with cardiovascular disease and-or diabetes when compared to people with a healthy BMI.
But in the 20- to 39-year-old age group, life expectancy is reduced by an estimated 8.4 years and six years for men and women respectively. And the corresponding reduction in years of healthy life for very obese people in this age group hovers around 19 years.
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