We've long known that too much excess weight increases your likelihood of dying prematurely. Or does it?
Obesity picture from Shutterstock
A large review of the evidence published in the Journal of the American Medical Association (JAMA) in January found that overweight people live longer than leaner people.
But don't reach for the pie and chips just yet.
There is no denying that the high and rising prevalence of obesity and overweight is a major health problem. With 70 per cent of men and 56 per cent of women either overweight or obese, Australia is among the worst-affected countries in the world.
We commonly measure body size with body mass index, or BMI: a person's weight in kilograms divided by the square of their height in metres. It is a good average measure of the amount of fat a person has in their body, which is useful for classifying people in population studies.
The World Health Organization (WHO) defines a "normal" BMI range as 18.5 to 24.9 and this corresponds to a weight of 49 to 65kg for Australian women of average height (1.62 m) and 58 to 77kg for men of average height (1.76 m).
Recent estimates have shown for the first time that a high BMI is now responsible for the greatest burden of disease in Australia and New Zealand, ahead of smoking and high blood pressure.
But the challenges represented by the obesity epidemic have raised the question of what the ideal BMI is for life expectancy.
Two large and well-conducted studies in the Lancet and New England Journal of Medicine examined this issue by gathering data on more than two million people who had their BMI calculated and were followed for their risk of dying over a defined time period.
The large numbers of participants and detailed individual data in these studies means the researchers were able to look at how small differences in BMI relate to the risk of death, accounting for a range of other factors known to influence this relationship, including illnesses that could potentially affect BMI.
Despite the diversity of populations covered by the studies, and the differences in methods, their findings are remarkably consistent: people with a BMI at the upper end of the WHO "normal" level (22.5 to 24.9) have the lowest death rates.
You can see this visually represented by the J-shaped curve in the graph below: as BMI goes up in increments of 2.5 above and below the 22.5-to-24.9 category, so do death rates.
Why do people with a BMI at the upper end of the "normal" range have the lowest risk of death?
People with BMIs above this optimal level have an increased risk of dying, especially from heart disease, most likely due to increases in blood pressure, cholesterol levels and diabetes caused by excess body fat. They also have an increased risk of dying of cancer.
People with BMI levels below the optimal level also have increased death rates, particularly from respiratory diseases (such as chronic bronchitis) and cancer. The increased risk of death in the people with lower BMIs may also be because chronic illness has caused them to lose weight.
So with this evidence in mind, how does the JAMA paper reach such a different conclusion?
It comes down to the way the data from each of the studies have been collated and presented.
The JAMA review used broad classifications for underweight, normal weight, overweight, and obese and very obese, rather than the 2.5 increments of BMI. People with a BMI of 18.5 to 24.9 were included in the "normal weight" category; we can see from the graph above that this broad category includes people with the lowest risk of death, combined with people with a higher risk of death.
The "normal weight" category was then used as the comparison group for the studies, and has an average risk of death that is higher than the risk in the broad "overweight" category.
This skews the optimal weight finding and changes the shape of the curve, from J-shaped to tick-shaped.
The result? One that suggests being overweight makes you live longer.
There are varying reasons why the researchers might have used these broad groups, including the fact that many studies are too small to be able to present statistically reliable results according to finer gradations in BMI.
The bottom line is that too much fat is bad for your health and increases your risk of dying prematurely. We still have a long way to go in our fight against obesity and, if anything, we need to redouble, not reduce, our efforts.
Emily Banks is Scientific Director of the 45 and Up Study at Sax Institute. She receives research funding from the National Health and Medical Research Council and the Australian Primary Health Care Research Institute. Rosemary Korda is a Research Fellow at the National Centre for Epidemiology and Population Health. She receives funding from NHMRC and ARC. Grace Joshy is a Research Fellow at the National Centre for Epidemiology and Population Health. She does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.