Being overweight or underweight could knock four years off of your life expectancy, according to a new study of nearly two million people in the UK.
One of the largest studies of its kind looking at people’s body mass index (BMI), the report found that from the age of 40 people within the healthy BMI range had the lowest risk of dying from disease but people at the top and bottom ends risked having shorter lives.
The difference in life expectancy was as much as 4.2 years for obese men and 3.5 years shorter for obese women. And 4.3 years shorter for underweight men and 4.5 for underweight women.
BMI (body mass index) is the metric used to determine whether someone falls within a healthy weight range, and is calculated by dividing an adult’s weight by the square of their height. A healthy score ranges from 18.25 to 25.
There are some limitations with BMI – the NHS says your BMI can tell you if you’re carrying too much weight, but it can’t tell if you’re carrying too much fat.
It also can’t tell the difference between excess fat, muscle or bone – so if you are particularly muscular this may register as being overweight when you are actually fit.
Additionally, adults who lose muscle as they get older may fall into the “healthy weight” range even though they may be carrying excess fat. And pregnant women won’t be able to judge their health based on their BMI.
But overall, medical experts say it is the best method they have of working out whether someone is obese because it is accurate and simple to measure.
In the study, BMI was associated with all causes of death categories (except transport-related accidents), including cancer, cardiovascular diseases and respiratory diseases.
Although being in the under or overweight categories puts you at higher risk of a shortened life span, this does not mean that everyone in the healthy bracket is exempt from disease.
Report author Dr Krishnan Bhaskaran told the BBC: “For most causes of death we found that there was an ‘optimal’ BMI level, with risk of death increasing both below and above that level.
“At BMIs below 21, we observed more deaths from most causes, compared with the optimum BMI levels. However, this might partly reflect the fact that low body weight can be a marker of underlying ill-health.”