People who are over or underweight may be at greater risk of death than previously expected, scientists say.
London School of Hygiene and Tropical Medicine researchers analysed data from around 3.6 million people to make the conclusion.
They found having a high BMI – over 25 – is linked to a higher risk of death from every major cause, except transport accidents.
While being underweight was also found to increase the risk of death for conditions such as cancer and heart disease, according to the study.
People who are over or underweight may be at greater risk of death than previously expected, scientists at The London School of Hygiene and Tropical Medicine say
Lead author, Dr Krishnan Bhaskaran, said the work is among some of the first in this field.
He said: ‘We know that BMI is linked to the risk of dying overall, but surprisingly little research has been conducted on the links to deaths from specific causes.
‘We have filled this knowledge gap to help researchers, patients and doctors better understand how underweight and excess weight might be associated with diseases such as cancer, respiratory disease and liver disease.
‘We found important associations between BMI and most causes of death examined, highlighting that body weight relative to height is linked to risk of a very wide range of conditions.
‘Our work underlines that maintaining a BMI in the range 21-25 is linked to the lowest risk of dying from most diseases.’
The report found BMI was linked to death from every major cause except transport-related incidents.
The lowest risk of cardiovascular death was in those with a BMI of 25.
Every five-unit increase in BMI above this was associated with a 29 per cent higher risk, said the report, published in The Lancet Diabetes and Endocrinology journal.
The lowest risk of cancer death was with a BMI of 21 – with every five-unit increase associated with a 13 per cent higher risk.
BMI outside the healthy range was associated with up to several years of lost lifespan, with most of the absolute mortality burden driven by obesity – those with a BMI of at least 30.
The report said the expected age of death for a 40-year-old of healthy weight who had never smoked was 82.2 years for men, and 84.3 for women.
Obesity was associated with knocking 4.2 years off that figure in men, and 3.5 years in women.
Class three obesity – those with a BMI of at least 40 – was associated with shortening life expectancy by 9.1 years in men and 7.7 years in women.
BMI calculators use factors such as age, weight and height to determine whether a person is of a healthy weight.