Death risk for overweight and underweight may be higher than previously thought

People who are over or underweight may be at greater risk of death than previous expected, scientists have said.

Those with a body mass index (BMI) of 21-25 were associated with the lowest risk of dying from cancer and heart disease, according to peer-reviewed analysis of 3.6 million people and nearly 370,000 deaths.

The London School of Hygiene and Tropical Medicine, which led the study, said that while BMI - a measure of body fat - has long been a key indicator of health, the findings shine new light on the extent to which being over or under a certain BMI could increase susceptibility to a shorter life expectancy.

Lead author, associate professor Krishnan Bhaskaran, 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 causes included cancer, cardiovascular disease, neurological issues and self-harm.

The lowest risk of cardiovascular death was in those with a BMI of 25. Every 5-unit increase in BMI above this was associated with a 29% 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 5-unit increase associated with a 13% 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.

Responding to the study, Dr Michelle McCully, head of research evidence and interpretation at World Cancer Research Fund, said: "Our research shows that overweight and obesity increases the risk of developing 12 cancers. Alongside our own research, this new study adds to the evidence that body weight influences both risk of, and survival from, cancer.

"WCRF calls on governments to take action in creating health-enabling environments that support people in following our cancer prevention recommendations, of which one is maintaining a healthy weight."