A new study by researchers from the United Kingdom has warned that being overweight or underweight can reduce a person's life expectancy.

The biggest of its kind, the new study by the London School of Hygiene and Tropical Medicine has linked a healthy body mass index or BMI to a lower risk of dying from heart conditions, cancer, or other life-threatening illnesses.

Meanwhile, having too high or too low BMI is linked to an increased risk of dying from every major cause except transport accidents.

BMI Linked To Life Expectancy

The study published in the journal The Lancet on Tuesday, Oct. 30, used anonymous data from the Clinical Practice Research Datalink that included BMI from primary care records. This is also linked to the data collected by the Office of National Statistics mortality database that also lists information on causes of death.

The researchers then looked at the association between BMI and various causes of deaths. They adjusted the data based on different other important factors such as sex, age, smoking habits, alcohol consumption, and socioeconomic status. The BMI is based on a person's height and weight.

The study revealed that obese men have a life expectancy that is 4.2 years shorter compared to those who are in a healthy weight range. Meanwhile, obese women are facing a life expectancy that is 3.5 years shorter than their peers.

Those who are considered underweight are not any better. Men and women who have lower BMIs knock off 4.3 years and 4.5 years from their respective lifespans.

"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," stated Krishnan Bhaskaran, lead author of the study and an associate professor at the London School of Hygiene and Tropical Medicine.

"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."

Keeping A Healthy BMI

The U.S. National Institute of Health recognizes BMI as a useful measure to determine whether a person is overweight or obese. In an unrelated study from 2010, public health officials found that the healthy body mass index of non-smoking adults is 20.0 to 24.9.

To calculate the BMI, divide weight by the square of the person's height. The NIH also has a BMI calculator on its official website.