Being overweight might not raise your risk of dying but being obese or underweight could, a study suggests

• Being overweight was not linked to a higher risk of dying in a study. 
• There was a higher risk of dying for those who were obese or overweight, however.
• A co-author of the study said it confirms that a person's BMI doesn't always reflect their health.

Being overweight on the body mass index was not linked to a higher risk of dying in a study, while being underweight or obese was. 

BMI is calculated by dividing your weight in pounds by your height in square meters. It is widely used as a tool to assess a person's risk of developing certain health conditions linked to weight.

For adults, a BMI of between 18.5 and 24.9 is considered healthy, 25 to 29.9 overweight, and 30 or more obese. 

Having a higher BMI is typically associated with a higher risk of diabetes, cancer, and heart disease, among other conditions, but there has been debate over the measure's effectiveness among the medical community for years.

As Insider previously reported, the American Medical Association recently announced that using BMI to measure health risk has significant limitations and it should be used in conjunction with other measurements, such as body composition, waist circumference, and genetic factors.

Overweight people did not have a higher risk of dying in the study

The study, published in the journal PLOS ONE, investigated whether there is a link between BMI and the chance of dying using data on 554,332 American adults taken from the 1999-2018 National Health Interview Survey and the 2019 US National Death Index. 

BMI was calculated using the participants' height and weight, and researchers compared BMI levels with what the participants died of over the 20-year period. 

The results showed no significantly increased risk of dying for those aged over 65 with a BMI between 22.5 and 34.9, spanning the overweight and obese categories, and the same applied to younger adults with BMIs between 22.5 and 27.4, meaning they were overweight.

Aayush Visaria, co-author of the study and an internal medicine resident at the Rutgers Robert Wood Johnson Medical School in New Jersey, told NBC: "Our results are basically confirming a lot of other studies that have been done in recent years — not at this large of a level, but basically confirming that BMI by itself is really a poor indicator of health risk."

However, Dr. Adam Jacobs, senior director of biostatistical science at clinical research organization Premier Research, who was not involved in the study said the lower mortality risk for overweight older adults may be explained by a "healthy survivor bias."

"In other words, older patients whose obesity puts them at particularly increased mortality risk are probably not in the study as they would already be dead before they reach that age," he said. 

But overall, men and women across all races and ethnicities who had a BMI of 30 or over, meaning they were obese, were found to have a 21% to 108% increased risk of dying. Those with a BMI of less than 18.5, who were classed as underweight, also had a higher risk of dying.

This reflects what's known as the U-shaped relationship between BMI and deaths, where people are most at risk at the highest and lowest ends of the scale.

However, experts note that a person's risk of dying may ultimately be an irrelevant measure if their quality of life is low. 

Professor Naveed Sattar, an expert in cardiovascular and metabolic Health at the University of Glasgow, UK, who was not involved in the study, said: "We know from other evidence that the higher our weight, the greater the risks of developing multiple conditions such as diabetes, arthritis, and heart failure. These conditions in turn adversely influence people's quality of life and their happiness."