“BMI doesn’t reliably predict risk of disease or mortality.”
According to a new paper, the waist-to-hip ratio is a better indicator of healthy weight and should be taken into consideration as a superior measure of early death than BMI.
The body mass index (BMI) is commonly used to determine a person’s healthy weight, with a BMI between 18.5-24.9 kg/m2 considered to be within the healthy weight range.
Irfan Khan, a medical student at the College of Medicine and Health, University College Cork, Cork, Ireland, who conducted the research with colleagues in Canada, notes that BMI “doesn’t take into account fat distribution.”
“It doesn’t consider where fat is stored – whether it’s accumulated around the hips or the waist. As a result, BMI doesn’t reliably predict risk of disease or mortality.
“We wanted to find out whether waist-to-hip ratio (WHR) or fat mass index (FMI) would more reliably predict mortality across different fat distributions.”
First, the researchers looked into whether having more fat (adiposity) leads to higher mortality rates or is just correlated with it.
Analysis of data from UK Biobank participants with genes known to increase the risk of weight gain/obesity (i.e. genetically-determined adiposity) showed that higher levels of fat actually cause more people to die, rather than just being linked to it.
After that, the researchers used information on 25,297 Caucasian men and women whose health had been monitored as part of the UK Biobank study until their deaths as well as 25,297 controls from the same study who were matched for age, sex, and genetic ancestry to apply genetically determined adiposity measures to the data. These measures included information on the genes associated with BMI, WHR, and FMI. (Participants’ average age was 61.6 years, with 59.3% men.)
The results indicated a linear association between WHR and death from any cause, indicating that the probability of early death was lowest for individuals with the lowest WHR and gradually increased with increased WHR.
As opposed to this, BMI and FMI had a J-shaped connection with all-cause mortality, which meant that people with either an extremely high or low BMI or FMI had a higher mortality risk than people with a moderate BMI or FMI.
Compared to BMI or FMI, WHR had a stronger correlation with all-cause death. For instance, compared to an increase in BMI or FMI, the odds of early death increased by approximately twice as much for each unit increase in WHR.
Males had a stronger correlation between WHR and all-cause mortality than females.
Unlike BMI and FMI, the relationship between WHR and death from all causes was the same for people with different amounts of fat.
The main flaw in BMI, according to Mr. Khan, is that it ignores variations in fat distribution. This could indicate that, despite the health dangers of abdominal fat, someone who has acquired “fat around their waist will have the same BMI as someone of the same age and height who stores their fat around the hips”.
However, WHR more accurately measures amounts of abdominal fat, particularly visceral fat, which wraps around the internal organs and increases the risk of a number of diseases, including type 2 diabetes and heart disease.
“With WHR the message is simply: the lower the WHR, the lower your mortality risk.”
He also says that the results need to be confirmed in people with different genetic backgrounds, but they show that having a lower WHR is better, no matter what population was studied.
“This is a further advantage over BMI, where optimal BMI varies from one population to the other,” adds Mr. Khan.
Although genetically determined WHR was employed in the analysis, the study’s authors claim that their findings still hold true for the traditional definition of WHR.
How is waist-to-hip (WHR) calculated?
To get this quick and easy measurement, divide the circumference of the waist by the circumference of the hips.
They conclude that the waist-to-hip ratio is a better indicator of health than the body mass index.
According to Mr. Khan, clinical advice and interventions should place more emphasis on establishing appropriate WHR goals than generic BMI targets.
“A more accurate measure of a healthy body shape may make a significant difference to the ill health and deaths caused by type 2 diabetes, heart disease, some cancers and numerous other conditions.”
Source: EASD Stockholm 2022, Abstract 65
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