Medical experts have suggested that ditching BMI and focusing more on waist-to-hip ratio could help keep health problems at bay.
BMI is a measure of your weight to your height, and is often used to determine if a person is overweight or obese.
But data presented at the European Association for the Study of Diabetes in Stockholm found that measuring stomach fat is a much better predictor of early death than looking at BMI.
This news comes after the National Institute for Health and Care Excellence (Nice) said a few months ago that people should make sure that their waist measurement is less than half of their height, in order to keep healthy.
The institute said that any adult with a BMI of under 35 should keep track of their own waist measurements in a bid to tackle obesity.
A BMI of 18 to 25 is considered a healthy weight, 25 to 30 is overweight, and over 30 is obese.
Nice said that by using a waist-to-height ratio, together with BMI, people can work out if they are carrying excess fat around their stomach.
This "tummy fat" is known to heighten the risk of type 2 diabetes, high blood pressure and heart disease.
For example, a 5ft 4in woman with a waist circumference of 29 inches would have a healthy ratio, but 32 inches would push them into the unhealthy range.
Irfan Khan, a medical student at the University College Cork in Ireland, who carried out the new study with colleagues in Canada, said: "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."
In the new study, which hasn't been peer-reviewed yet, researchers first explored whether higher levels of fat increased the risk of death, or if they were just linked.
Using data from UK Biobank participants who had genes known to raise the risk of weight gain and obesity, they found that higher levels of fat actually increased the risk of early death.
Further work on 25,297 men and women whose health had been tracked as part of the study, matched with the same number of people acting as controls, showed that the relationship between waist-to-hip ratio and death from any cause increased in a linear way.
This meant the risk of an early death was lowest for those with the lowest waist-to-hip ratio, before steadily increasing as the waist-to-hip ratio increased.
In contrast, those with either an extremely high or low BMI or fat mass index (another measure of fat) had a higher risk of death compared to those with a moderate BMI or fat mass index.
Waist-to-hip ratio was also more strongly associated with deaths from any causes than BMI or fat mass index.
Irfan said: "BMI’s major limitation is that it doesn’t take into account differences in fat distribution.
"This could mean that someone who has accumulated fat around their waist will have the same BMI as someone of the same age and height who stores their fat around the hips, despite the health risks of abdominal fat.
"Waist-to-hip ratio, however, better reflects levels of abdominal fat, including visceral fat, which wraps around the organs deep inside the body and raises the risk of a range of conditions, including type 2 diabetes and heart disease.
"With waist-to-hip ratio the message is simply: the lower the ratio, the lower your mortality risk."
Professor Nick Finer, from the University College London Institute of Cardiovascular Science, said: "It is well established that BMI is an imperfect measure of the risks of having overweight or obesity, and that many (not all) of these risks are driven not just by how much fat is stored in the body but where it is stored.
"However, there is a problem about relying on the waist-hip ratio as a measure of obesity severity, because it changes little with modest weight loss and so does not necessarily reflect health improvements as a result of obesity treatments that lead to weight loss."
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