Slim waist is a better indicator of health than body mass index, researchers say.

According to the scientists, keeping waist less than half of the weight can increase lifespan. The study, conducted by the City University London, scientists finds that waist to height ratio is a better indicator of health than the more commonly use Body Mass Index (BMI).

BMI or body mass index was developed by Belgian scientist Adolphe Quetelet in the 1830s. It is a number calculated according to person's weight and height. A person is underweight if BMI is below 18.5, normal if it is between 18.5 and 24.9, overweight if it is between 25.0 and 29.9 and obese if it is 30.0 and above. Health experts say that BMI isn't a good indicator of health as it fails to account for fat.

For the study, the researchers compared the effect of central obesity, which is measured using WHtR with that of total obesity (measured in BMI) on life expectancy. The team looked at health data from more than two decades. They found a strong link between weight to height ratio and mortality rates.

According to their estimates, an average 30-year-old woman, who is about 5'4" tall, can lose 1.4 years of her life if she lets her waist size expand from 32" to 38.4". If her waist size increases to 51", then she'd die 10 years earlier.

Similarly, a 30-year-old man, who is about 5'10" tall, could die 20.2 years earlier if lets his waist expand from 35" to 56".

The study shows that doctors need to reassess obesity standards.

"There is now overwhelming evidence that government policy should place greater emphasis on WHtR as a screening tool. Current UK policy tends to be restricted to BMI and, to a lesser extent, waist circumference. Focusing on WHtR, which is more globally useful than waist circumference, will identify those with central obesity and ensure resources are focused on those most at risk," Les Mayhew of Cass Business School, according to a news release.

This isn't the first time that researchers have found an association between flab around the tummy to potential health risks. Belly fat has been linked with increased risk of fractures and even kidney diseases. Health experts say that "apple shaped" people, with fat around their abdomen are at a greater risk of long-term health complications than other people who store fat in different parts of the body.

Other researchers have also found that body shape rather than body mass could be a better indicator of death risk in overweight or obese people.