Up to 60 per cent more adults could be classed as obese under a radical shake-up of the body-mass-index (BMI) system, experts have warned.

The current standards indicate a healthy BMI ranges from 18.5 to 25, while a score of 25 to 29 signifies being overweight, and a BMI of 30 or higher is considered obese. This is the threshold where the danger of significant health issues sharply increases.

But 58 international specialists have proposed an overhaul of how obesity is diagnosed, arguing BMI alone is too blunt a tool. 

They suggest adding waist size and weight-to-height ratio to give a fuller picture of unhealthy body fat.

Researchers from Harvard University and Massachusetts General Hospital have discovered that by applying a revised definition, the number of individuals categorized as obese could climb by nearly 60 percent. Their analysis included data from over 300,000 American adults.

In Britain, where around 13 million adults are currently obese, the same calculation could see that figure soar to nearly 21 million.

Specialists described these findings as ‘significant’, emphasizing that the ‘considerable increase in obesity rates’ could result in major economic and public-health challenges.

According to the updated guidelines, obesity would be classified by a BMI over 30 plus an additional elevated metric, like a higher waist circumference or waist-to-height ratio, or a BMI surpassing 40.

Researchers labelled these subtypes of obesity as BMI-plus-anthropometric obesity.

Individuals could also be considered obese even if their BMI is below 30, provided they exhibit at least two elevated physical indicators, referred to as anthropometric-only obesity.

In the US research, 206,361 individuals fit the new obesity standards. Only 678 people (0.2 percent) who were formerly classified as obese based on BMI alone were excluded under the new criteria due to normal scores on other metrics.

Writing in JAMA Network Open, the Harvard and Massachusetts General Hospital researchers said: ‘The prevalence of obesity increased by 60 per cent when using the new definition compared with the traditional BMI-based one.

‘This rise was entirely driven by inclusion of individuals with “anthropometric-only obesity”,’ they added.

Around a quarter of all those classed as obese under the new definition fell into this group.

Although their BMI appeared healthy, these individuals had a significantly higher risk of organ dysfunction and diabetes than people without obesity – and were more than three times as likely to suffer organ damage, the scientists found.

The researchers also discovered that nearly 80 per cent of participants aged 70 or over were classed as obese under the new criteria – double the current rate.

Earlier this year, 50 experts from around the world proposed a ‘radical overhaul’ of how obesity is diagnosed and treated by including other measures such as weight-to-height ratio and waist circumference alongside BMI 

‘Our findings support inclusion of anthropometric-only obesity within the new obesity definition,’ they concluded.

The definition was first proposed in January in a report by 58 global experts published in the prestigious Lancet Diabetes & Endocrinology journal. 

They argued that BMI alone is not a sufficiently ‘nuanced’ way to assess obesity.

At the time, their recommendations were endorsed by numerous organisations, including the Royal College of Physicians.

It comes as a raft of long-awaited anti-obesity measures came into force earlier this month.

Under new Government laws, buy one, get one free deals on sweets, crisps, sugary drinks and other snacks have been outlawed in England, along with free refills of fizzy drinks in restaurants and cafés.

The crackdown will be followed in January by a ban on online adverts for unhealthy food and drink, and restrictions on TV advertising before 9 pm.

Ministers say the policies are designed to curb Britain’s growing obesity crisis.

A sobering report last year warned that Britain’s spiralling weight problem has fuelled a 39 per cent rise in type 2 diabetes among under-40s, with around 168,000 young adults now living with the disease.

Excess weight has also been linked to at least 13 types of cancer and is the second-biggest preventable cause of the disease in the UK, according to Cancer Research UK.