There is no such thing as ‘fat but fit’, scientists claim

The idea that a person can be ‘fat but fit’ may be a myth, according to a major study of more than 150,000 Britons.

Researchers found that obese adults with no obvious metabolic problems – meaning no high blood pressure, diabetes or abnormal cholesterol – were still significantly more likely to develop heart, liver and kidney disease than those of a healthy weight.

The findings challenge the widely debated idea that it is possible to be overweight yet remain healthy, suggesting that excess body fat alone may be enough to drive long-term disease risk.

The study led by experts at Imperial College London, analysed data from more than 157,000 participants in the UK Biobank and followed them for a median of 12.6 years.

Participants were grouped according to body mass index (BMI) and whether they had metabolic abnormalities such as high blood pressure, diabetes or high cholesterol – allowing researchers to compare those considered ‘metabolically healthy’ with those already showing signs of ill health.

Among the group, whose average age was 56.5, around a quarter were classified as obese and more than two-thirds had at least one metabolic abnormality.

The results showed that even in the absence of any other warning signs, obesity carried substantial risks.

Men with obesity but no metabolic problems were 46 per cent more likely to develop heart disease, 63 per cent more likely to experience heart failure and more than twice as likely to develop fatty liver disease compared with healthy-weight men.

Women in the same category were 34 per cent more likely to develop heart disease, 69 per cent more likely to develop heart failure and more than four times as likely to develop fatty liver.

Once metabolic abnormalities were present, the risks rose sharply.

Men with obesity and conditions such as high blood pressure, diabetes or abnormal cholesterol were more than twice as likely to develop cardiovascular disease, nearly three times more likely to suffer heart failure and more than six times more likely to develop fatty liver disease.

Women faced almost four times the risk of heart failure and more than eight times the risk of fatty liver disease, while the likelihood of kidney disease and early death also increased significantly.

Overall, the researchers concluded that obesity without metabolic abnormalities is not a benign condition and is linked to a wide range of adverse cardiometabolic outcomes.

They said the findings highlight the importance of early intervention – before metabolic problems develop – to reduce long-term risks.

The study comes as obesity rates continue to climb.

In the UK, nearly two-thirds of adults are overweight and more than a quarter are obese – equivalent to around 14 million people.

Obesity is already known to increase the risk of a range of serious conditions, including type 2 diabetes, heart disease and respiratory illness, and is estimated to cost the NHS more than £11 billion a year, with further losses linked to reduced productivity and welfare spending.

At the same time, experts are increasingly questioning whether current measures of obesity are fit for purpose.

Body mass index, or BMI, remains the most widely used tool, with a score of 18.5 to 25 considered healthy, 25 to 29 classed as overweight and 30 or above defined as obese.

However, critics argue it is too blunt an instrument and does not account for how fat is distributed around the body – a key factor in determining risk.

A group of 58 international specialists has recently proposed a broader approach that includes measures such as waist circumference and weight-to-height ratio to better capture harmful fat levels.

Analysis of data from more than 300,000 adults in the US suggests that adopting these additional measures could increase the number of people classified as obese by almost 60 per cent.

In the UK, that could mean the number rises from around 13 million to nearly 21 million.

The researchers behind the latest study say their findings reinforce a clear message: even when traditional markers of metabolic health appear normal, carrying excess weight is associated with significantly higher risks of serious disease.

Maintaining a healthy weight, they suggest, remains one of the most important steps for protecting long-term heart, liver and kidney health.