The term 'fat but fit' is a fallacy that risks accelerating the spread of global obesity, according to new research which throws cold water on a commonly held medical belief.

Medical scientists from the University of Birmingham have countered the theory that people can be fat but medically fit with new research which claims that obese people remain at greater risk of developing heart disease, strokes and heart failure than people of normal weight.

The study, as yet unpublished, was presented at the European Congress on Obesity in Portugal, this week. It looks at the medical records of 3.5 million people in the U.K. between 1995 and 2015 to assess the legitimacy of the 'fat but fit' theory.

Medical research has previously suggested that obesity can have little impact on a person's chances of contracting various harmful diseases if they are otherwise medically healthy. However, the latest research, which tracked obese but "metabolically healthy" people, found that they continued to be at higher risk of developing diabetes and heart-related diseases later in life.