They compared this group with people who had a BMI in the "normal" range and analyzed the risk of both groups developing four cardiovascular diseases-coronary heart disease, stroke, heart failure and peripheral vascular disease (PVD), where fatty deposits build up in the arteries, restricting blood supply to the legs.
The team looked for markers of being metabolically healthy while also being obese.
"This is the largest prospective study of the association between metabolically healthy obesity and cardiovascular disease events,", said Dr. Rishi Caleyachetty, lead scientist of the study.
A large study of 3.5 million people has found that apparently "healthy" obese people are still at higher risk of cardiovascular disease events than the general population.
The study, which is as yet unpublished but was discussed at the European Congress of Obesity in Portugal this week, analysed medical data from 3.5 million people in the United Kingdom over a 20-year period between 1995 and 2015.
Folks dubbed healthy obese don't have metabolic problems typically associated with obesity - such as high cholesterol, poor blood sugar control, diabetes or high blood pressure.
'What was new from this study for me is that it showed that people who were overweight or obese were at increased risk of heart disease even though they may have been healthy in every other respect.
Overweight people who fall into the obese category have an increased risk of stroke and nearly double the risk of heart failure, the research found.
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The idea of "healthy obesity" is a myth, a study of 3.5 million Britons suggests. They also had a 7 per cent higher risk of stroke and a 96 per cent increased risk of heart failure.
Being "fat but fit" does not reduce the risk of having a heart failure or stroke, according to a study by British scientists. It is generally agreed to be imperfect because athletes and very fit people with dense muscle can have the same BMI as somebody who is obese.
The findings held true also when factors such as smoking were taken into account.
Experts have long debated whether people can truly be "healthy obese" or "fat but fit", characterised by having normal markers of metabolic health despite having a body mass index (BMI) of 30 or more.
In addition, excluding smokers, those with metabolically healthy obesity had an 11% higher risk of developing peripheral vascular disease. According to Caleyachetty, at the population level, the so-called metabolically healthy obesity is not a harmless condition. "It's actually better not to use this term as it can create a lot of confusion".
"Can you be fat and fit?"
'Unfortunately, the Birmingham researchers appear not to have factored in recent genetic evidence that goes a long way to explaining why so many fat people appear not to suffer ill health.
And other studies have suggested that is not always the amount of fat that matters but where where the excess fat is carried on the body that can affect fitness and health.