"The idea that people can be fat but medically fit is a myth," reports BBC News.
The story is based on research from scientists at the University of Birmingham, reported at a medical conference but not yet published.
The researchers wanted to know if this group, sometimes called "metabolically healthy obese" people, had a raised risk of cardiovascular disease compared to people of recommended weight (a BMI of 18.5 to 24.9).
The research found they had a higher chance of heart disease, stroke or transient ischaemic attack (mini stroke) and heart failure, compared to those of recommended weight. However, their risk was not as high as for obese people who also had diabetes, high blood pressure or abnormal fats.
The research is unpublished, which means we can't check the validity of the study. However, it confirms that keeping to a healthy weight is likely to lower your chances of cardiovascular disease, which is not a surprising finding.
The study was carried out by researchers from the University of Birmingham. It was presented at the European Congress on Obesity in Portugal.
Sources of funding were not declared.
Some sections of the UK media seized on the study with glee. "Think you're fat AND fit? There's no such thing!" crowed the Daily Mail, illustrating its article with fat-shaming photographs of overweight people at the gym.
Most of the coverage repeated the line that it is not possible to be overweight and healthy, which is not what the study found.
The study results showed obese people were at an increased risk of certain diseases, but that doesn't mean they will all get these diseases.
This was a prospective cohort study.
This type of study is good at finding links between factors – such as weight, metabolic indicators and cardiovascular disease, in this case – but cannot prove that one causes another.
So the study does not prove that being obese but metabolically healthy causes cardiovascular disease, only that there's a link between the two.
Researchers used electronic health records from 1995 to 2015 from the Health Improvement Network database of UK general practice records.
They looked at records of people aged 18 and over, without cardiovascular disease at the start of the study. People were grouped according to their BMI and whether they had any of three metabolic risk factors: diabetes, high blood pressure or abnormal blood fats.
Researchers then calculated the relative risk for each group of getting one of four cardiovascular disease conditions:
They compared the risks of people with recommended weight and no metabolic risk factors with people who were obese and had no, one, two or three metabolic risk factors.
They adjusted their figures to take account of confounding factors including age, sex, smoking and socioeconomic status.
Of the 3.5 million people in the study, 766,900 (21.9%) were obese – of whom 518,000 (14.8%) were obese with no additional risk factors (metabolically healthy).
The researchers found that, compared to people of recommended weight, metabolically-healthy obese people were:
The findings for peripheral vascular disease were mixed. Overall, metabolically-healthy obese people were 9% less likely to get peripheral vascular disease. However, excluding smokers, the risk was 11% higher.
Metabolic risk factors raised the chances of getting any of these conditions, in addition to obesity.
Compared to recommended weight, metabolically-healthy people, those who were obese and had all three risk factors were:
The researchers said their study showed that "metabolically-healthy obese individuals are at higher risk" of the diseases studied and that "The priority of health professionals should be to promote and facilitate weight loss among obese persons, regardless of the presence or absence of metabolic abnormalities."
Study author Dr Rishi Caleyachetty added: "So-called metabolically healthy obesity is not a harmless condition and perhaps it is better not to use this term to describe an obese person."
The question of whether someone can be "fat but fit" has been much debated. If you're obese but exercise, eat well and don't have metabolic risk factors, the theory goes, you could be just as healthy as someone of recommended weight. This study suggests that may not be true.
It is definitely worth adopting a healthy lifestyle, whatever your weight. The study found that, the more metabolic risk factors people had, the more likely they were to develop heart disease, cardiovascular disease and so on. Metabolic risk factors do make a difference.
But in this large study, on average, people who were obese with no metabolic risk factors had a higher risk of disease than people of recommended weight with no metabolic risk factors.
The study has some strengths. It is very large, and uses data from records that are thought to be reasonably reliable. However, we need to remain cautious about the strength of the study until we can see the full data. The researchers say the paper is under peer review and is expected to be published in a medical journal.
If you are worried about your weight, talk to your doctor and take a look at our weight loss programme.