The idea that one could be obese but still fit took root in 2008, in a study carried out at the University of Tübingen, Germany
Fat but fit: Can you be metabolically healthy while being overweight?
Doctors call it “metabolically healthy obesity”. Newspaper headlines trim it down to “fat but fit”. But whatever you choose to call it, since 2008, those of us carrying around a few extra kilos have been able to console – or, possibly, deceive – ourselves with the “fact” that, while we might wobble a bit when we run, at heart we are healthier than we look.
Last week a team of researchers from over 40 institutions across Europe poured cold water on the whole idea, with a paper concluding that “‘metabolically healthy’ obesity is not a benign condition”. Or, if you prefer, the whole concept of “fat but fit” is nothing but a myth.
The idea that one could be obese but still fit took root in 2008, in a study carried out at the University of Tübingen, Germany. Professor Norbert Stefan and colleagues were the first to identify and describe a group of patients who, though technically obese, showed few of the telltale signs that they were heading for heart problems.
There are five key markers: obesity, high blood pressure, high levels of fats and low levels of ‘good’ cholesterol in the blood, and rising levels of blood sugar, itself a precursor to diabetes. A patient with any three or more of these symptoms is in the grip of a condition known as metabolic syndrome, and on the fast track to coronary heart disease.
Obesity is defined by your score on the internationally recognised Body Mass Index, or BMI. This is your weight divided by your height squared. Under 18.5 is deemed to be underweight, 18.5 to 24.9 healthy weight, 25 to 29.9 overweight, and 30 and over obese.
But writing in the journal Archives of Internal Medicine in 2008, Stefan and colleagues described “a metabolically benign obesity that is not accompanied by insulin resistance and early atherosclerosis”, or blockage of the arteries. Even more surprising, they found that in a proportion of obese patients, this “metabolically benign obesity” could actually “protect from insulin resistance and atherosclerosis”.
Though they conceded that the mechanisms involved were not unclear, that paper, says Stefan, chair of clinical and experimental diabetology at the University of Tübingen, Germany, and head of the Department of Pathophysiology of Prediabetes at the Institute of Diabetes Research and Metabolic Diseases, Munich, “did put the subject on the stage”.
Now it appears to have been discounted by a paper published last week in the European Heart Journal. A team led by scientists from Imperial College London and the University of Cambridge looked at 7,637 people enrolled in the large European Prospective Investigation into Cancer and Nutrition, who over a 12-year period had gone on to contract coronary heart disease.
They found that, “irrespective of metabolic health, overweight and obese people had higher risk than lean people”. The extra kilos raised the risk of heart disease by 28 per cent and, they concluded, “‘metabolically healthy’ obesity is not a benign condition”.
Put another way, don’t get too comfortable in your overstretched skin – right now you might be free of the danger signs, such as increased blood pressure and raised blood glucose levels, but that isn't going to last. All it means, says co-author Dr Ioanna Tzoulaki, from Imperial’s School of Public Health, is that “people with excess weight who might be classed as ‘healthy’ haven't yet developed an unhealthy metabolic profile”.
The findings, says lead author Dr Camille Lassale, from University College London, “challenge the concept of the ‘healthy obese’… those overweight individuals who appear to be otherwise healthy are still at increased risk of heart disease”.
But Stefan, father of the “fat but fit” concept, insists that the new paper is flawed because the researchers define metabolic unhealthiness – fat but unfit – differently to other researchers, including him. It all comes down to how many of the qualifying characteristics for metabolic syndrome – obesity, insulin resistance, glucose intolerance and high levels of ‘bad’ cholesterol, and low levels of the ‘good’ kind – that you count.
“We suggest someone who is obese is unhealthy if they have more than one of these factors,” says Professor Stefan. “But they allow an individual to have two.”
So while Stefan and other researchers over the past decade have concluded that about a quarter of obese people are fat but fit, the new study more than doubles this percentage. The consequence is that far more of them end up having heart problems – because by the terms of the original definition they were never “healthy” in the first place.
Not so, says Tzoulaki. “We chose a definition of metabolic syndrome as suggested by international guidelines,” she insists. What’s more, “even when we defined healthy by the absence of all four metabolic factors, the results were similar”.
But Stefan points to a table in the paper showing the relative risk of heart disease for five different categories of individuals. “The risk for the metabolically healthy obese is given as 28 per cent higher, but the risk for the metabolically unhealthy obese is 254 per cent,” he says. But if there really were no such thing as fat but fit, he says, the risk for both sets of people should be about the same.
“Clearly, even by their definition, you are much more safe being metabolically healthily obese than metabolically unhealthily obese,” says Stefan. “So when they conclude that the concept of metabolically healthy obesity is not correct, I would say these data don’t show that.”
For some, the back and forth of scientific debate, played out in newspaper headlines, is not helpful for ordinary people concerned about their health. Academics arguing about whether or not “fat but fit” is a myth serves only to drown out a simple, but important message, says Duane Mellor, dietician and spokesperson for the British Dietetic Association: “It doesn’t matter what size you are, you can do something to improve your health.”
The most helpful message for people concerned about their weight, he says, is to “be more active and try to eat better – maybe reduce the size of portion sizes and eat less food that’s high in fat and sugar, and more that is less refined – wholegrain foods, vegetables and fruit”.
Oh, and lose those scales.
“Don’t worry about the weight – we don’t have that much evidence that weight loss on its own is the best thing,” he says.
“If you are more physically active and eat better, your weight might change a little bit, but you will tend to notice that your trousers or your skirt fit better, and that’s probably more important than the number on the scales.”