New evidence shows that healthy obesity is a myth

Exercise is important, but so is weight loss. Pressmaster/Shutterstock

There is no such thing as being “healthy at every size”. Our latest study shows that if you’re obese but metabolically healthy (so-called “fat but fit”), you are still at a greater risk of cardiovascular disease compared with metabolically healthy people who are a normal weight.

The fat-but-fit debate – which has been raging for more than a decade – is important, from a public health perspective. More resources can be focused on those at a high risk of cardiovascular disease (such as heart attacks, heart failure and stroke), while fewer resources need to be expended on those who are at low risk of developing cardiovascular disease.

So, if you’re obese but don’t have type 2 diabetes, high blood pressure or high cholesterol, perhaps you should be treated the same as someone who has normal weight and doesn’t have any of these metabolic abnormalities. Or so the thinking went. But mounting evidence suggests that being obese is a risk factor for future ill health, regardless of current metabolic health.

A recent observational study, with 18,000 participants from ten European countries, found that obese people had a higher risk of coronary heart disease than metabolically healthy normal weight people. Our study adds to that evidence, and goes much further.

Largest study on the topic

Ours is the largest prospective observational study on the metabolically healthy obese so far. We used data on 3.5m people, aged 18 years or older, from a large primary care database in the UK.

We were not only interested to know if there was an association between being obese and metabolically healthy and later developing coronary heart disease, we also wanted to know if there was an association with stroke, mini-stroke (also known as a transient ischaemic attack or TIA), heart failure and peripheral vascular disease. We also assessed whether healthy obese adults maintain their metabolically healthy profile over a follow-up period.

To be classified as “metabolically healthy obese”, participants had to have no diabetes, abnormal blood fats or high blood pressure at the start of the study.

Out of the 3.5m participants, who were initially free from cardiovascular disease, about 15% were classified as metabolically healthy obese. During an average follow-up period of five years, of the people who were initially metabolically healthy obese, about 6% developed diabetes, 12% had abnormal blood fats and 11% developed high blood pressure.

Compared with normal weight people with no metabolic abnormalities, people who were metabolically healthy obese had a 50% increased risk of coronary heart disease, a 7% increased risk of stroke and a double risk of heart failure. These results couldn’t be explained by age, sex, smoking or socio-economic status as we took these factors into account in our calculations.

A body mass index of 30 or greater is considered obese. Igor Chus/Shutterstock

Not a benign condition

So can you be fat but fit? If being fit equates to cardiovascular health, then no. Taking into account the genetic differences related to obesity among people, we can perhaps imagine that within the population of obese people there are those who may not face an increased risk of obesity-related complications, such as cardiovascular disease. But enough evidence has now accumulated to accept that, at the population level, metabolically healthy obese is not a benign condition. Future research work should therefore focus on finding the most effective population-wide obesity prevention and control strategies.