You've probably heard someone say, "I'm fat but fit." Several recent studies have suggested this statement could be true. But a new review of existing studies published this week in the Annals of Internal Medicine may put a stop to the rumor.

"Healthy obesity" is just a myth, the study authors say.

Some background

Scientists know that overweight people can be what they call "metabolically healthy." This means that despite having a high body mass index, or BMI, someone can have a small waistline, normal blood pressure and low cholesterol levels, and show little to no risk for developing diabetes. The opposite is also true; thin people can be metabolically unhealthy, with high blood pressure, high cholesterol and fat that accumulates only around their middle, which is a known risk for heart disease.

This kind of paradox highlights "the complexity of the relationship between weight and mortality," the authors of this new meta-analysis write. A lot of factors impact a person's cardiovascular health, including how much they exercise and when they put on the weight.



The study

The researchers evaluated the eight studies, which included a total of more than 60,000 participants. All of the studies had recorded participants' BMI and their metabolic status, as well as any fatal or nonfatal cardiovascular events, such as a heart attack.

Data analysts split the participants into five groups:

- Metabolically unhealthy and normal weight

- Metabolically unhealthy and overweight

- Metabolically unhealthy and obese

- Overweight but metabolically healthy

- Obese but metabolically healthy

They then compared the number of cardiovascular events that happened in each of these groups with their control group: a sample of normal weight, metabolically healthy people.

The results

As they expected, the researchers found that their control group - the normal weight, metabolically healthy people - had the fewest cardiovascular events of all the groups.

They also found that being metabolically unhealthy puts you at a higher risk for cardiovascular disease, even if you're in the normal BMI range. In fact, the metabolically unhealthy thin people had the same risk as the metabolically unhealthy obese group in the short-term.

Perhaps more surprising was that they found little difference in the risk for heart disease between metabolically healthy overweight individuals and metabolically healthy normal weight people. The same was true for metabolically healthy obese people.

So why do they say healthy obesity is a myth?

Because when researchers looked specifically at studies that followed participants for at least 10 years, it showed the metabolically healthy obese group had an increased risk of death and cardiovascular events compared to those of normal weight.

And why have some previous studies suggested you can be overweight and healthy?

The study authors say those studies included metabolically unhealthy individuals in the "normal weight" control groups, which influenced their results.

Caveats

Every study has a caveat - something the scientists couldn't control for or didn't analyze that may be affecting the results. The studies analyzed by these scientists did not always have adequate information on the participants' health behaviors, such as what they ate or whether they smoked, and did not collect data about the participants' weight gain over time. Plus, not every age group was represented equally in these studies.

Takeaway

Excess weight will affect your body, even if the damage isn't apparent now, the study authors say. And normal weight people shouldn't consider themselves healthy without checking their cholesterol, blood pressure and glucose levels.

"Obesity is taking a toll on the health and well-being of Americans," Drs. James Hill and Holly Wyatt write in an accompanying editorial in the journal. "Accepting that no level of obesity is healthy is an important step toward deciding how best to use our resources and our political will to develop and implement strategies to combat the obesity epidemic."