According to the National Institute of Diabetes and Digestive and Kidney Diseases, in 2013–2014, up to 40.4 percent of adult women in the United States had been diagnosed with obesity.
Still, some women have what is often known as "metabolically healthy obesity," as, despite their high body mass index (BMI), they do not have many of the additional health conditions that obesity is a major risk factor for.
But as specialists have pointed out, there is currently no clear definition of metabolically healthy obesity, so different studies investigating the specifics of this condition may each describe it differently.
Research published recently in The Lancet Diabetes & Endocrinology opts for a definition that characterizes it as obesity in the "simultaneous absence of hypertension, dyslipidaemia, and diabetes."
The authors of this study, however, were interested in addressing a question that is a frequent point of debate when considering metabolically healthy obesity: "Does it affect the risk of cardiovascular disease (CVD), and if so, to what extent?"
Lead researcher Prof. Matthias Schulze — from the German Institute of Human Nutrition Potsdam-Rehbruecke in Nuthetal — and colleagues hypothesized that, while at first glance, obesity free of metabolic diseases may not heighten a person's risk of CVD, in the long-term, it may lead to the same negative effects on cardiovascular health as other types of obesity.
Is metabolically healthy obesity 'healthy?'
At first, the research team identified previous studies that had looked at the effects of metabolically healthy obesity on health, focusing on those that followed up the participants over a long period of time (over 12 years).
A systematic review of these studies revealed that obesity — whether or not it is accompanied by metabolic diseases — does heighten the risk for cardiovascular conditions.
To confirm these preliminary observations, Prof. Schulze and team went ahead and conducted a large cohort study involving 90,257 women — recruited using the ongoing Nurses' Health Study — who were free of CVD at baseline.
The participants were then split into different health categories based on their BMIs, their baseline metabolic health status, and change in metabolic health status. Then, they were followed up for a period of 30 years (1980–2010).
In order to monitor any changes in health, the women were sent questionnaires every couple of years, so they were able to report any relevant medical information.
For their analysis, the researchers also adjusted for influencing factors, including the participants' age, levels of physical activity, smoking and drinking habits, ethnicity, aspirin use, and any family history of heart attack or diabetes.
Larger CVD risk for all women with high BMIs
"Our large cohort study confirms that metabolically healthy obesity is not a harmless condition, and even women who remain free of metabolic diseases for decades face an increased risk of cardiovascular events," notes Prof. Schulze.
Their analysis revealed, unsurprisingly, that all women with a metabolic disease — notwithstanding BMI — were at a heightened risk of also developing a cardiovascular condition.
More specifically, women with a normal BMI but who were metabolically unhealthy had a 2.5 times higher risk of CVD compared with peers with both normal weight and no metabolic conditions.
As for women with metabolically healthy obesity, they had a 39 percent higher risk of CVD than healthy study participants.
But the findings did not stop here. Eighty-four percent of the women who had metabolically healthy obesity at baseline, as well as 68 percent of the metabolically healthy women with a normal BMI, developed a metabolic condition over a 20-year period.
"[W]e observed," Prof. Schulze goes on to say, "that most healthy women are likely to develop type 2 diabetes, high blood pressure, or high cholesterol over time, irrespective of their BMI, putting them at much higher risk for cardiovascular disease."
But even the women who had managed to steer free of metabolic diseases over those 20 years nevertheless had a 57 percent higher risk of CVD if they had obesity than healthy women with a normal weight.
Preventive measures always important
The study authors note that all these observations suggest that even women who believe that they are "in the clear" when it comes to diabetes, hypertension, and — by association — CVD would do well to take preventive measures by paying more attention to their diets and exercising habits.
"Long-term maintenance of metabolic health is a challenge for overweight/obese, but also for normal-weight women," Prof. Schulze says.
"Our findings highlight the importance of preventing the development of metabolic diseases, and suggest that even individuals in good metabolic health may benefit from early behavioral management to improve their diet and increase physical activity in order to [avoid] poor metabolic health."
Still, the team admits that the study faces a number of limitations that should be taken into account. First and foremost, the authors say, it should be noted that the research only observed a set of associations, so relationships of cause and effect would benefit from further investigation.
Moreover, most of the study participants were women of European descent, which may mean that women of other ethnicities, as well as men, may not encounter the same effects observed in this study.
Finally, since definitions of "metabolically healthy obesity" differ between studies, other projects may reach different conclusions by working with alternative descriptions of this condition.
Yet this study's most obvious strengths, note Prof. Schulze and colleagues, are the fact that it followed up a large cohort over a long period of time, and that relevant measurements were repeated as necessary.