Countless epidemiological studies have shown that as you move from a normal body weight towards obesity the risk of many chronic diseases increases exponentially. However, more and more research suggests that the relationship between body weight and health is much more nuanced than previously thought.
Recent estimates suggest that approximately one in three obese individuals remain metabolically healthy (displaying normal blood glucose, blood lipids, blood pressure, and a healthy cytokine profile) despite their excess weight. These same individuals may have a similar risk of developing heart disease and diabetes as their skinny counterparts and they may actually become less healthy by losing weight!
Thus, as we shall see below, there is certainly more to health than the number on one’s bathroom scale.
It all started in 1965. Two researchers, Albrink and Meigs, were performing a general investigation into the health of factory workers, and noted a rather counterintuitive result: that many obese men had healthy levels of triglycerides in their blood.
In 1982, Ethan Sims first included the designation of "healthy obese" subtype in his classification of obesity, thereby first identifying a unique subset of obese individuals that appear to be at least partially protected from the development of the metabolic disturbances generally attributed to obesity.
Unfortunately, despite awareness of the metabolically health obese phenotype for close to 30 years, there currently exist no established criteria by which to define these individuals.
What defines a metabolically healthy obese individual?
The defining characteristics of the metabolically healthy obese phenotype, in contrast to obese individuals with metabolic risk, include limited abdominal, particularly visceral fat accumulation, an earlier onset of obesity (<20 years) and high levels of physical activity. Additionally, black obese individuals have a greater tendency of being metabolically healthy in contrast to white obese.
Japanese sumo wrestlers are often used as a popular example of metabolically healthy obese. They are morbidly obese and yet due to their high level of activity have very little visceral fat accumulation, tons of muscle mass, and a healthy metabolic profile—until they stop training, that is. Once their activity drops off, so does their fitness, and they begin to accumulate excess fat in deleterious locations, matched by a worsening in their metabolic profile.
Risk of heart disease and diabetes among healthy obese individuals
Ok, so maybe there are some people with excess weight who somehow manage to maintain a reasonable healthy metabolism. But what about the chances of developing chronic diseases such as diabetes or cardiovascular disease—two common ailments tied to carrying excess weight?
Two large studies have sought to answer this question, and their results may surprise you.
In the first study, 2902 men and women were subdivided into different groups based on their weight and metabolic status and were prospectively followed for up to 11 years to see who would develop type-2 diabetes or cardiovascular disease.
And what say the results?
In terms of risk for developing cardiovascular disease or type 2 diabetes, metabolically healthy obese were at NO higher risk in comparison to their equally healthy, but normal weight individuals.
The authors of this study provide the following conclusion [emphasis added]:
"[...] in the absence of metabolic abnormalities, obesity itself did not increase risk for cardiovascular disease and was a relatively weak risk factor for incident diabetes."
A similar conclusion was reached by Canadian researchers following a group of 1824 men for a duration of 13 years:
"The results of this prospective population-based study indicate that the risk of ischemic heart disease associated with a high body mass index depended entirely on whether features of insulin resistance syndrome were simultaneously present."
Thus, it appears that 1 in 3 obese individuals is not only metabolically healthy, but their risk of disease is equal to that of their leaner counterparts.
A Caveat: Are metabolically healthy obese individuals REALLY healthy?
But, is there more to health than the level of triglycerides in one’s blood, or their risk of diabetes or cardiovascular disease? And if we consider other, potentially more telling outcomes—such as mortality—will metabolically healthy obese individuals still be considered healthy?
A 2009 study by Canadian researchers, asked this important question using prospective data on 6,011 men and women from the Third National Health and Nutrition Examination Survey.
Their results provided the first chink in the armour of the metabolically healthy obese concept.
In describing the major finding, Dr. Kuk, the primary author of the study, said this:
"Individuals who are obese and do not have common diabetes and heart disease risk factors die at the same rate as those obese individuals who do. This means being overweight alone puts you at higher risk for dying, even though you do not high blood pressure, high cholesterol or high blood sugar. This highlights the negative health impact of body weight alone".
Why the discrepancy?
Quite simply, there are numerous health issues brought on by carrying excess weight that are not always metabolic (i.e. joint problems due to excess load, skin infections, depression, etc.). Additionally, obese individuals are less likely to survive a trauma as compared to normal weight individuals despite similar injuries, due to longer transport times due to their higher body weight, and difficulty assessing and treating the injuries. Further, they are less likely to see their physicians regularly, which may be in part why cancer is generally diagnosed in obese individuals at later stages.
Thus, it is now believed that despite being metabolically healthy, obese individuals may still be far from optimal health.
Should healthy obese people lose weight?
In 2008, in response to a recent study, the answer to this question may have been no.
In this small study, researchers reported a VERY paradoxical finding: when otherwise healthy obese women lose weight via dieting their insulin sensitivity (a marker of diabetes risk) worsens. That is, they become less healthy in response to weight loss.
Even the authors of the study had no idea how to explain the findings.
Being intrigued with this finding, I conducted a similar study in a larger sample of metabolically healthy men and women as part of my PhD thesis.
In the recently published study, a total of 63 metabolically healthy obese men and women and 43 metabolically unhealthy obese men and women participated in three to six months of exercise and/or diet weight-loss intervention.
As you’d expect, body weight and body fat were significantly reduced in all subjects.
More importantly, insulin sensitivity improved after weight loss in both the metabolically healthy (by about 20 percent) and metabolically unhealthy obese individuals. However, the improvement was greater in the metabolically unhealthy subjects.
Thus, the recent results suggest that while the metabolic benefit of weight loss among healthy obese may be less drastic, there is certainly no reason to fear a deterioration of metabolic health.
Take home message:
Although a fair number of obese individuals may have a perfect metabolic profile, it appears they may still experience negative consequences of their excess weight. Furthermore, weight loss achieved via lifestyle intervention appears to still bring about some metabolic benefit among previously healthy obese individuals. Given the numerous nonmetabolic benefits of weight loss, all obese individuals certainly have something to gain from a modest reduction in body weight as achieved by a healthier lifestyle.
Image credit: Magnus D, from Flickr
About the author: Peter Janiszewski has a PhD in clinical exercise physiology. He is a published obesity researcher, medical writer, science blogger and an advocate of new media in scientific knowledge translation. He blogs at Obesity Panacea and Science of Blogging and regularly tweets as @PMJaniszewski.
The views expressed are those of the author and are not necessarily those of Scientific American.