Can You be “Fit but Fat?”: Study Debunks Metabolically Healthy Obesity

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One out of every 3 adults (around 36%) is obese, and there is no way around the fact that the world is undergoing an epidemic of poor metabolic health — which is unlikely to reverse itself anytime soon.

The health consequences of obesity are talked about widely and well-understood at a scientific level. Obesity is associated with a greater risk for cardiovascular disease and diabetes, shorter life expectancy, and overall worse health outcomes. The causes of this are several, but involve inflammation, elevated blood pressure, and poor glucose and insulin control, among other mechanisms.

Despite the consistent associations of obesity with adverse health outcomes, there is a “debate” as to whether one can be obese but otherwise healthy. This is often referred to as “metabolically healthy obesity” (MHO) or more crudely the “fit but fat” phenomenon.

Proponents of MHO claim that obesity is not an indicator of poor health per se — rather that weight can (and should) be separated from other parameters of metabolic health. Metabolically healthy obese individuals are characterized as having a BMI in the “obese” range, but normal levels of risk factors including blood pressure, lipids, inflammation, and insulin and glucose-related measures.

And sure — there are individuals who have an “obese” BMI yet are still “fit” by most standards and “healthy” in the sense of being free from disease.

But whether metabolically healthy obesity really represents a “healthy” phenotype is highly controversial. In essence, to say MHO is real is to conclude that someone who is obese and has the same risk factor profile as someone who is non-obese will have similar health outcomes and incidence of disease. But is this true?

A recently published study analyzed data from a prospective cohort of ~380,000 individuals to answer this question. How does MHO relate to health outcomes, and how do healthy outcomes of individuals with MHO compare to those in other BMI and metabolic health categories?

Brief study methods

Data for this study were gathered using the UK Biobank, which is a register containing over 500,000 participants who participated in the UK Biobank prospective cohort study (2007–2014.)

A total of 381,363 individuals had complete data on study-related outcomes which included: height and weight (and body-mass index or BMI), blood pressure, C-reactive protein (a marker of inflammation), triacylglycerols, LDL-cholesterol, HDL-cholesterol, and HbA1c (a marker of long-term glucose control.)

All participants were placed into one of four categories based on their BMI and metabolic status

  1. Metabolically healthy non-obesity (MHN)
  2. Metabolically healthy obesity (MHO)
  3. Metabolically unhealthy non-obesity (MUN)
  4. Metabolically unhealthy obesity (MUO)

Metabolic “health” was defined as a participant who had normal levels of at least 4 of the 6 metabolic biomarkers listed above.

Comparisons of health outcomes between groups were made using hazard ratios (HR) — which tells us the probability of an event (in this case type 2 diabetes, atherosclerotic cardiovascular disease, stroke, myocardial infarction, heart failure, respiratory disease, and COPD) occuring in one group compared to another.

Other statistical analyses and tests were run, but we won’t go into detail on those at this moment.

Results

Individuals with metabolically healthy obesity had higher rates of:

  • Diabetes (~4x greater)
  • Atherosclerotic cardiovascular disease (~1.2x greater)
  • Myocardial infarction (~1.2x greater)
  • Stroke (~1.1x greater)
  • Heart failure (~1.8x greater)
  • Respiratory diseases (~1.3x greater)
  • COPD (~1.2x greater)

Cardiovascular and respiratory disease outcomes were the highest in metabolically unhealthy obese (MUO) individuals, followed by metabolically unhealthy non-obese (MUN) and metabolically healthy obese (MOH) individuals.

Rates of heart failure and respiratory disease-related outcomes were, perhaps surprisingly, higher in metabolically healthy obesity compared to metabolically unhealthy non-obese individuals.

All-cause mortality was higher in metabolically healthy obesity vs. metabolically healthy non-obesity

Metabolically healthy obesity had higher rates of diabetes, heart failure, and respiratory diseases compared to individuals without obesity — regardless of their metabolic status.

Participants who were metabolically healthy but obese but transitioned to metabolically unhealthy obesity over 3–5 years of follow up significantly increased their risk of atherosclerotic cardiovascular disease.

I think these findings are best summed up by some direct quotes from the authors themselves, who state that “the risk of many important outcomes, such as heart failure and respiratory disease, is elevated in people with obesity even if they have a normal metabolic profile.”

Making some even more powerful (controversial) language, they go on to say that: “Using the label ‘metabolically healthy’ to describe this group in clinical medicine is misleading and therefore should be avoided.”

This is a pretty bold statement, especially given the current cultural climate which seems to subscribe to the “healthy at any size” hypothesis. These results (and others) would suggest that no, any size is not “healthy” and being overweight should be considered an independent health risk.

I think it is necessary to add that I think the health risks of overweight and obesity need to be separated from the cultural stigma surrounding obesity. To claim that there are health risks associated with being overweight is NOT to disparage, condemn, or fault those who may suffer from obesity. It has nothing to say about the causes of obesity, nor the social, biological, or psychological underpinnings that underlie this ‘disease.’

With that being said, it should be a public health imperative to improve the metabolic health and weight of our population — this is obvious. If the startling findings that obesity is an independent risk factor for COVID-19 infection tell us anything, it’s that impaired metabolic health is not just an issue of weight, but of population-wide health outcomes and perhaps the future of humanity as we see it. No one is “fat shaming” the individual if they are simply pointing out the deleterious consequences associated with excess fat itself.

Health and medical professionals who warn against the risks of obesity are not “stigmatizing” the disease (well…most aren’t), but only wish to improve the health of individuals and the community as a whole.

This isn’t an easy issue to fix, and simple changes in diet, physical activity, and lifestyle are probably not enough to solve our metabolic crisis. Sure, they’re a start — but larger changes in culture and the food/activity environment are needed if we are ever to reverse the profound harms we’ve done to our health. I don’t have the answer, but I only hope to stimulate discussion and change by spreading knowledge and (hopefully) insightful information.

Study Cited

Zhou, Z., Macpherson, J., Gray, S.R. et al. Are people with metabolically healthy obesity really healthy? A prospective cohort study of 381,363 UK Biobank participants. Diabetologia (2021). https://doi.org/10.1007/s00125-021-05484-6

PhD candidate at the University of Florida — Science writing with a particular focus on exercise and nutrition interventions, aging, health, and disease.