Author: Dr. Sean Wheatley, PhD – Science and Research Lead/15 August 2017
On August 15th the media widely reported that the idea someone can be “fat but fit” is not true, based on some newly published research. The BBC version of this story can be found here. But is this really what the study showed?
What was the study?
The study in question was published in the European Heart Journal, and used data from a large study of 520,000 Europeans followed up for 12 years on average. For their analysis the researchers grouped the participants based on whether they were normal weight, overweight, or obese AND whether they were metabolically healthy or not. They were metabolically “unhealthy” if they had 3 or more of the following risk factors:
Elevated blood pressure
High triglycerides (elevated amounts of fat in the blood)
Hyperglycaemia (high blood glucose)
Elevated waist circumference
This meant there were 6 groups being compared; healthy-normal weight, healthy-overweight, healthy-obese, unhealthy-normal weight, unhealthy-overweight, and unhealthy-obese
What did they conclude?
The study showed that those who were overweight or obese had an increased risk of having a cardiovascular event- such as a heart attack- compared to those who were normal weight, even if they didn’t have poor metabolic health. This was only true when compared to the normal weight participants who weren’t metabolically unhealthy.
It was this finding that the media focused on, using it to claim that there was no such thing as being “fat but fit” and that all people who are overweight and obese had an increased risk irrelevant of other factors.
So is there a problem with this conclusion?
Yes and no. Although there was an increased risk for the metabolically healthy overweight and obese participants, this increase wasn’t actually that big. The hazard ratios of 1.26 and 1.28 for the overweight and obese participants respectively basically mean that the increased risk was 26% and 28% in each group.
The limitations of observational research are well documented though. We’ve addressed some of them before, but key issues include the inability to judge cause and effect and the fact it is almost impossible to control for all the potential factors that might influence the outcomes. In relation to these methodological limitations it is debatable whether hazard ratios of this size are actually meaningful, with some people suggesting a hazard ratio of more than 2 is needed to demonstrate a clear impact.
The data does actually tell a more important story however, and that is that whether somebody was metabolically healthy or not was a MUCH better indicator of their health than their body weight. The hazard ratios for the “healthy” overweight and obese participants pale into insignificance next to those of the metabolically unhealthy groups, including those whose who were normal weight.
The graph below, figure 3 in the actual paper, shows this quite clearly. The further to the right the black square is, the greater the risk for that subgroup. The squares for all the metabolically unhealthy groups are much further over. It is worth noting all of these hazard ratios were above 2, making it more likely they are meaningful and important.
Essentially the real story, in my opinion, is that whether somebody has the metabolic syndrome is more important than whether someone is overweight or not. The headlines could just have easily been that being normal weight doesn’t protect you against cardiovascular disease if you have poor metabolic health!
Any other issues?
It isn’t the point of this blog to fully critique this research; and it is an interesting, informative and well conducted study (as far as observational research can be!). One other slight flaw in how the study was reported however was the claim that it showed you can’t be “fat but fit”.
Generally speaking this terminology is used to refer to someone who has good physical fitness despite carrying excess body weight, for example if someone is still active and able to take part in physically strenuous activities despite having an elevated BMI and/or WC. From this perspective, the study didn’t even look at “fitness” in the way many people might assume they did!
So what’s the bottom line?
Although the study did show that there is a slightly increased risk associated with being overweight or obese the research actually found that the markers of metabolic health were a much more important indicator of cardiovascular risk. Losing some weight may well reduce your risk of cardiovascular events if you are overweight or obese, but the study didn’t even look at “fitness” in the traditional sense.
Essentially, the findings don’t debunk the “fat but fit” story line quite as clearly as the headlines suggest!
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