“People can be fat yet fit, research suggests” was the BBC headline.
“People can be fat AND fit as study finds obesity doesn’t automatically lead to ill-health” claimed the Daily Mail.
“Fat but fit people as healthy as normal weight ones: research” was The Telegraph’s take on the study.
You would imagine that the study had followed a number of fit and fat people and a number of unfit and fat people, over a long period of time and looked at absolute differences in disease (heart disease and cancer were the major focus of the study) that they went on to suffer. You would be wrong.
There are only a few studies in the world that have been done with a large number of participants over a long period of time. A couple of examples are the Health Professionals Follow-up Study (1986-2008) involving 49,934 men and the Nurses’ Health Study (1980-2008) involving 92,468 women. As these take a lot of time and are very expensive, the data is made available to other researchers to look at things that they may want to investigate.
This is the case in this latest article in the European Heart Journal. The team have taken the Aerobics Center Longitudinal Study (ACLS). This study is 98% Caucasian, well educated, and comprises people who have worked in executive or professional positions, so the researchers do accept at the outset that it may not be applicable to the whole population.
There were 43,265 participants in this study (24% female). They joined the study any time between 1979 and 2003 and were followed up for an average (median) of 14.3 years.
When each person joined the study they had one health check – I repeat – one health check. This health check (called the baseline assessment) gathered data on blood pressure, blood lipids, blood glucose, questionnaires for smoking/alcohol – usual stuff – and recorded BMI and Body Fat. The participants also took part in one fitness test – a treadmill exercise – which they all had to score at least 85% on, or they were excluded from the study. Hence a level of fitness was pre-selected. This was not a study comparing fit and unfit. It was a study looking at 85% of maximal performance on a treadmill test and higher.
This is confirmed in the researchers’ own words. The journal article states: “All participants completed a detailed questionnaire and underwent an extensive clinical evaluation, including a physical examination, fasting blood chemistry analyses, personal and family health history, body composition, smoking and alcohol use, and a maximal exercise treadmill test between 1979 and 2003.” i.e. each participant did one treadmill test between 1979 and 2003. Participants were only included in the study if they were “achieving 85% or more of the individual’s age predicted maximal heart rate during the graded modified Balke treadmill exercise testing.”
The participants were not then allocated to groups according to fit vs. super fit (85% on the test vs. say 100%). They were allocated to two groups depending on the number of characteristics of the metabolic syndrome that they displayed at the time of this one-off baseline health check. (The metabolic syndrome is the term given to people displaying a number of health symptoms – high blood pressure, high blood fats, high blood glucose and so on – which are believed to be an indicator of overall health.)
Those who had 0 or 1 condition associated with the metabolic syndrome were deemed metabolically healthy. Those who had 2 or more of these health markers were deemed metabolically abnormal (no judgement there!)
Once people had been allocated to these two groups – the researchers for this article looked at how these two groups performed on the one treadmill test done when they joined the study. Figure 1 in the paper (if you want a copy of the original article please let me know) shows the different performance on this test between metabolically healthy and obese and metabolically abnormal and obese people. The comparison is made in METs, metabolic equivalents, (1 MET = 3.5 mL of oxygen uptake/kg/min.) Reading across on the graph, it looks like the MET figure for the metabolically healthy and obese is 9.5 and the MET figure for the metabolically abnormal and obese is 8.9 – not huge and this is a one-off baseline test.
So – the original cut for the data was not about fitness. It was about the metabolic measurements taking at the baseline assessment. This would be like allocating people to those under 5′ 5″ and those 5′ 5” or over and then measuring their chocolate consumption. The headline might then be “People who consume more chocolate are shorter”. No! The chocolate consumption is what it is – the two groups being reviewed are those with 0-1 metabolic factor and those with more than this.
Absolute vs. relative risk
We then have the absolute classic error, made in every journal article that I review – researchers will present relative, not absolute risk. I can double your chance of winning the lottery by advising you to buy two tickets. You still have a 1 in 7 million chance of winning – but that is twice as good as a 1 in 14 million chance of winning. If I played with this the other way round and told you I could half your risk of heart disease – this would make headlines. However, I will have played with the numbers in exactly the same way as this lottery example. It is bad science. It is omnipresent and, frankly, unforgivable from so-called scientists.
There were 1,779 deaths during the study period i.e. over an average 14.3 year follow up of 43,265 people. This is an annual incident rate of 0.28% i.e. fewer than 3 in 1000 people. Claiming that the risk factor is 30-50% lower for ‘fat & fit’ as opposed to ‘fat and not fit’ can mean that 2.8 people in 1,000 died in any one year in the fat & fit group and 3-4 people in 1,000 died in any one year in the ‘fat and not fit’ group. Hold the front page!
The article claimed that “no significant differences were observed between metabolically healthy but obese and metabolically healthy normal-fat participants”. This is what gave rise to the headlines such as “Fat but fit people as healthy as normal weight ones: research.” This is the researchers playing with absolute, and not relative, risk when it suits them.
Table 3 in the article shows that metabolically healthy but obese people have a higher all cause mortality rate vs. metabolically healthy normal weight (Body Fat comparison) people. 216 metabolically healthy, but obese people died during the study, out of 5,959 in this category. That’s a 3.6% overall death rate, or a 0.25% death rate per year. 584 metabolically healthy normal weight people died during the study out of 21,023 in this category. That’s a 2.8% overall death rate or 0.19% death rate per year. The absolute risk differentials look very small – 0.25% vs. 0.19%. However, playing with the numbers in the way that the researchers did to get their headline – 0.25% is 30% higher than 0.19%. We should therefore say that metabolically healthy but obese people have a 30% greater chance of dying from any cause than metabolically healthy normal weight people. That’s not “Fat but fit people as healthy as normal weight ones”.
Do I think that it’s better to be fit and fat than fat? Yes – but a) this study has not proven this and b) it is still better to be normal weight. One treadmill test, since 1979, does not a fitness study make!
As a final thought – those who know me know how passionate I am about conflict of interest. Genuine research is sadly rare nowadays, as only those who stand to gain from something provide funding and people dependent on funding (the authors of this article, for example) don’t bite the hand that feeds them.
Would you be interested to know that this study was funded by “…an unrestricted research grant from The Coca-Cola Company”?! The key message that the ‘food’ and ‘drink’ companies want to get across is that – we shouldn’t worry about consuming their junk – the reason that we are fat is because we are lazy – not because of their empty calories. What a wonderfully authoritative article – in the European Heart Journal no less – to enable Coca-Cola to deliver their core message.