Can You Be Fit and Fat at the Same Time?

The high priest of fitness waxes evangelical about the benefits of… you guessed it: running.


As a RW reader, you are the ‘choir’, so you probably don’t need to hear this sermon. But just in case: The regular running you do is one of the most powerful forces for health known to medicine. It helps literally everything. It reduces your risk of chronic diseases such as heart disease and type 2 diabetes; it prevents cancers, protects against the effects of ageing, and even delays the declines in cognitive function that afflict us as we get older. Even Covid-19 outcomes are improved by fitness. It’s a ‘catch-all’ for health.

“Obesity, smoking, high cholesterol and diabetes COMBINED caused only slightly more deaths than being unfit!”

Yet there’s a debate in the medical world about the relative contribution of fitness to health, compared to being overweight. In how we assess people’s health, weight has been overvalued, whereas fitness is undervalued. Many doctors happily and regularly measure blood pressure, blood sugar and BMI (which do matter – no doubt about that), but almost never ask their patients, “Are you fit?”, or “Tell me about your regular runs?” That, at its core, is the debate about fitness vs fatness; and the conceptual question is: can a person be fat and fit at the same time? Without getting stuck on the exact definition of ‘fat’, research has shown pretty consistently that people who are ‘unfit’ (and again, let’s park the definition) have twice the risk of mortality compared to people classified ‘fit’, irrespective of their Body Mass Index (BMI, a measure of mass relative to height). Put differently: an obese person who would be regarded as even moderately fit is half as likely to suffer a cardiovascular-related death as a person who is ‘normal’ weight, but unfit.

Tried and Tested

In one study, scientists used clever methods that work out how many deaths would be avoided if all the people with so-called ‘bad habits’ (such as smoking, inactivity or diabetes) could be converted to having ‘good habits’ (such as becoming non-smokers, regular exercisers, or non diabetics). What was found is that low fitness scores contributed significantly more than all the other risk factors. In fact, obesity, smoking, high cholesterol and diabetes COMBINED caused only slightly more deaths than being unfit! So now we can say – backed by evidence – that our moderately fit obese person, who ALSO smokes and has high cholesterol and diabetes, has about the same risk of death as an unfit person who is normal weight! It really is remarkable. Only high blood pressure comes close in terms of mortality risk.

Clearly, fitness saves lives. And the finding that this is the case regardless of BMI shows how being fit can mask a number of other problems, so it’s worth making ‘get fit’ the goal that trumps all others when you train, even if your goal is just ‘health’. Of course, if your goal is to run a faster parkrun, or finish a marathon, then fitness is your goal, so you’re on the right journey already! Note that none of this downplays the fact that obesity remains a risk factor; so if you can encourage a person to lose weight safely, and move into a healthy weight range, you must. Being overweight is a risk, all by itself; and it creates practical challenges, like making it harder to exercise and reducing quality of life. The point is that your focus shouldn’t be on weight, but rather on fitness. Solve that, and many other problems disappear.

The Fine Print

Now for some caveats. The first and obvious one is that these two concepts, fitness and fatness, are often mutually exclusive; most often, a significantly overweight person is also going to be unfit, and vice versa. There are not that many fit overweight people. Practically, this means that for those in the normal weight range, maybe even slightly overweight for their height, this debate is basically moot. If you just keep running, working on your fitness, your weight will ‘take care of itself’, the natural consequence of your running. So too with all those other risk factors – high blood pressure, cholesterol, early signs of diabetes, they’re all positively influenced by your running anyway; so you’re killing many birds with one stone. On the other hand, if you’re obese, exercise on its own has been found to be less effective at improving health markers than diet; which tells us that weight is a key component of health, and should be considered very important. It also makes exercise easier.

The challenge for those who are significantly overweight is that running is a tricky way to get fit, because the load on your joints is that much higher, every step you take. This is when ‘walk before you run’ is the logical precaution to take; in some instances, it might even be worth starting with non-weight-bearing activities such as swimming and cycling. You can graduate to walking and then running once some weight is lost, some strength gained, and some fitness earned. But for everyone, irrespective of fitness and current weight, the truth is clear – you can be fit and fat; you should aspire to either maintain or improve your fitness level; and as a fitter person, you’re significantly protected. And thus endeth the sermon!


Prof. Ross Tucker is one of the world’s top sports scientists, and the co-host of the Real Science of Sport podcast.

Follow him on @scienceofsport

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