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A little science: difference between average and fast runners

In 2015, an interesting study on exercise physiology was published in the BMJ journal Open Sport & Exercise Medicine. Norwegian sports scientist K.J. Hetlelid and his team investigated differences between well-trained and recreationally trained runners. They were particularly interested in energy metabolism and substrate use responses (i.e. use of carbohydrates and fats).

Traditionally, the contribution of fat oxidation was thought to decrease once exercise intensity exceeded 60-75% of one’s VO2max, and even to be negligible above 85%. This has lead to an enormous emphasis on carbohydrate oxidation for most endurance sports, in other words how to train it and how diet can maximise it.

Hetlelid’s team were aware of a study done by Stepto et al., already back in 2001. This Australian research team had studied 7 competitive cyclists during a high-intensity interval training session (8x5min with 1min recovery between intervals). In these well-trained cyclists, fat oxidation was minor, but not negligible, and even increased as the interval session went on. Hetlelid wanted to see if differences could be seen between well-trained athletes, like in this study, and recreationally trained individuals.

The study

Rethinking the role of fat oxidation: substrate utilisation during high-intensity interval training in well-trained and recreationally trained runners.

Before we dive into results and conclusions, it’s important to briefly explain how the researchers set up their study. They took two groups of male runners, 9 well-trained (WT) and 9 recreationally trained (RT). The groups were similar for age, height, peak blood lactate and peak preceived exertion (on preliminary testing), and diet (although this was not discussed in detail). The groups differed when it came to VO2max (average 70 in WT vs. 55 in RT) and weight (WT were lighter), showing the clear difference in fitness level, which was an important feature of the two groups in order to investigate differences in energy metabolism.

They had both groups perform the same self-paced high-intensity interval session, consisting of 6x4min on a 5% incline treadmill with 2min recovery. Heart rate (HR) was measured continuously, as was gas exchange (O2 in vs. CO2 out). The latter provides the respiratory exchange ratio (RER), ranging from 0.70 to 1.00. Basically, it determines how many carbohydrates vs. how many fats are being used for fuel. 0.70 on one end of the spectrum means 100% fat and 1.00 on the other end means 100% carbs. Blood lactate was measured immediately after intervals 1, 3 and 6.


WT runners averaged 14,92 kph, whereas RT runners ran 10,99 kph. It is evident that this went together with a much higher energy expenditure (38,1 kJ/kg in WT vs. 28,6 kJ/kg in RT). The WT group also ran at a higher percentage of their VO2max. Relative O2 consumption was 94% in WT vs. 89% in RT. Interestingly, all runners, except for a few RT, performed way above the 85% mark, where fat as a fuel (FATox) was believed to be non-existent.

Despite the clear differences in fitness and performance, both groups showed some very similar characteristics. Perceived effort/intensity was the same, both groups stating it going from “hard” to “very hard”. Lactate and HR responses were very similar as well, at 93-95% of max HR. The authors concluded that these parameters are not what separates elite from average runners.

Now, this is where the study got interesting. A very different RER proved that use of carbs and fat was different (0.88 in WT vs. O.95 in RT). Actually, carbohydrate oxidation was pretty similar, even slightly lower in WT runners (3,61 vs. 3,79 g/min). FATox was what explained the much higher energy expenditure in the well-trained, and therefore much faster running. The authors calculated FATox to account for 33% of total energy expended.

The authors concluded that “the capacity to oxidise fat at high exercise intensities is a supremely advantageous adaptation for endurance athletes, [since] highly trained participants sustain higher work rates with substantially greater fat [burning], but similar carbohydrate burning.”

My additional thoughts

I think it is needless to say that a well-trained runner was faster mainly because he had trained much more for several years. Now, if a moderately trained individual would want to move towards a well-trained fitness level, he would have to train more for a considerable amount of time, right? How would this runner have to train? For sure an individual thing when it comes to fine-tuning. However, this study shows that training to be a better fat-burner should play a major part in his training schedule. I think that this runner will stagnate sooner when too many runs are anaerobic – and this doesn’t even have to be above lactate threshold.

One finding that struck me in this study is a noticeable difference in relative oxygen consumption. Well-trained runners were able to perform closer to their physiological limit, at 94% of their VO2max, even though workout duration was exactly the same for both groups. It’s not like they went harder either – it felt just as hard and lactate/heart rate were the same. Moreover, the WT did not weaken, like the RT did a bit in the second half. They even had enough in the tank for a strong finish. Perhaps their better trained fat burning is what allowed this. Their experience in pacing a hard effort will have helped as well.

As I was reading through the paper, I was thinking ahead on how to exploit this fat burning thing. And I must agree with the authors’ final paragraphs: “If the capacity for fat [burning] is important for athletes exercising at high intensities, […] the logical progression would be to question how this quality might be enhanced.” In my mind, the answer is simple: by choosing foods and training intensities that stimulate fat burning, while limiting factors that hinder it. Eating too many carbs when sugar reserves (glycogen) are already high will hinder proper fat burning. This only gets worse when you avoid fats to stay lean, in which case you may not stay lean. Training above your maximum aerobic heart rate too often will do the same.

A final thought with future research in mind is the specific influence of food on fat burning, in elite athletes as well as us, amateurs. I believe everyone has a different carbohydrate intolerance before fat burning diminishes. It is therefore important to be honest with yourself, when you’ve exceeded your limit.

I’ll just carry on blogging about that. Stay tuned. Subscribe.

2 thoughts on “A little science: difference between average and fast runners

  1. Interessante blog.


    1. admin says:

      Merci, Kris! En nogmaals bedankt voor de fotoshoot.

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