I read an article saying you should always negative split a marathon to preserve carbs as long as possible by keeping the heart rate low, It said that the body couldn’t rely on carbohydrate to fuel the marathon distance and that this is why most people miss their best times. Let’s discuss this idea and why it doesn’t stand up to science.
The argument for fat use in marathons
The author of the article suggested that people “burn out” in the latter miles of a marathon because they’ve run out of carbohydrates. Whilst this isn’t necessarily wrong, the chances of that happening are variable depending on the runner and how long they take to complete the 26.22 miles.
The average human body can store around 1600 calories of carbohydrates, but this can be increased through muscle mass gain and is higher in more athletic individuals like runners. The argument that carb limits performance assumes that who people running faster use only carbohydrates for energy, when in fact the body would be using fat as well as carbohydrate in ratios defined by energy expenditure and genetics.
Fuelling a sub-3 marathon
My first marathon used an estimated 2800 calories in a little under 3hrs. I consumed 7 gels and some orange juice, adding around 240g carbs (960 cals). Now if we assume that I store a little above average carbohydrate at 1800 calories worth, I theoretically had 2760 calories to use so I’d have completely run out of energy with around half a mile to go which obviously I didn’t.
Fat metabolisation has a limit of around 270-400 calories per hour for average to elite athletes. That means fat provided a maximum of around 1100 calories for me, further going to show that the author’s theory isn’t correct.
How long will you take to run your marathon?
If you’re looking at running say 5-6 hours, your body will be working hard for a longer time, so will absolutely need more fat fuel than those running faster times, but you can consume both fat and carbs or use carb supplements like gels and drinks, whilst relying on your stored fat for fuel more as the race goes on.
For ultramarathons fat adaptation has a great deal of value, as you just can’t consume enough calories to fuel these longer slogs. For those races you want your body to turn to its fat stores more readily and metabolise the fat you consume on the run more easily, whilst still carb fuelling for an extra boost.
I won’t deny that people run slower than they could for a marathon because they go out too fast and don’t pace it as they could, but assuming it’s down to the fuel their body uses is wrong. Running at low heart rates like the author suggested, would have meant slowing by over 1 minute per mile for me, adding at least 26 minutes to my marathon.
Whatever you do, keep eating carbs for your long runs and marathons. Consider your expected finish time and experiment from there. You don’t have to negative split your marathon to run your best time. Although negative splitting is often associated with PBs for us normal runners, it will always mean something was left on the road.
Written by Kyle Brooks, Running Coach based in Norwich, Norfolk