If you plan on running your event in zone 3 or above, use simple carbohydrates like gels, sports drinks, and Vert. Zone 2 and below, feel free to experiment with pizza, bacon, whatever.

Race day nutrition for marathon runners versus ultrarunners are very different beasts. You can see this reflected in the attitudes of athletes towards nutrition. Rarely do you hear a marathon runner say they will just eat whatever they have at the aid stations. Generally, they will have trained on the on-course product to ensure it sits well. The last marathon I did, I ran with a trail vest as the on-course product I knew gave me gastro. However, trail runners are notorious for eating a handful of Skittles, a slice of pizza, and washing it down with pickle juice. What explains the difference?

Most of the difference in fueling strategies comes down to the intensity of the exercise. As a general rule, the higher the heart rate, the greater the proportion of glucose that the body is using. During zone 1 (50-60%) and 2 (60-70%), fat is the primary fuel used to fuel exercise, zone 3 (70-80%) is a mix of carbs and fat, and Zone 4 (80-90%) is all glucose, as is Zone 5 (90-100%). Most road marathons are run at zone 4, whereas an Ultra is run at Zone 2 with periods of 3 and 4 (the easy rolling hills).

The intensity/fuel hypothesis can be further evidenced by what elites say they eat based on distance. Courtney Dauwalter, for example, consumes nothing but sports drink for events up to 50km. Above 50k, she adds in stroopwafels and chews, north of 100k she adds in mashed potato. Killian Jornet follows a similar approach fueling with gels for short events like Zegama and for longer events like UTMB and Hardrock, he adds in avocados, trail mix, dark chocolate. 

The other factor to consider is that in Zone 3 and above, gastric emptying slows down. Digesting things like pizza will take longer than it would in zone 1 and 2. It will sit in your stomach sloshing around. Whereas in Zone 2, digestion is practically normal so you can get away with eating more “real” food. The higher the intensity of your exercise, the more you want to rely on gels as opposed to sports drink as gastric emptying slows and you have less volume to deal with.

So from an energy substrate point of view and a digestion point of view, eating bacon or pizza makes zero sense during a road marathon but might make more sense in a 100k Ultramarathon. However, if you are going to push it and run your 50k or 100k in  zone 4, or if you have gastro issues, stick to simple carbohydrates like gels (Vert) or sports drink.

April 12, 2024 — Ben Paris