Eating for a Marathon: Using Fats as Fuel

As it turns out, nine months is a long time to be training for one event. So like most athletes, I do follow a periodized training plan, where different phases of training have slightly different goals. Being a sports nutritionist, I also have different nutrition goals to support my training during my different phases. And one of my first goals was to improve my body’s ability to use fats as fuel.

The Why

As a sports scientist I love data. So in January, I booked in for some physiology testing at Loughborough University. The information you get from this type of testing can then guide your training – do I need more long slow runs to build my aerobic base, or more high intensity intervals to improve VO2 max?

Aside from showing how little running fitness I had in general, the testing showed I was relatively poor at using fats for fuel, even at very low speeds. This is a particular problem for slower marathoners such as myself. See, even highly trained athletes can only store enough carbohydrates to get through 60-90 minutes of exercise. Of course, you can top up your carbohydrate stores during exercise by having energy gels or sports drinks, but most runners can only tolerate 60g of carbohydrates per hour (or less).

Based on previous experience, I will be burning at least 60Kcal per Km, which is over 2500Kcal for a whole marathon. Assuming I have enough carbs stored in my muscles for the first hour (about 550 Kcal), then take in 60g of carbs per hour (about 960 Kcal), that leaves 1000Kcal my body has to get from somewhere. If I can teach my body to use fats as a fuel over carbohydrates, then I am less likely to run out of energy before the end of the race.

The How

The easiest way to force your body to use fats as fuel is to eat a high fat, low carbohydrate diet. However, I’ve been down that road before, and I can’t say it appeals. Especially as I would have to maintain such a diet for over 9 months of training! Plus, research has shown that while high fat diets can increase time to exhaustion in endurance tests, top end sprint speed and power tend to decrease1. You wouldn’t think that matters in the marathon, especially for someone targeting four and half hours. But another result of my testing was a need to improve my VO2 max and overall running speed. That meant regular sprint intervals and faster paced training sessions. To get the most out of these sessions, I would need full carbohydrate stores.

An alternative way to encourage fat burning is to periodize your carbohydrate and fat intake2. This involves eating more carbohydrates on days with high intensity, high speed sessions, and fewer carbohydrates on rest and easy days. You can also teach your body to use fats as fuel by fasting and incorporating fasted training into your plan3. This is where you go for a run first thing in the morning before breakfast, after your usual overnight fast (about 12 hours since dinner). As you will have used up most of your carbohydrates as you sleep, your body is forced to rely on fats for fuel. It does feel harder, which is why these runs tend to be less than an hour at an easy pace.

Here’s an example of how I incorporated periodized carbohydrate intake with fasting into my training (in March, April and early May):

  • Monday: Moderate to high carbohydrate diet. Sprint interval session on hills in the afternoon.
  • Tuesday mornings: Fasted easy run before breakfast, then eat normally the rest of the day (moderate carbohydrate ~4g/kg).
  • Wednesdays: Rest day. Small, very low carbohydrate breakfast (usually a two egg cheese omelette) and fasted for the rest of the day (only water and green tea).
  • Thursday: Higher carbohydrate breakfast and lunch (more than 1g/kg carbohydrate in each meal) before a marathon pace run on Thursday afternoon. Back in March, marathon pace was a Tempo run (a pace I could only sustain for 15-20 minutes).

The Result

I would not recommend the above schedule to everyone, but it seemed to work for me. I was able to increase the pace of my long, slow runs while keeping the same effort (I use a heart rate monitor), indicating I was getting better at using fats for fuel. It did have it’s drawbacks though, which I will discuss in the next post.

When it comes to sports nutrition, I it very true that everyone is an experiment of one.


As always, there are many papers on this topic, but I have tried to provide open access ones.

  1. Burke et al. 2020. Adaptation to a low carbohydrate high fat diet is rapid but impairs endurance exercise metabolism and performance despite enhanced glycogen availability.
  2. Jeukendrup, A. 2017. Periodized Nutrition for Athletes.
  3. Aird, TP et al. 2018. Effects of fasted vs fed-state exercise on performance and post-exercise metabolism: A systematic review and meta-analysis. 10.1111/sms.13054

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