Exogenous carbohydrate needs and the marathon

Spoiler: I went from requiring 80-100 grams/hour of carbohydrates to requiring 0 grams/hour of carbohydrates at ~3-hour marathon pace*.

In my blog on periodized nutrition (see links below), I wrote an example of how my exogenous carbohydrate needs for a 100 km ultramarathon have changed dramatically after improving my metabolic flexibility through diet. In particular, I went from requiring over 100g of carbs/hour - an impossible task for my gastrointestinal system - to requiring less than 20 g/hour, for a hypothetical 100 km race.

Diet, it turns out, matters much more than exercise when it comes to substrate utilization, but this aspect is often overlooked by recreational athletes (and maybe not only recreational ones).

It’s marathon season, and therefore, today I wanted to look at the implications for the marathon. The marathon is much higher intensity than a 100 km ultramarathon, and yet, despite “common thinking” on the infamous crossover point, we can still rely on fat (and spare scarce carbohydrate stores) at those intensities, as I have shown in my own data (nutrition is incredible!).

In particular, in a previous blog where I had documented my 14-year journey to break 3 hours in the marathon, I wrote the following:

“my consumption at ~3-hour pace intensity is ~940 kcal/hour, hence, I need 2820 kcals to get to the end in less than 3 hours, and I have ~2000 in storage, assuming full glycogen stores. An extra 1000 kcals means 250-300 grams of carbs over 3 hours or 80-100 grams/hour (I am intaking quite a bit less).“ - at that time, I was deriving nearly ~100% of my energy from carbs at marathon intensity.

What about now?

*Having improved metabolic flexibility and fat oxidation rates even at high intensities, I now expend 875 kcals/hour and still oxidize 0.85 grams/minute of fat (and at least 50% of total oxidation comes from fat now), near marathon pace. Hence, I only burn through half my carbohydrate stores (the remaining 50% of total oxidation). I, therefore, use 875*0.5 = 440 kcals/hour from carbs, and for a 3-hour race, I need (440*3) 1312 kcals overall from carbs. At full glycogen stores (2000 kcal), I am now more than covered without any exogenous carbohydrates. (please see the image below for fat and carbohydrate oxidation rates at different intensities during two tests done while similarly fit, but with dramatically different nutrition).

As I wrote above, I went from requiring 100 grams/hour, to requiring 0 grams/hour for a 3-hour marathon.

Let’s be clear - there are other good reasons to intake carbs during prolonged exercise, from maintaining blood glucose to “tricking the brain” - however, while I used to get fully depleted, cramp, finish while slowing down, get nausea, etc. - my long runs now have become the opposite: strong finishes, no signs of cramping, no gastrointestinal distress. My carbohydrate intake during these marathon pace sessions is 20g every 8 km, which makes for about 35g/hour.

The experts I had talked to always suggested one solution to my previous problems (the cramps, the slowing down, etc.): try to eat more carbs. I cannot blame them. When you underperform in the marathon, with respect to shorter distances, not fueling properly might be the issue. However, in my case, it turns out I needed the opposite strategy, or to eat fewer carbs outside of training, to improve my metabolic flexibility as I was otherwise depleting my body too fast.

Don’t get me wrong, carbohydrates are great and essential to endurance performance. I fuel properly (i.e., with high carb meals and in-training nutrition) all my high-intensity sessions and clearly see the benefits of doing so. However, like most things, dosage and periodization matter (if you need a parallel: high-intensity training is also great and essential, but doing it every day is a recipe for disaster). Heavily periodizing carbohydrate intake was the answer to my issues.

Before jumping on the “high-carb all the time” train, ask yourself: What are my limiters? How can I test them? and most importantly, how can I get better, given those limiters?

Maybe you do need more carbohydrates, or maybe you need fewer. No need to guess; we can test and get clear answers to all of those questions.

My point is that we need to think critically about the individual and assess their specific situation without bias and without always pushing the only tool that we know or what is hip at a given point in time. Otherwise, we simply risk doing our athletes (or ourselves) a disservice.

Happy training!

• My latest marathon pace workout: strava.com/activities/1…

• Learn more about periodized nutrition: marcoaltini.substack.co…

• If you are interested in working with me, learn more and apply, here: marcoaltini.substack.co…