When to Cram Carbs In an Ultra-Endurance Event

photo credit: Kyle Thornbill

By Dr. Luke R. Bucci and Jeff Feliciano

INTRODUCTION

One of the most frequent questions we get is how much carbs do I need to take (or, often, how much can I take) during my long-duration events, races, or training? This blog is a follow-up to the previous blog about increasing carbs/hour during endurance exercise, where Dr. Bucci answered that question. One thing he didn’t cover in that last blog is that, with higher carb intake, timing is everything, so in this blog, Dr. Bucci addresses when to go big on carbs.

Sweet science

This Blog is short and sweet (pun intended, because sugars are the topic). If you read the previous blog, you’ll know how to train your gut to take in higher amounts of carbs than traditionally used in training (that’s more than 60g carbs per hour), you’ll have our conversion charts to help dial carb load to your bodyweight, and you’ll know to use training sessions to fine tune a carbs/hour load that works best for you – the what and the how of cramming carbs. So now, let’s get the other important point – when – nailed down.

The simple answer is to start ~30 minutes before the start of exercise, and then continue taking in more carbs at 20 or 30 minute intervals during the first hour and every hour thereafter.

Carbing up before exercise and during the first two hours is critical, as that time period lets you take advantage of optimal uptake of carbs (and water & electrolytes) while your gut is still at maximal absorptive capacity. You want to top up your glycogen stores and supply your muscles and brain with a stable blood glucose supply early on. That means keeping your carb intake high early in an event, as much as you can, without incurring GI issues (again, use training sessions to find this ceiling for yourself).

After two hours of intense exercise, your gut is starting to change, even if you do not feel anything wonky. Your muscles are demanding most of your blood supply from each heartbeat, and the redistribution of blood to muscles means less for everything else, including your GI tract.

With less blood flow to the small intestines, the rate of absorbing water, electrolytes, and carbs is slower, reaching a plateau later in a long-duration event, usually at 4-6 hours. Even so, you still need to maintain your carb intake as much as possible, using your GI tract status as a gauge of how often and how much.

We’ll be looking closer at what happens to your GI tract during long-duration exercise and how to deal with it in future blogs, because it is an important part of performing your best. (Spoiler: non-solid carbs are typically best once you hit that 4-6 hour mark.) But for now, suffice it to say that your stomach emptying slows down – although less so when you have trained your gut to keep processing higher carb loads.

CONCLUSION

To summarize, begin carbing up 30-45 minutes before exercise, and then focus on maintaining carb intake levels – while also maintaining your personal GI tract’s comfort level, which you should ascertain during test periods while training – during the first two hours of intense exercise. Your body will gradually lose absorptive capacity, so carb consumption in the later hours (4+) of exercise will become trickier, but front-loading in those first two hours will help ensure your glycogen stores are topped off for late-race heroics.