Does Carb Loading Work for Every Runner?

Translating the research and dispelling myths so you can run your best on race day

From a research perspective, the benefits of glycogen supercompensation (carb loading) for the marathon and triathlons are clear. Manipulating your diet to eat 8-10 grams of carbs per kilogram of bodyweight improves performance by 2-3%. That’s about 3-4 minutes for a 4-hour marathoner, assuming all other variables equal.

From a coaching perspective, I’ve seen a range of athlete responses. Some athletes feel amazing and perform incredibly well coming off a standard carb load (three days at 8 g carb per kg of bodyweight). Other athletes report bloating, stomach distress, and other undesirable symptoms. Does carb loading not work for these runners, or are there adjustments they can make to gain that 2-3% performance advantage?

Then, you have your social media claims: women shouldn’t carb load, only certain types of carbs store as glycogen, you shouldn’t run during the carb load, and more.

So let’s dive into the science and practice of carb loads. In this article, I’ll examine what research actually says about carb loads - and if those social media claims hold up. Additionally, I’ll share evidence-based approaches for carb loading, including what to do if a regular carb load leaves you running for a bathroom on race day.

What is a Carb Load?

Firstly, let’s begin this discussion with a clear definition. A carb load is a deliberate increase in carbohydrate intake in the two to three days prior to a marathon or other long endurance event. The research consensus is 8-10 grams of carbohydrate per kilogram of bodyweight for two to three days leading up to an endurance event such as a marathon. To be fully effective, a carb load must be coupled with a reduction in exercise - or the final days of the pre-race taper.

Glycogen (from both the muscles and the liver) is a preferred fuel source for energy production during the marathon. The body is able to use glycogen and glucose to rapidly produce large amounts of ATP - enough to keep up with the energetic demands of running at a marathon.

After 90 minutes to two hours of running at marathon intensity, your glycogen stores run low. You never truly deplete them - your body will slow you down first for protection, since true depletion would be life-threatening. However, as they run low, you are forced to slow down so that your body can use fat oxidation (a slower, less efficient process) to produce energy.

In addition to intra-run fueling and a race morning breakfast, a carb load optimizes your glycogen stores for performance. Glycogen is a preferred energy source at marathon pace. If you start with your race with higher glycogen stores, and provide glucose (intra-run fuel) throughout, you will not “hit the wall” and have a better chance of sustaining race pace throughout.

Does Carb Loading Work for All Runners?