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What are carbohydrates and how do they work?

Icon of calendar08/11/2023

High-carb products are extremely popular with endurance athletes and for a very good reason. But power endurance (rugby, football, field/ice-hockey, rowing, American football) and high-intensity strength athletes (CrossFit, sprint-training, MMA, boxing) benefit equally from taking in carbs during training sessions or during events or matches. But what exactly are carbs, how do they work, and why do you need them?

Different energy sources

First, it's important to understand that the body uses two different nutrients as its fuel source: carbohydrates and fats. In this article, we will focus on the first. 

Carbohydrates are stored in your body as glycogen, primarily in the liver and in muscle tissue. If your diet is good, the liver (typically) holds 80-100g of glycogen, and muscle tissue holds somewhere between 300-500g of glycogen. That doesn't mean both storages are available for exercise though. The glycogen stored in your liver is used to maintain blood glucose levels, and it's the glycogen stored in your muscles that is used during exercise.

During low-intensity sessions (zone 1), your body typically uses fat as its main energy source. So, if you want to burn fat, keep your training intensity low. But as soon as you dial up the intensity (zone 2-5), regardless of the type of sport, your body automatically switches to carbs, as they’re a more efficient fuel source. At medium intensity, you burn around 3 grams of glycogen per minute, and at high intensity, you burn around 4 grams of carbohydrates per minute. Given that you have around 300-500 grams of carbs stored in your muscles, why the need for extra? Well, there are numerous reasons.

The impact of glycogen on performance

If you train more than three times a week, preserving your muscle glycogen is crucial because you need certain minimum levels of muscle-stored glycogen to maintain your training intensity throughout the week. Therefore, you want to finish each training session as "fresh" as you started it, nourishing your "glycogen threshold" to maximise the benefits of every training.

That “glycogen threshold" is a concept in exercise physiology that refers to a critical level of muscle glycogen necessary for optimal athletic performance. It's not a fixed amount of glycogen per se, but rather a point at which glycogen stores become low enough that they can no longer support high-intensity exercise, leading to a significant drop in performance (also known as "bonking" or "hitting the wall"). So, as muscle glycogen levels decrease, the ability to sustain high-intensity exercise declines rapidly, and when glycogen levels drop, the body must rely more on fat oxidation, which is a much less efficient energy source. And during high-intensity or prolonged sessions, when oxygen is scarce, relying on glycogen becomes even more important. So, simply put, it is important to keep your glycogen levels in check during each training if you want to maximise your training intensity through the rest of the week.

Another reason you want to keep those glycogen levels in check is that low glycogen levels trigger the body to reduce its energy output to preserve remaining stores, leading to feeling fatigued, which results in a natural reduction in exercise intensity, limiting you from pushing your limits. This process is like a chain reaction that starts with your body's hormone levels changing. When your body runs low on its glycogen stores, it raises your cortisol levels, which, in turn, causes your body to break down proteins. On top of that, when your body's response to insulin changes, it can impact how effectively it uses the carbohydrates that you have left for energy. This can lead to feeling more tired than usual and may decrease your endurance or performance in physical activities even more. 

Talking about a domino effect... 

Carb ratios and carbs per hour

Now that we covered how carbs function and you understand their importance, we should discuss another straightforward yet often overlooked topic: how many carbs should you consume per hour and why? 

As explained earlier, how much you should consume per hour, and why, completely depends on intensity and duration, but the general rule of thumb is simple:
  • For a medium-intensity session shorter than two hours: consume 30-60 (45g avg.) grams per hour (1 unit per hour)
  • For a high-intensity session OR sessions longer than two hours: consume 90/135 grams per hour (2-3 units per hour)

That doesn't mean you can consume any gel, chewable or drink mix at these rates, as that will inevitably lead to stomach and GI issues. Whether or not you can process these quantities heavily depends on the "carbohydrate ratio" used in the product. That carb ratio is the relative proportion of different types of carbs present, and how your body processes these carbs has everything to do with two transporters: the SGLT1-transporter, responsible for processing glucose (and maltodextrin) and galactose, and the GLUT-transporters, responsible for processing fructose. 

Each transporter has a somewhat fixed upper limit of what it can process per hour: for the SGLT1, that's around 60 grams per hour, and for the GLUT-transporter, it's about 30-40 grams per hour. So, if you were to maximise both transporters, you can rather easily process up to 90-100 grams of carbs per hour without any gut discomfort. However, if you stress one of these transporters by consuming more than they can handle, you'll experience gastrointestinal problems like bloating, cramps, and nausea, which can impair performance. While many endurance athletes are familiar with these problems, the issue isn't the number of carbohydrates you consume per hour, but how you mix and match different carb ratios.

Many old-school brands such as SiS, Hammer, and Powerbar, and young, emerging brands still offer gels and drink mixes containing only a single type of carbohydrate, seriously limiting how much you can process per hour. If you try to consume 90 grams of carbs per hour coming from one single carbohydrate source, you may overload one of the transporters, leading to gastrointestinal issues and significantly limiting your performance.

It is known that SGLT-transporters have the ability to process 60-90 grams of glucose per hour, while GLUT-transporters can handle 30-50 grams. When glucose is consumed alone, the absorption rate is 1-1.1 gram per minute (60-66g/h). However, when glucose is combined with fructose, the absorption rate increases to 1.5-2.0 g/m (90-120 g/h). This results in the well-known 2:1 ratio.

Train with what you compete with

We can’t stress this enough, but getting your body accustomed to taking in large quantities of carbohydrates per hour takes training. If you plan to compete using carbs as your main fuel source, consistently using them during your weekly runs, rides or workouts is as important as putting in the hours. Then, when race day comes, do not switch brands or suddenly increase your hourly intake. Your body will already be under significant stress, and overloading the earlier-mentioned transporters is not a wise decision. This is what the off-season is for: read up and try new brands and their products. 

02 During

We developed 02 During in response to an extremely confusing and broad offering of gels and drink mixes already available.  Despite the clear science and available information, many products still contain sub-optimal carb ratios and confusing quantities, making it difficult for athletes to optimize their energy management. How can we be expected to consume 90-135 grams per hour when gels typically contain between 18-25 grams of carbohydrates each, especially when such a low-carb gel costs around €2,00-3,50 per unit? This makes things too expensive and highly impractical for most—not to mention the challenge of carrying all those gels.

Both our 02 During drink mix and gel contain 45 grams of carbs per serving, simplifying intake planning and energy management optimisation while significantly reducing cost per hour/training - even when aiming for 135 g/h. 

Our fuel guide

To help you further, we've put together a comprehensive fuel and hydration guide. If you're interested in getting a personalised intake plan, just follow the link below, answer a few questions, and our performance coaches will tailor a plan for you.