High-Carb Fueling: How High Can You Go?

After playing second fiddle to fat, carbohydrates are now back in the limelight for endurance athletes. In recent years, Keto-adapted or fat-adapted diet trends have dominated headlines and prompted polarizing debates on social media about how to fuel endurance events.

Perhaps it’s the recent introduction of glucose monitoring devices such Super Sapiens that have propelled the return of carbohydrates now to center stage as a crucial component for performance in endurance racing. When it comes to converting nutrients into energy, carbohydrates are the fastest and most efficient form of fuel for athletic performance—or as this Aussie would say, “carbs can help you go bloody quick!”

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If you want to go fast, you need carbohydrates

Science has time and again demonstrated that as exercise intensity increases, fuel utilization shifts from predominantly fats to predominantly carbs. This shift occurs at around 70-75% of an athlete’s VO2max. If you are working at a high enough intensity, your body will benefit from exogenous (ingested) carbohydrates to maintain blood glucose levels at a stable level and attempt to spare liver and muscle glycogen. The consumption of carbohydrates also positively influences the central nervous system, reducing the feelings of fatigue. Under the right conditions, carbohydrates will ultimately improve performance and offset fatigue.

In this article, I’ll delve into why carbohydrates are important for performance, when you should use carbohydrates, factors to consider when using carbohydrates for training and racing, fueling with carbohydrates in practice, as well as how many carbohydrates might be too many.

When should you eat carbohydrates?

Not every training session requires carbs. Ask yourself, “What is the purpose of the session today?” Only after answering that question, will it be clear how you should fuel.

For sessions lasting over 90 minutes at an intensity of at least 70% of VO2max (high aerobic zone 2 and above) the consumption of carbohydrates will help offset fatigue and improve your performance.

During lower intensity endurance sessions (i.e. longer than 90 minutes with low intensity), the consumption of large amounts of carbohydrates is not required. Indeed, if you’re looking to improve your ability to utilize fat as a fuel source, keeping carbohydrate consumption low during these sessions can be beneficial. You are effectively reducing your respiratory exchange ratio to improve metabolic flexibility and fat utilization as a fuel source. Another reason to minimize carbohydrates during these sessions could be if you are aiming to influence body composition or lose weight through caloric deficit. Reducing carbohydrate intake is one way of reducing total caloric intake while training.

The exception to this rule would be short, high-intensity sessions where the sole purpose is to perform at your absolute best. In this situation, carbohydrate consumption could prove useful. However, this improvement in performance appears to be neurally driven (brain-derived) and could be an example of when mouth rinse carbohydrate fueling might be used. That’s a topic for another day, so for now I’ll focus on carbohydrate consumption.

How much carbohydrates should you consume? 

Before we dig into how many carbohydrates are appropriate to consume and the factors surrounding this topic, I should note that a lot of research has shown small amounts (i.e. 22-40g/hr) of carbohydrate consumption from a single source, namely glucose, is beneficial and is not necessarily superior to higher amounts. However, most of this research has been performed in relation to a sprint at the end of extended exercise (two to three hours) rather than continuous race pace intensity. In essence, the data surrounding high carbohydrate fueling for triathletes does not exist. We are starting to see research being published around high carbohydrate fueling for cyclists, but even this research is relatively new.

What influences how much you can consume?

From a practical perspective, palatability is a crucial aspect of fueling. Can you eat certain carbohydrates while training and racing? The flavor, consistency, and overall palatability of what you plan to eat is an important factor to consider and to practice to ensure you can swallow the nutrition. This is often not discussed in scientific papers as participants usually don’t get a choice. As an athlete, you can pick and choose based on personal preferences.

The optimal amount of carbohydrates that you consume should coincide with the maximal rate of carbohydrate oxidation without causing you to feel sick. This does not occur in isolation to fatty acid oxidation and the rates at which it occurs will vary depending on the intensity and the duration of the session, as already discussed. The oxidation rate of carbohydrates appears to be relatively similar among fluids, gels, chews or block performance nutrition, although the data on this is limited. From personal experience, working with hundreds of triathletes, the tolerance of high amounts of carbohydrates appears to favor gels, chews/blocks, liquids, and finally bars or solid food, in that order. 

The difference in oxidation rates between types of carbohydrates is unequivocal with glucose, sucrose, and maltodextrins having much higher rates of oxidation compared to fructose and amylose for example. This has implications on your choice of carbohydrates.

The rate at which carbohydrates can be oxidized and used appears to be limited based on the type. Single source carbohydrates are limited to around 60g/hr. Even if you continue to ingest a greater amount, you are not necessarily using them, rather you are increasing your risk of gastro-intestinal (“GI”) distress. Many “energy” products on the market combine glucose, maltodextrin, and fructose in order to increase carbohydrate oxidation and avoid this rate limiting effect. Rates of oxidation increases of up to 45% have been observed by using sources of carbohydrates that require multiple transporters.

Figuring out what works for you

The science is interesting but a critical aspect of high carbohydrate fueling is how you put it in practice. Testing and trialing carbohydrate intake in training is just as important as testing any other piece of equipment you might use in a race. You wouldn’t ride a time trial bike for the first time in a race; you’d practice and tweak your position over multiple rides until you have confidence and comfort riding it. It’s the same process for race nutrition. Test, adjust, and repeat until you have a formula that works for you.

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To begin with, I recommend using single source glucose or maltodextrin products such as chews or blocks. This reduces the risk of GI distress. Do not attempt to consume more than 60g/hr in your first few trials if you have not done so previously, as it will likely end in a trip to the loo! Take a data-driven approach to the process by recording the session duration, session intensity, total consumption, the source of carbs (gels/chews/fluids/bars), brands, and whether you experience any upper or lower GI issues. (Shameless plug: We do this for athletes in the FuelIn app) Repeat the process until you are comfortable with the level before increasing. Once you are consuming 60g/hr, you can trial a higher carbohydrate intake that includes some product with a 2:1 glucose/maltodextrin:fructose ratio in the ingredients. “Some” is key here since if all of them are of the 2:1 ratio you risk experiencing GI distress due to the increased amounts of fructose. While the addition of fructose can improve the total amount of carbohydrates that can be oxidized, fructose does have a slower gastric emptying effect on the gut, which can be problematic. 

Research indicates that oxidation rates are similar across carbohydrate formats such as gels, chews, and liquids, but we also know that highly concentrated carbohydrate liquids pull water into the intestine from the bloodstream in an attempt to reduce osmolality. This typically increases the risk of GI distress—either upper GI in the form of belching, burping or vomiting, and/or lower GI in the form of flatulence, cramping, bloating, or diarrhea. Concentrated carbohydrate fluids are unlikely to be an issue during the cycling leg of a triathlon and most of the research relating to high carbohydrate fueling has been performed on male cyclists. However, that research does not take into account the mechanical stress placed on the stomach when running. A bouncing diaphragm combined with a stomach full of a hypertonic solution often results in a trip to the bathroom rather than a direct route to the finish line! In addition, in colder climates when sweat rates and the need to drink are lower, it may be more challenging to consume high amounts of carbohydrates in liquid form. 

For Olympic and half-Ironman distance events, athletes can, with practice, consume 90-120g/hr via gels and chews/blocks while using electrolytes and water to maintain adequate hydration. For Iron-distance events, carbohydrates in liquid form could be consumed during the first half of the bike leg to allow adequate time for gastric emptying before the run. Athletes might also consider eating a bar of mixed macronutrients (carbs/fat/protein) every hour in longer distance events to boost caloric intake and palatability. You will never match your caloric expenditure through ingestion of carbohydrates during a long race, but you do need to consume calories in whatever form your body can handle. Again, this should be practiced in training and repeated multiple times in training to ensure it works for you.

RELATED: The Triathlete’s Guide to Race Fueling for Every Distance

High-carbs: How high is high enough?

From my experience with triathletes, combined with the research, there appears to be an upper limit of ~140-150g/hr of exogenous (ingested) carbs. This level is achievable for both males and females although it may not be necessary for smaller athletes to fuel beyond ~100g/hr on the bike due to lower overall body mass. Above the 140-150g/hr, the body’s ability to oxidize carbohydrates is maxed out and additional intake does not improve oxidation rates nor perceived effort or performance. Of course, the total amount will vary between individuals but these levels, I believe, (until research indicates otherwise) are sufficient to allow you to perform at your best.

While these intake levels can work well on the bike, from my experience with athletes at FuelIn, carbohydrate intake during the run will be lower. The higher carbohydrate consumption rate on the bike sets up the run nicely and the consumption rate on the run can drop to 50-90g/hr based on testing and self-assessment.

The final considerations in all of this are your body size and the intensity of the racing. If you are not racing above 70% of VO2max, either due to experience, choice, or ability, then this is not applicable. If you are looking to go fast, hit PRs, and do your absolute best, then systematically practicing and implementing a high carbohydrate fueling protocol in training and racing is recommended.

Scott Tindal is a performance nutrition coach with 20 years of experience working with pro and amateur athletes. He has a Masters degree in sports medicine and a post-graduate diploma in performance nutrition. He is the co-founder of FuelIn, an app-based personalized nutrition coaching program.