How Many Carbs Should Endurance Athletes Really Consume Per Hour?

The Recent Epiphanies Reshaping Carb Intake

Carbs used to be so simple – just take in plenty of sugar or glucose or maltodextrin or a combination thereof. This is what EFS, EFS-PRO, and Liquid Shot were created to deliver, and they do it well (more on this later). But slamming more and more carbs into your body has limits, even with ultra-endurance distances and despite the recent research opening the floodgate. Before getting into the nuts-and-bolts of how to carb up, let’s get down with some history about carbs and exercise.

Glucose Legacy

Twentieth-century exercise physiologists focused on glucose (dextrose) for carb delivery because that’s what’s in your blood, that’s what’s stored as glycogen in muscle and liver, and that’s the primary fuel “burned” by muscles. Given that basic fact, exercise physiologists were looking for various ways to squeeze more glucose into you for decades, but limits were quickly found as endurance events became longer and longer. The limit was getting glucose past the gut and into your bloodstream without causing GI distress from unabsorbed carbs, but those early researchers had a limited understanding of how your gut really works to absorb and metabolize sugars.

Fortunately for athletes, science marches on, and game-changing gut adaptation epiphanies have recently been pinned down, making an impact on fueling for endurance events. Here are some of the bigger epiphanies.

Epiphany #1: MTCs Were a Carb Revolution – All About GLUT5 (recently renamed SLC2A5)

MTCs means Multiple Transportable Carbohydrates, which really gathered steam in the mid-2000s and are now the norm. In its simplest form, MTC formulas add large amounts of pure fructose to other carbs, usually glucose or glucose polymers (maltodextrins), in ratios of 2:1 or 1:1. These high fructose ratios were studied early in order to maximize the newly understood effect of sneaking fructose into the gut with a forgotten transport receptor on your absorptive gut lining cells (enterocytes). These receptors are called GLUT5 (we’ll stay with the more familiar GLUT5 name for this blog). GLUT5 prefers to pull pure fructose out of the inside of your gut and throw it into your enterocytes. Since there are two major glucose transporters (SLGT1 & GLUT2), adding GLUT5 to the mix is where the MTCs name derived.

Adding enough fructose to glucose sources increases overall carb delivery by effectively circumnavigating your body’s glucose uptake limits – letting fructose in the back door of intestinal lining cells while the front door is blocked by the glucose queue. Once it’s absorbed, fructose is then converted into more glucose, up to the amount of your gut’s ability to convert it.

MTC carb studies on endurance exercise showed that carb grams per hour can be increased to previously untenable amounts, especially for exercise lasting three hours or more. A consensus of research shows that sustained carb intakes of 90g/h as Glc:Fru combinations (usually 2:1 or 1:1) usually leads to improvements in endurance performance, which in turn led to a plethora of Glc:Fru products for endurance athletes that we have today. However, other ratios of Glc:Fru products for endurance athletes have not received attention recently at higher intakes. Of course, First Endurance was ahead of the game as usual, and capitalized on MTCs in the mid-2000s when EFS, and later, EFS-PRO were introduced. Our field testing backed up the laboratory research that MTCs can improve long-duration performance.

Epiphany #2: Instant Adaptation Opens Glucose Floodgates

Very recently, another research epiphany overturned intestinal dogma by showing that GLUT2 (recently renamed SLC2A2), another transporter for glucose, can rapidly show up on the apical, or inward absorptive side of gut enterocytes – within minutes after ingestion of high glucose loads. This on-the-fly adaptation lets your body get more glucose from the gut into the gut lining and, thus, into the bloodstream than before.

Normally, GLUT2 is how gut enterocyte cells dump glucose absorbed from the gut out the other side (basal side) into circulation; however, GLUT2 was not seen on the absorptive (apical) side of gut enterocytes because researchers did not see it during rest or normal meals. It was thought that the SGLT1 transporter (recently renamed SLC5A1) was sufficient to internalize normal glucose intakes. But researchers never looked for GLUT2 in extreme conditions, like during endurance exercise or high carb intakes. This finding is so new that gastroenterology textbooks still show GLUT2 receptors only on the basal side of gut enterocytes, but this will change with time.

Here’s the new news: When your gut gets a lot of glucose quickly, your gut nervous system sends cellular commands to make more GLUT2 glucose receptors on the absorptive apical side, and this happens quickly! In minutes! It was evident that your enterocytes normally keep a supply of GLUT2 receptors hanging out inside themselves, just waiting to be directed to top (apical) or bottom (basal) sites depending on how much glucose you just ate. As soon as the inside-gut levels of glucose drop to more manageable levels, the added apical SLC2A2 receptors disappear just as quickly. GLUT2 is still pushing absorbed glucose out of enterocytes into circulation on the basal side, as always.

This normal phenomenon has only recently been elucidated, but it explains 90g/h or higher carb intakes being manageable. But of course it’s not that simple. Can you simply glug down more carb drinks per hour and perform longer and stronger? Not so fast – ramping up to 2-3 times more carbs than usual at once will still lead to GI disasters. There is another, much more important and quantitative way to increase your gut carb uptake during endurance exercise: Gut Training.

Epiphany #3: Gut Training for Increased Carb Intake

Studies have shown that training your gut to accept higher carb intakes during exercise can be accomplished in two weeks. One two-week study (Costa 2017) had athletes ingesting 30g of carbs every 20 minutes for two hours running at 60% VO2max, five days per week, while maintaining their normal training loads. That shakes out to 90gCHO/h or 180gCHO during each two-hour session. The subjects (29, but only10 women and 15 men completed the study) ingested carbs as a 2:1 Glu:Fru combination in solid (“disc” aka chewable tablet) form with 400 ml flavored liquid. It’s important to note that the subjects’ body weights were 67-76kg (~150lb).

Subjects also ran under placebo (flavored water) conditions and again with the same amount of carbs as a food (AmazeBalls from Australia) with 4g protein and 4g fat per feeding. Running performance and blood glucose levels were improved by the Glu:Fru discs but not by the food balls, compared to placebo conditions.

In this study, 29 men and women had four dropouts (DNFs) from adverse GI complaints (14%). For the 25 finishers, although GI complaints were reduced when retested after the study was completed, there were still some GI complaints, but close to normal levels seen with lower carb intakes (<60gCHO/h), indicating that the gut receptors were trained to handle larger carb loads. Compared to the first carb trial, both carb sources (discs and food balls) had reduced GI complaints at the end of two weeks.

This study presented a model for training the gut to more easily handle larger carb loads for ultraendurance athletes. Individual biology is also obviously important, so the hourly amount isn’t universal and should be adjusted individually according to your body weight to prevent GI distress.

Individualization for Increased Carb Intake

Don’t simply follow the study protocol listed above, as it only applies to a specific demographic: well-trained athletes weighing ~150lb. If you are under 140lb or more than 160lb, those gCHO/hour don’t match the research and may push you into forbidden territory for results – and for GI side effects.

So how do you tailor this approach to yourself, to determine what you actually need to consume to reach the magic grams/hour numbers? And how do you keep this process simple and make its application relevant to your preferred fuel source? You individualize!

We’re providing the handy table below so you can see how to individualize your carb intake using EFS & EFS-PRO powder amounts to match the gCHO per hour for 150lb (70kg) persons used in the Costa study for gut training for 90 gCHO/h over 2 hours. We are also showing the original recommendation for 60 gCHO/h and increased amounts for 120g CHO/h for heavy-duty CHO intake.

To keep this simple, we calculated how many scoops of EFS or EFS-PRO each bodyweight needs, and how much water (in fl oz like most water bottles) is needed to be added to the scoopfuls to reach a 10% CHO concentration. The goal is to drink at least that many fl oz every hour, preferably in 3 equal installments every 20 minutes. Keep in mind that you may need more fluid per hour than the fl oz in the Table – simply add water based on your traditional needs. It’s OK to drink more dilute concentrations if that works out better logistically. For body weights under 154lb, you may need to ingest additional fluids, preferably water, each hour.

Table 1: How to Use EFS or EFS-PRO for Gut Training by Your Body Weight

Note 1: grams CHO have been rounded up to the nearest whole number.

 If you encounter GI distress that’s too much to handle, try using an amount in between the gCHO/hour that you know you can handle and the amount that was too much. For example, if you cannot tolerate 90 gCHO/h, try 75 gCHO/h by using the halfway amounts between 60 and 90 gCHO/h.

If gut training and training exercise simulating a real event go well, you should be ready. Gut training this way you can max out your performance by maximizing your fuel intake, staying hydrated, and not losing electrolytes during long duration events. We also calculated the electrolyte intakes and even at the highest CHO intake (120 gCHO/h), they are not excessive and within actual usage and controlled study intakes.

The other important piece to this puzzle is when to start ingesting higher amounts of carbs. Current advice is to start early, with a dose 30 minutes before starting exercise, then start ingesting your drinks at 20 minutes after the start for 3 servings per hour, as in Gut Training. Others argue to use your usual practices until 2 or 3 hours into an event, and then ramp up your carb intake. This issue is why you need to try increasing carb intake, even after Gut Training, before you participate in an actual event. You know what you can take better than we do, especially when pushing your limits.

Conclusion

Using gCHO/kg/h is a good starting point to individualize how much carbs you can take during exercise because endurance athletes aren’t all uniformly lean. Getting close to a carb dose per hour that should be ideal for you lets you fine-tune your training to determine what really works for you, faster, sooner, and with less bonking or belly sloshing and GI distress.

Gut Training is the “new” message for achieving and maintaining higher-than-usual carb intakes during endurance exercise longer than three hours to improve performance without running into GI tolerance problems. Benefits of reaching 90g carbs or more per hour after gut training are improved performance and less GI distress, but not zero GI discomfort or symptoms. The unique EFS-PRO formula has solved these issues based on extensive feedback over years of successful usage.

References

Alam YH, Kim R, Jang C. Metabolism and health impacts of dietary sugars. J Lipid Atheroscler. 2022 Jan;11(1):20-38.
Arribalzaga S, Viribay A, Calleja-Gonzalez J, Fernandez-Lazaro D, Castaneda-Babarro A, Mielgo-Ayuso J. Relationship of carbohydrate intake during a single-stage one-day ultra-trail race with fatigue outcomes and gastrointestinal problems: a systematic review. Int J Environ Res Public Health. 2021 May27;18(11):5737.
Bourdas DI, Souglis A, Zacharakis ED, Geladas ND, Travlos AK. Meta-analysis of carbohydrate solution intake during prolonged exercise in adults: from the last 45+ years’ perspective. Nutrients. 2021 Nov24;13(12):4223.
Costa RJS, Hoffman MD, Stellingwerff T. Considerations for ultra-endurance activities: part 1 – nutrition. Res Sports Med. 2019 Apr-Jun;27(2):166-81.
Costa RJS, Miall A, Khoo A, Rauch C, Snipe R, Canoes-Costa V, Gibson P. Gut-training: the impact of two weeks repetitive gut-challenge during exercise on gastrointestinal status, glucose availability, fuel kinetics, and running performance. Appl Physiol Nutr Metab. 2017 May;42(5):547-57.
de Oliveira EP, Burini RC. Carbohydrate-dependent, exercise-induced gastrointestinal distress. Nutrients. 2014 Oct13;6(10):4191-9.
de Oliveira EP, Burini RC, Jeukendrup A. Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations. Sports Med. 2014 May;44 Suppl 1(Suppl 1):S79-85.
Drozdowski L, Thomson ABR. Intestinal sugar transport. World J Gastroenterol. 2006 Mar21;12(11):1657-70.
Ferraris RP, Diamond J. Regulation of intestinal sugar transport. Physiol Rev. 1997 Jan;77(1):257-302.
Ishihara K, Uchiyama N, Kizaki S, Mori E, Nonaka T, Oneda H. Application of continuous glucose monitoring for assessment of individual carbohydrate requirement during ultramarathon race. Nutrients. 2020 Apr17;12(4):1121.
Jeukendrup AE. Training the gut for athletes. Sports Med. 2017 Mar;47(Suppl 1):101-10.
Jeukendrup AE, Moseley L. Multiple transportable carbohydrates enhance gastric emptying and fluid delivery. Scand J Med Sci Sports. 2010 Feb;20(1):112-21.
King AJ, O’Hara JP, Morrison DJ, Preston T, King RGJ. Carbohydrate dose influences liver and muscle glycogen oxidation and performance during prolonged exercise. Physiol Rep. 2018 Jan;6(1):e13555.
Koepsell H. Glucose transporters in the small intestine in health and disease. Pflugers Arch. 2020 Sep;472(9):1207-48.
Miall A, Khoo A, Rauch C, Snipe RJ, Canoes-Costa VL, Gibson PR, Costa RJS. Two weeks of repetitive gut-challenge reduce exercise-associated gastrointestinal symptoms and malabsorption. Scand J Med Sci Sports. 2018 Feb;28(2):630-40.
Pfeiffer B, Stellingwerf T, Hodgson AB, Randell R, Pottgen K, Res P, Jeukendrup AE. Nutritional intake and gastrointestinal problems during competitive endurance events. Med Sci Sports Exerc. 2012 Feb;44(2):344-51.
Rumessen JJ, Gudmand-Hoyer E. Absorption capacity of fructose in healthy adults. Comparison with sucrose and its constituent monosaccharides. Gut. 1986 Oct;27(10):1161-8.
Scow JS, Tavakkolizadeh A, Zheng y, Sarr MG. Acute “adaptation” by the small intestinal enterocyte: a posttranscriptional mechanism involving apical translocation of nutrient transporters. Surgery. 2011 May;149(5):601-5.
Tiller NB, Roberts JD, Beasley L, Chapman S, Pinto JM, Smith L, Wiffin M, Russell M, Sparks SA, Duckworth L, O’Hara J, Sutton L, Antonio J, Willoughby DS, Tarpey MD, Smith-Ryan AE, Ormsbee MJ, Astorino TA, Kreider RB, McGinnis GR, Stout JR, Smith JEW, Arent SM, Campbell BI, Bannock L. International Society of Sports Nutrition Position Stand: nutritional considerations for single-stage ultra-marathon training and racing. J Int Soc Sports Nutr. 2019 Nov7;16(1):50.
Urdampilleta A, Arriblazaga S, Viribay A, Castaneda-Babarro A, Seco-Calvo J, Mielgo-Ayuso J. Effects of 120 vs. 60 and 90 g/h carbohydrate intake during a trail marathon on neuromuscular function and high intensity run capacity recovery. Nutrients. 2020 Jul15;12():2094.
Viribay A, Arribalzaga S, Mielgo-Ayuso J, Castaneda-Babarro A, Seco-Calvo J, Urdampilleta A. Effects of 120 g/h of carbohydrates intakes during a mountain marathon on exercise-induced muscle damage in elite runners. Nutrients. 2020 May11;12(5):1367.