October 13, 2022

Endurance training for your digestive system

Often, one of the greatest limiters for athletic performance is proper fueling.

You may feel like your training is insufficient, or you are just having a bad day, but the root cause could be your ability to maintain fuel supply. Properly training your digestive system can take your performance to new heights especially where endurance is a factor.

“Eat more, go faster” is commonly quoted training advice, that's somewhat true when done correctly. The key to that is training your gut to consume more calories, and doing so to utilize those calories, which is important for any endurance sport.

We’ve asked our nutritionist Spencer Miller for advice on training your digestive system.

Why is digestive training something that you need to think about?

It's difficult to just suddenly consume more carbs per hour and immediately have the benefits, often the digestive system rejects a huge influx of food during or before an effort, either causing discomfort or even expelling the stomach content it can’t manage, and even if it doesn’t do this, it may not be able to process all the calories available. Gut training, which is teaching your body to be able to handle and utilize more energy, is something that takes time and must be done correctly. When you do it correctly, you may find that you were never reaching your full potential and will see increases purely from fueling, not just additional training.

How the body processes energy

Most of us know that the energy source which you break down during exercise (carbs or fats) is highly dependent on many things, including the intensity of your training, how well “fat-adapted” you are, and the type of energy you consumer before and during the training.

In general, there are 2 types of energy for the body

1. Fat

2. Carbohydrates


Breaking down of fat for energy is called lipolysis. The process of lipolysis normally provides free fatty acids (FFA) at a rate in excess of that required to supply resting energy requirements. At the start of low intensity exercise, lipolysis increases further, thereby providing sufficient FFA to provide energy substrates in excess of requirements.


Sources of carbohydrates for the muscle can be broken down further into blood glucose, muscle glycogen, and liver glycogen.

Blood Glucose

Blood sugar, or glucose, is the main sugar found in your blood. It comes from the food you eat and is your body's main source of energy. Your blood carries glucose to all of your body's cells to use for energy.

Muscle Glycogen

Muscle glycogen serves mainly as a source of metabolic fuel for your muscles.

Your muscles require lots of energy to function in order for you to move. If your muscles relied on glucose from your bloodstream for this energy, your body would quickly run out of glucose as it is only available for a limited period following digestion and is also required by other organs to function.

Because of this, your body stores three-quarters of your total glycogen in all of your skeletal muscles so they have a consistent supply of energy, especially during exercise, without dramatically affecting the levels of your blood glucose.

The rate at which your muscle glycogen reduces is primarily related to the intensity of physical activity — the greater the exercise intensity, the greater the rate at which muscle glycogen runs out. As a result, high-intensity activity, such as repeated sprinting, can quickly lower glycogen stores in active muscle cells, even though the total time of activity might be relatively brief.

The way to restore muscles with glycogen is by consuming carbohydrates, especially after intense exercise. The rate of glycogen replenishment is highest in the 2 hours after training.

Liver Glycogen

Your body mainly uses the store of glycogen in your liver to help regulate your blood glucose (sugar) levels.

Your body normally carefully regulates your blood glucose primarily with the hormones glucagon and insulin. When your blood glucose levels fall too low (hypoglycemia), your pancreas releases more glucagon. Glucagon, in part, triggers glycogen in your liver to convert back to glucose so it can enter your bloodstream. This process is called glycogenolysis. When glucose is in your bloodstream, cells throughout your body can use it for energy.

The glycogen stores in your liver also partially help with muscle activity and exercise. At the start of exercise, your liver begins breaking down glycogen to maintain blood glucose levels as your working muscles use it for energy. However, your muscles primarily use their own glycogen stores to function.

Training your gut

During endurance or lower intensity rides, your body will get most of its energy needs from the breakdown of fat. Of course, this depends on the person, what you ate before, and if you intake carbs during the training session. Consuming higher GI, faster-digesting carbs will almost “turn off” the fat burning and switch your body to breaking down carbs for energy.

One pound of fat has roughly 3,500 calories, so even the leanest athletes have plenty of fat stores to fuel long, slow endurance exercise. However, most reading this may race, or compete in events where you also go into higher intensity and utilize carbs for energy.

The body needs to break down carbs via anaerobic glycolysis. This is great to be able to do high intensity efforts, but the problem is that your body can only store roughly 300-500g carbs in the form of glycogen. One gram of glycogen is 4 calories, so you can see the issue here. You can only go so long relying on glycogen without consuming more fuel to keep up with the burn rate you are exercising at.

Based on the 300-500g glycogen stores available, this is only 1,200-2,000 calories, not enough to fuel a long race, or really any sort of high intensity ride over 2 hours.

You can of course keep up with the demand by consuming carbs in the form of drink mix, gels, bars, etc. however the body can only access so much, without training the gut to be able to use more than the generally accepted 60g carbs per hour.

This is highly dependent on an athlete’s body composition and how much they have already trained their gut. Some athletes run into trouble at just 40g per hour without any training, and others can handle upwards of 90g + per hour after training their gut and utilizing a glucose-fructose mix which we will talk more about below.

The bottom line here is that everyone should work on training their gut to be able to handle and utilize more so that they can ride harder, access more calories, and be able to replenish those calories without any GI distress or bonking.

What does Digestive training do?

In a nutshell, training the gut allows you to consume more, so you can ride harder for a longer period without exhausting your digestive system.

“Studies show that regular intake of 60-90 g/hr of carbohydrate during training can increase tolerance to aggressive feedings,” says Patrick Wilson, author of the acclaimed The Athlete’s Gut: The Inside Science of Digestion, Nutrition and Stomach Distress. “Some of the adaptations probably relate to stomach emptying, while others involve upregulating the expression transporters that shuttle sugar molecules into intestinal cells.”

The reasons that gut training helps are that it...

• Improves gastric emptying

• Improves intestinal absorption

• Improves stomach comfort

• Reduces GI problems

How to train the digestive system and how long does it take?

Now you know how the body processes energy, fat vs carb breakdown during training, and the benefits of training your gut. So, now to answer the question of how to do it and how long does it take?

It is best to gradually increase carbohydrate intake once or twice a week. It doesn’t have to be anything crazy, just a small increase. It is much better to do this little by little over multiple weeks than try and go to fast and get the GI distress that comes with it.

If you are currently only doing 30g carbs per hour in a ride, then try upping that by 10g per hour to a total of 40g per hour. Then, see how that feels. If there are no issues, the next week jump up to 50g carbs per hour. Try that for a week. Again, if there are no issues, jump up another 10g per hour to 60g per hour. Follow this trend until you reach your desired intake per hour.

Additionally, this isn’t something that you need to do year-round. You don’t need super high carb intake every hour of training year-round. The high intake is more important for key sessions and racing. So, over the winter you can relax a little bit during the short turbo sessions and begin to train the gut tolerance back up as it gets closer to race season.

There is no set amount of time on how long it should take. It all depends on your starting point and your end goal. Some may just want to try and increase 10-20g carbs per hour. Others may not be doing many at all but want to get to a high level to really fuel those hard races and they may need more time to reach their maximal carbohydrate intake per hour.

You can really go by feel. If you start to get GI distress, then you are likely going to quick. Back it down a bit and try that for a longer amount of time then try to increase again. If it feels good, then you know you just needed more time to adapt. If it still doesn’t feel comfortable and you are having stomach issues, it is likely you have found your body’s max and you can’t go any further than the level you are currently at.

Different sources of carbohydrates for maximum absorption

You have probably seen many different high-carb drink mixes on the market nowadays. Most of the ones that you should be looking at are a mix of glucose and fructose. Why glucose and fructose? This comes down to the transporters that shuttle the carbs to the muscles.

The transporter for glucose to the working muscle, SGLT1 (sodium/glucose cotransporter 1) is responsible for glucose absorption in the small intestine. Exercise studies have provided indirect, but strong evidence that the delivery of carbohydrate is limited by the transport capacity of SGLT1.

Intestinal fructose uptake is not regulated by the same transport system, as it largely depends on GLUT5 as opposed to SGLT1 transporters. Combining the intake of glucose plus fructose can further increase total exogenous carbohydrate availability and, as such, allow higher exogenous carbohydrate oxidation rates (Fuchs CJ, Gonzalez JT, van Loon LJC).

Ingesting a mixture of both glucose and fructose can improve endurance exercise performance compared to equivalent amounts of glucose (polymers) only. Fructose co-ingestion can also accelerate post-exercise (liver) glycogen repletion rates, which may be relevant when rapid (<24 h) recovery is required

For example, you may have been limited at 60g carbohydrate per hour with just glucose. But do a mix of glucose and fructose and you may see that you can handle 90g carbs per hour without any issues.

Different studies show slightly different things, but it has previously been accepted to do a 2:1 ratio of glucose to fructose (maltodextrin to fructose). However, new research has shown that a 1:0.8 ratio of maltodextrin allows for even more carbohydrate absorption than the original 2:1.

Refueling from the workout

Don’t stop consuming carbs in the last hour

Now you know just how important it is to train the gut and how to do it. Making sure your gut can handle a high amount of carbs per hour, and then following through with consuming them, is extremely important. It is not only important for that ride, but for the next ride. In a stage race, you may have heard a coach say that you are “eating for tomorrow”. We say this same thing to our athletes, whether racing or training.

Just because you get to the last hour of a ride and you feel good does NOT mean you should stop eating. You don’t want to completely deplete your glycogen stores because that is difficult to recover from, and exactly what you don’t want when you have to do another hard session (or race) tomorrow.

How many carbs to replenish glycogen stores?

You also want to make sure that you are consuming enough carbohydrates post-exercise, when the body is primed and ready to absorb those carbs and shuttle it to the muscles (and liver) to replenish glycogen stores and begin the recovery process.

The generally accepted protocol for replenishing carbs intake is to consume 1.2g/kg bodyweight every hour for the first four hours after training (so for a 75kg athlete this would be 90g carbs per hour). After these four hours, then the athlete would want to resume normal carb intake based on their needs for the day and what is coming the next day.

Some people, however, might have trouble consuming such large amounts of carbs. Now, research has shown that you can co-ingest protein along with those carbs in the amount of 0.2-0.4g/kg/hr in protein along with 0.8g/kg/hr in carbs.

However, we know most of us reading this have lives, jobs, families, and things that may not allow them to eat every hour for four hours after exercise. The big thing to ensure glycogen stores are replenished is to consume enough carbohydrates in the 24 hours after exercise before the next bout of exercise, “In the context of recovery from exhaustive exercise, it is known that ingesting 6–12 g carbohydrate·kg−1 is sufficient to restore endogenous glycogen reserves when the recovery time is ≥24 h” [Alghannam AF, Gonzalez JT, Betts JA.]

Key takeaway

It really doesn’t matter how perfect your amounts of carbs directly post-exercise. Of course, you want to prioritize them after. However, the important thing is that you simply consume enough of them in the hours after your session and before the next session.

You don’t want to overcomplicate it, and sure, it might be slightly better to exactly follow the protocols above, but they certainly aren’t required to still recover and train well the next day.

Practical recommendations and takeaways

Training the gut is something that takes time and must be carefully considered before race day. Slowly increasing total grams of carbs per hour over time will allow the gut to adapt to the increase so that you can utilize a higher amount of carbs without GI distress.

The reasons that gut training helps are that it...

• Improves gastric emptying

• Improves intestinal absorption

• Improves stomach comfort

• Reduces GI problems

Although it is generally believed that gastric emptying is not a limiting factor, it is likely that when a combination of factors (for example heat, high carbohydrate intake and high intensity exercise) all act together, gastric emptying may be compromised. Therefore, it is important to practice a race nutritional strategy in training and get used to higher volumes of intake or higher carbohydrate intakes.

Since the gut is highly adaptable, it seems wise to regularly (at least once a week) ingest carbohydrate during exercise and mimic a feeding strategy that would be used in competition. The gut will be “trained” to absorb more carbohydrate, resulting in higher rates of exogenous carbohydrate oxidation, less GI distress and most likely better performance.

Utilizing dual carb sources (glucose and fructose) can allow you to absorb more carbohydrates per hour than just glucose alone due to the transporters for each type of carb (SGLT1 and GLUT5). Transporters for glucose (SGLT1) get saturated at more than 60g carbs per hour, so by adding fructose which has a different transporter (GLUT5), carbohydrate intake can be increased.

You never want to stop eating towards the end of the ride because you are “saving calories and almost home”. On the bike is never the time to cut calories, and by not eating in the last hour, you can impair recovery and your workout the next day.

Once the ride is over, especially if you are doing a race or key session the next day, you want to make sure you refuel with sufficient carbohydrates (and protein) to recover and replenish glycogen stores.


Alghannam AF, Gonzalez JT, Betts JA. Restoration of Muscle Glycogen and Functional Capacity: Role of Post-Exercise Carbohydrate and Protein Co-Ingestion. Nutrients. 2018 Feb 23;10(2):253. doi: 10.3390/nu10020253. PMID: 29473893; PMCID: PMC5852829.

Cox GR, Clark SA, Amanda J. Cox AJ, Halson SL, Hargreaves M, Hawley JA, Jeacocke N, Snow RJ, Yeo WK, Burke LM. Daily training with high carbohydrate availability increases exogenous carbohydrate oxidation during endurance cycling. Journal of Applied Physiology Published 1 July 2010 Vol. 109 no. 1, 126-134 DOI: 10.1152/japplphysiol.00950.2009

Fuchs CJ, Gonzalez JT, van Loon LJC. Fructose co-ingestion to increase carbohydrate availability in athletes. J Physiol. 2019 Jul;597(14):3549-3560. doi: 10.1113/JP277116. Epub 2019 Jul 2. PMID: 31166604; PMCID: PMC6852172.

Jeukendrup AE. Training the gut for athletes. Sports Medicine.

O'Brien WJ, Stannard SR, Clarke JA, Rowlands DS. Fructose-maltodextrin ratio governs exogenous and other CHO oxidation and performance. Med Sci Sports Exerc. 2013 Sep;45(9):1814-24. doi: 10.1249/MSS.0b013e31828e12d4. PMID: 23949097.

Vandenbogaerde TJ, Hopkins WG. Effects of acute carbohydrate supplementation on endurance performance: a meta-analysis. Sports Med. 2011 Sep 1;41(9):773-92. doi: 10.2165/11590520-000000000-00000. PMID: 21846165.