Carbohydrate intake: Critical for performance. Especially during long training sessions and races.

But increasing the body’s capacity to ingest higher amounts of carbohydrates is not a quick adjustment. Meeting your body's metabolic needs (and doing so efficiently) for going faster or longer or faster longer requires intentional practice and time. Our capacity to intake, digest, absorb, and transport high amounts of carbohydrates has its limits... if you've experienced bloating, stomach pains, or worse then you know what I'm talking about.

Yet we're always striving to increase the boundary of what our body can tolerate. Because pushing boundaries: It’s what we do.

If you’ve read our articles on Ronan McLachlan or heard Jan Frodeno mention his 120g/hr carbohydrate intake, you might think: “That amount of intake makes me feel sick just thinking about it.”

It’s not an uncommon or surprising reaction. But, know this: In all cases of taking more than even 60-90g/hr of carbohydrates, you'll the need to test it in training before race day. Or, more likely, test it over a period of training so that the gut gets used to handling this amount of intake, aka 'training the gut.'

Why Do I Need To Train My Gut?

No one just takes in that much fuel out of the blue. Our digestive and absorption systems are used to normal eating habits. Trying to intake increasingly large amounts of carbohydrates in a relatively short time window can be a shock to your body.

During exercise, the maximum rate of glucose absorption by the small intestine ranges between 50 and 90g/h on average. The rate of exogenous glucose oxidation is mainly limited by your rate of digestion, absorption, maximal hepatic glucose output, and transport of the glucose in circulation rather than by glucose uptake and oxidation in the muscle. That's why you should focus on increasing your body’s absorption capacity.

Interestingly, intense aerobic exercise derives, on average, 80% of its energy requirement from carbohydrate breakdown. That means about 4g of carbohydrates are metabolized per minute (1g of carbohydrate = 4 kcal). This energy demand equals to a carbohydrate requirement of roughly 240g/h, which cannot be matched by its ingestion rate during intense endurance events.

In addition, once in the muscle cells, glucose breakdown provides only 30-35% of its energy to sustain output. The rest is dissipated in other ways, such as heat. These facts highlight the important implications for practicing your carbohydrate feeding strategy and endorse the growing body of evidence that support that you can the increase the body’s capacity to absorb exogenous glucose.

What Limits Our Carbohydrate Absorption Capacity?

Gastric emptying is the first important step towards delivering exogenous carbohydrates and fluids to the working muscle.

For this reason, it is not only essential that higher carbohydrate intake can be digested and tolerated, but also able to be emptied from the stomach into the intestines for absorption.

Although it is generally believed that gastric emptying is not a limiting factor for fuel efficiency, it is likely a combination of factors that include gastric emptying that limit capacity. For example, heat, high carbohydrate intake, and high-intensity exercise, which are all factors known to impact gastric emptying will act together, thereby compromising gastric emptying. Therefore, it is important to practice nutritional strategies that aim to improve digestion in training and get your body used to higher volumes of carbohydrate intake, especially if your race-day nutrition plan dictates this!

Once emptied from the stomach, most sugar absorption will take place in the small intestine where the main absorption barriers exist. Exercise studies have provided indirect but strong evidence that the delivery of carbohydrate is limited by the transport capacity of some glucose transporters in the intestine (specifically sodium-dependent glucose transporter (SGLT1)). This capacity becomes a limiting factor when dietary carbohydrate exceeds a certain level.

One of the main limitations to increasing the amount of carbohydrates we ingest is the occurrence of gastrointestinal (GI) complaints. For example nausea or GI distress.

Bolstering the case for gut training is the relatively common occurrence of such issues as events that go beyond two hours or so. These can be relatively debilitating and can derail your race pretty quickly.

GI problems are very common amongst athletes: 30–50% of all athletes experience such problems regularly. The root causes are still largely unknown but appear to be partly genetically determined and highly individual. The mechanisms are likely different for upper and lower GI problems. The symptoms are more likely to occur in high heat environments and are exacerbated by hot weather conditions and dehydration.

Although a direct link to nutrition intake is not always identified, certain practices have been found to correlate with the incidence of GI problems: fiber intake, fat intake, and highly concentrated carbohydrate solutions seem to increase the prevalence of GI problems. On the other hand, there are also interesting strategies to train your GI system to maximize and better tolerate nutrient intake.

Kristian Blummenfelt training his gut to absorb more carbohydrates

So, What Is Training The Gut?

Training the gut is a progressive stress on the gastrointestinal system to improve tolerance of intake and increase the rate of intake possibilities. It is not a new concept, and is based on the fact that the gut is highly adaptable and can therefore be ‘trained’. (Note, gut training should always be undertaken in the context of the broader training plan.)

More specifically, training the gut is a practice by which an athlete trains their adaptability to digest food and absorb nutrients in a setting that is closely related to the event they are training to perform best at. By this, the athlete prepares their gut to deal with more and more easily with food ingestion during the event, before and during training sessions, and has less or zero negative side effects by being more adapted to digest high amounts of carbohydrates.

Gut training  can take many forms with different goals. Some of these focus on minimizing gastrointestinal discomfort, and some on increasing nutrient absorption. These are not necessarily mutually exclusive either. It may be eating before training when you usually wouldn't, ingesting more than the normal  amount of carbohydrates, fluid, or a combination of the two, or increasing habitual carbohydrate intake.

At present, the link between daily carbohydrate intake and the transport capacity for glucose in the human intestine is uncertain. Although the exact magnitude of effects in athletes who are already consuming a high-carbohydrate diet may be uncertain, it seems fair to conclude that those athletes who are not practicing a high carbohydrate diet can benefit substantially when increasing average carbohydrates consumption. Especially when athletes are carbohydrate restricting. These athletes would be advised to include some high carbohydrate days in their training.

Do I Need To Train My Gut?

The easy answer: Anyone who is looking to increase their intake beyond 60g/hr of carbohydrates should train their guy.

Additionally, those who have previously struggled with gastrointestinal issues or are traveling to a warmer climate to compete should train their gut, as this adds gastrointestinal stress. Remember, if you decide to take more than 60g/hr of carbohydrates, you need to consider the source of the fuel – it should be a mix of glucose and fructose, not just a single sugar source. This is because the transporters (those SGLT1s we mentioned before) for glucose become oversaturated. But since fructose is absorbed via different transporters (GLUT5), you can avoid some of those issues by fueling with a mixture.

How Can I Train My Gut?

There are many ways that you can decide to train the gut, some of which depend on your reasoning for doing so. For example, if you’re concerned about climatic conditions and are training the gut to avoid issues due to heat stress, it seems prudent to use heat stress for at least a portion of your gut training protocol. On the contrary, if your aim is to train the stomach to increase gastric emptying, it is reasonable to progressively augment the amount of food you normally intake. Self-experimentation is key. Start with what you can easily tolerate and increase it step by step by small portions.

Gut Training Protocols for Specific Situations

Challenge: General, non-specific GI upset in races

Potential Training Methodology:

  • Easy training close to a meal once per week
  • Training with more fluid intake than is required once a month for non-key but longer training sessions
  • Race day nutrition plan for race simulation training

Challenge: Wanting to increase carbohydrate intake in race

Potential Training Methodology:

  • Increase habitual carbohydrate intake
  • Race day nutrition plan for race simulation training and key workouts

Challenge: Preparation for a hot race

Potential Training Methodology:

  • Easy training close to a meal once per week
  • Increase habitual carbohydrate intake
  • Training with more fluid intake than is required once a month for non-key but longer training sessions
  • Race day nutrition plan for race simulation training and key workouts

Gut training methods and benefits
From Jeukendrup in 2017: A summary of methods to “train the gut," the adaptations that may occur in the gut, and implications for performance.

When Should I Train The Gut?

Of course it may depend on the specific method you choose, but you can definitely incorporate "training your gut" throughout all parts of a training cycle.

Alternatively, you could focus on a specific block of training leading into a race. For example, starting to mimic race day fueling about 3 months before competition. One caveat with this method is that this time period is critical for your training. Make sure you don't ramping up fuel intake too much too fast so that it impacts your training goals.

Ensuring your nutrition strategy is dialed in so race day doesn't get derailed by gastrointestinal issues is key. When planning your next training block, make sure you consider how you will incorporate some gut training so you can fuel for your performance goals.

References

  1. Burke, Louise. 2021. Clinical Sports Nutrition. https://nla.gov.au/nla.obj-2988918349.
  2. McArdle, William D., Frank I. Katch, and Victor L. Katch. 2001. Exercise physiology: energy, nutrition, and human performance. Philadelphia: Lippincott Williams & Wilkins.
  3. 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. doi: 10.1007/s40279-014-0153-2. PMID: 24791919; PMCID: PMC4008808. https://pubmed.ncbi.nlm.nih.gov/24791919/
  4. Hoogervorst D, van der Burg N, Versteegen JJ, Lambrechtse KJ, Redegeld MI, Cornelissen LAJ, Wardenaar FC. Gastrointestinal Complaints and Correlations with Self-Reported Macronutrient Intake in Independent Groups of (Ultra)Marathon Runners Competing at Different Distances. Sports (Basel). 2019 Jun 7;7(6):140. doi: 10.3390/sports7060140. PMID: 31181655; PMCID: PMC6628076. https://pubmed.ncbi.nlm.nih.gov/31181655/
  5. Murray R. Training the gut for competition. Curr Sports Med Rep. 2006 May;5(3):161-4. doi: 10.1097/01.csmr.0000306307.10697.77. PMID: 16640953. https://pubmed.ncbi.nlm.nih.gov/16640953/
  6. Jeukendrup AE. Training the Gut for Athletes. Sports Med. 2017 Mar;47(Suppl 1):101-110. doi: 10.1007/s40279-017-0690-6. PMID: 28332114; PMCID: PMC5371619. https://pubmed.ncbi.nlm.nih.gov/28332114/
  7. Jeukendrup A. A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Med. 2014 May;44 Suppl 1(Suppl 1):S25-33. doi: 10.1007/s40279-014-0148-z. PMID: 24791914; PMCID: PMC4008807. https://pubmed.ncbi.nlm.nih.gov/24791914/