Digestive problems: how to avoid stomach aches while running?

In the world of endurance sports (marathons, triathlons, ultra-trails, cycling), an uncomfortable truth circulates among the runners: it's not always the legs that give out first, but the stomach. The statistics are telling: between 30% and 50% of long-distance runners experience gastrointestinal (GI) problems during competition.

For an athlete, the question "What nutrition is best for a sensitive stomach?" isn't simply a dietary query; it's a search for a solution to salvage months of training. The answer doesn't lie in a magic formula, but in a deep understanding of physiology and the selection of truly suitable products.

This guide aims to become your go-to reference. We will deconstruct the mechanisms of digestive intolerance, analyze why commercial sports nutrition often fails, and demonstrate why Mulebar's natural approach is the most effective solution for athletes with sensitive stomachs.

Why do we get stomach aches when running?

To solve the problem, we need to understand why a digestive system that is healthy at rest becomes dysfunctional under stress. It's a physiological "storm".

Ischemia: decreased blood flow to the stomach and intestines

As soon as you start running, your body needs to oxygenate your muscles and cool your skin. To do this, it diverts blood away from organs that are "non-essential" at that moment: the stomach and intestines. Intestinal blood flow can drop by 80%.

As a result, digestion stops. Food stagnates in the stomach (bloating, nausea) or is no longer absorbed in the intestine (diarrhea).

"Leaky Gut"

Due to a lack of oxygen, the intestinal wall becomes permeable. This allows toxins to pass into the bloodstream, triggering nausea and inflammation.

Mechanical shocks specific to running

With each stride, your internal organs experience a shockwave. This constant "shaker" disrupts digestion and promotes gastroesophageal reflux (GERD), especially if you have ingested liquids that are too acidic or too concentrated.

The "false friends" of standard sports nutrition

Why do you experience digestive problems with certain energy gels? Well, it's often due to ingredients that are unsuitable for a stomach that's been put under stress.

Excess fructose

Many brands use inexpensive glucose-fructose syrups. Fructose uses a specific transporter (GLUT5) that quickly becomes saturated. The unabsorbed fructose then ferments in the colon, causing gas, bloating, and pain.

Ultra-processed corn maltodextrin

Although popular, corn maltodextrin has an explosive glycemic index (>100) which causes insulin spikes followed by reactive hypoglycemia (hunger pangs + nausea).

Chemical colorings, flavorings, and preservatives

Check your labels. Do you see Sodium Benzoate (E211) or Potassium Sorbate (E202)? These are antibacterial agents. On an intestinal lining weakened by exertion, they act as powerful irritants.

As a general rule, as with your everyday diet, avoid ultra-processed products containing "questionable" ingredients.

Solutions for digestive disorders in competition

The FODMAP diet

FODMAP is the acronym that describes a category of carbohydrates that are difficult for some people to digest: Fermentable Oligosaccharides, Disaccharides, Monosaccharides, and Polyols.

Originally designed to reduce digestive issues, this diet is increasingly used by endurance athletes to prevent cramps, bloating, and intestinal discomfort during exercise. It involves temporarily restricting certain fermentable carbohydrates that are difficult to digest, particularly in the 24 to 48 hours leading up to a race. This approach improves digestive tolerance at a time when the digestive system is most vulnerable, as blood flow is directed to the muscles rather than the intestines.

By choosing simple, natural, low-FODMAP foods and products free of polyols and complex sugars, athletes significantly reduce the risk of gastrointestinal problems and optimize their comfort during races. This is a targeted, effective strategy that is increasingly adopted in endurance sports.

Gut Training

One of the most important principles in sports nutrition is simple: never test a drink, gel, or dietary strategy on the day of a competition. This is the whole point of Gut Training: to train your digestive system the same way you train your muscles. During exercise, digestion is compromised because blood flow is redirected to the muscles; introducing an untested food or drink increases the risk of cramps, diarrhea, nausea, or bloating. The body must gradually learn to tolerate a certain type of carbohydrate, a precise quantity, a specific texture (gel, compote, drink), and an appropriate osmolarity.

Gut Training therefore consists of replicating the same intake during training as in competition, so that the gut gets used to efficiently absorbing energy without causing digestive problems. It's a golden rule: anything that hasn't been tested in real-world conditions should never be used on race day.

Rice syrup vs. maltodextrin: which sugar should you choose for digestion?

Mulebar bucked the trend in the sports nutrition industry (historically manufactured by pharmaceutical companies) by focusing on natural ingredients and digestibility as early as 2008. Here's why it became the number one choice for those with sensitive stomachs.

Instead of maltodextrin, Mulebar uses brown rice syrup . It is composed of glucose (fast-acting), maltose (medium-acting), and maltotriose (slow-acting). It does not stimulate the fructose transporter, thus preventing fermentation. It is the safest fuel for irritable bowels.

100% natural & preservative-free

The recipes are simple, using as few ingredients as possible. No E211, no E202. Just fruit, seeds, rice syrup, and natural flavorings. No chemical irritants will be present to irritate your permeable intestinal lining.

The ultimate weapon against energy gels and compotes: The resealable cap!

This is a major ergonomic innovation for digestion.

The problem with other gels on the market is simple. You have to swallow 30g of sugar all at once (from non-resealable sachets or tubes). This creates a violent osmotic shock in the stomach.

The Mulebar solution: The resealable cap allows you to take a small sip every 2-3 km with a little water each time. This smooths out your sugar intake, prevents overloading your stomach, and avoids nausea.

Why don't other brands offer this type of packaging? Simply because it is very expensive compared to an extremely inexpensive flexible bag.

Mulebar is compatible with a "low FODMAP" approach

Which Mulebar product should I choose for my stomach?

For those with extreme sensitivity: energy-boosting fruit compotes

Mulebar energy compotes are a game changer. A smooth fruit puree, fruitier and more digestible than a gel (though a bit thicker), and much easier to swallow than a bar. Chewing while running is very difficult.

The star: organic banana compote. Bananas are a natural antacid rich in potassium. Their "baby food" texture soothes the stomach when nothing else seems to agree with it.

For performance: energy gels

Mulebar energy gels all contain fruit, caramel, or coffee concentrate, brown rice syrup, and Himalayan pink salt. This is the salt richest in trace elements in the world. Sodium helps with water absorption for better rehydration.

Cherry and apple gels are antioxidants, making them ideal for the start of a race. Salted caramel gel, a real treat, will delight your taste buds mid-race, while lemon and coffee gels, with 50mg and 100mg of natural caffeine (Guarana) respectively, will provide the final boost to reach the finish line.

It's worth noting, although unrelated to the digestibility of Mulebar gels, that they are the only ones with rounded edges to prevent punctures in technical fabrics, and whose tubes are reusable. Once rinsed with hot water, you can refill them with their eco-refills. An environmentally friendly and very economical choice. An eco-refill of 12 gels costs only the price of 8, representing a 33% saving.

You can also fill small, washable silicone vials (60ml, enough for 2 gels, or 150ml, enough for 6 gels) and make your own mixtures, such as half gel, half water for an ultra-liquid hydrogel-type blend. This is a better option for those concerned about excessive sugar.

For the start of the race: energy bars

The Mulebar range of 8 energy bars features only 100% natural recipes, with 6 of them being vegan and organic. Designed to support athletes in their efforts, whether training or competing, these bars are rich in slowly released carbohydrates and essential nutrients, providing sustained energy without spikes or crashes, to maintain performance without weighing you down. Thanks to their gentle digestion, they are perfect before, during, or after exercise.

The Lemon-Ginger bar is a favorite among athletes with sensitive stomachs, even though this might seem counterintuitive given the acidic image associated with lemon. Ginger is a clinically recognized anti-nausea agent. It's ideal to consume at the start of an event or just before the start to prevent stomach upsets.

Hydration during a race: which drink to choose?

This is a question that comes up very frequently in the Mulebar chat. Here's some information to help clarify things.

To be properly absorbed during exercise, a drink must have an osmolarity close to that of blood: this is called an isotonic drink. This equivalence allows for the rapid passage of water and electrolytes into the body without disrupting the digestive balance.

Conversely, a hypertonic drink (too concentrated in sugars or minerals) draws water towards the stomach through osmosis: this is osmotic shock. The result: bloating, nausea, heaviness, slowed digestion… and decreased performance. This is why overly sugary drinks, fruit juices, or certain sodas are not recommended during intense exercise. A true isotonic drink contains a controlled concentration of carbohydrates (generally 6 to 8%), sodium, potassium, and is digested very quickly.

Mulebar's sports and hydration drinks are designed to support your performance, especially during prolonged or intense efforts. They effectively rehydrate, replace electrolytes lost through sweat, and provide rapidly absorbed carbohydrates to maintain energy levels. Thanks to their natural and well-balanced formula, they help prevent cramps, dehydration, and fatigue, while also promoting easy digestion. Whether you're trail running, cycling, running, or participating in endurance sports, these drinks optimize endurance, recovery, and comfort. A true asset for maintaining peak performance from start to finish.

For more information, consult the guides:

For the race (trail running, cycling) or after the race: protein bars

After exercise, your body deserves effective recovery. Mulebar protein bars are designed for just that. Rich in high-quality plant-based protein, good carbohydrates, and essential nutrients, they support muscle repair, replenish your glycogen stores, and promote fast and lasting recovery. Thanks to their easily digestible and natural formula, they minimize post-workout discomfort and provide the necessary balance between energy and recovery.

"Zero Stomach Problems" Protocols

Marathon Protocol (Target time 3h30 - 4h30)

Before departure: 45 minutes beforehand, have an energy bar for stress relief and because it will take about twenty minutes to digest and provide energy for the first few kilometers. 10 minutes before, have a small sip of applesauce. Banana to line the stomach.
Km 5 to 30: One gel every 45 min. Take it in 3 or 4 doses (using the cap) and drink pure water immediately afterwards.
Km 30+ (The Wall): Switch to caffeinated gels Lemon or coffee. If your stomach is upset, dilute the gel in your water bottle!

Ultra-Trail Protocol

0h - 4h: Eat solid food such as lemon-ginger bars. Chew thoroughly and drink regularly.
4h - 10h: Alternating fruit purees and gels accompanied by hydration
Night/End of race: Liquid only. Dilute 2 Mulebar lemon or coffee gels in a 500ml flask of water. This is your isotonic "magic potion" that requires no digestive effort.

Don't let your stomach dictate your performance anymore. By switching to natural, preservative-free nutrition, you eliminate the main causes of digestive issues. Ready to try it? Discover the Mulebar Fruit Compote Discovery Pack and rediscover the joy of pain-free running.

This article is not medical advice. For chronic conditions, consult a sports medicine physician.