In the world of endurance sports (marathon, triathlon, ultra-trail, cycling), an uncomfortable truth circulates among the pack: it is 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 athletes, the question "What nutrition is right for a sensitive stomach?" is not a simple dietary query; it is a search for a solution to save months of training. The answer does not lie in a magic formula, but in a detailed understanding of physiology and the choice of products that are genuinely suitable.
This guide aims to become your go-to reference. We will break down the mechanisms behind digestive intolerance, analyze why industrial sports nutrition often fails, and demonstrate why Mulebar’s natural approach is the most comprehensive solution for athletes with sensitive stomachs.
Why does running cause stomach pain?
To solve the problem, you need to understand why a healthy digestive system at rest becomes dysfunctional during exercise. It is a physiological “storm.”
Ischemia: reduced blood flow to the stomach and intestines
As soon as you start running, your body must oxygenate your muscles and cool your skin. To do this, it diverts blood away from organs that are “nonessential” at that moment: the stomach and intestines. Intestinal blood flow can drop by 80%.
As a result, digestion stops. Food remains in the stomach (sloshing, urge to vomit) or is no longer absorbed in the intestines (diarrhea). This discomfort generally appears in amateur runners around the 30th kilometer and is known as the "marathon wall"
“Leaky Gut” (the leaky intestine)
Under the effects of oxygen deprivation, the intestinal wall becomes permeable. It allows toxins to pass into the bloodstream, triggering nausea and inflammation.
The mechanical impacts specific to running
With every stride, your internal organs are subjected to a shock wave. This constant “shaker” disrupts digestion and encourages gastroesophageal reflux disease (GERD), especially if you have consumed liquids that are too acidic or too concentrated.
The “false friends” of standard sports nutrition
Why do you experience digestive issues with certain energy gels? Well, it is often due to ingredients that are unsuitable for a stomach under severe strain.
Excess fructose
Many brands use inexpensive glucose-fructose syrups. Fructose uses a specific transporter (GLUT5) that quickly becomes saturated. Unabsorbed fructose then ferments in the colon, creating gas, bloating, and pain.
Too much fiber consumed before exercise
limiting fiber before exercise is largely based on the work of Rehrer, linking dietary fiber to intestinal cramps; the delayed gastric emptying associated with viscous fiber also plays a role, exacerbated by high-intensity exercise.
Ultra-processed corn maltodextrin
Although popular, corn maltodextrin has an explosive glycemic index (>100), causing insulin spikes followed by reactive hypoglycemia (hunger pangs + nausea).
Colorants, flavoring agents, and chemical preservatives
Check your labels. Do you see Sodium Benzoate (E211) or Potassium Sorbate (E202)? These are antibacterial agents. On an intestinal lining weakened by exercise, they act as powerful irritants.
As a general rule, as with your everyday diet, avoid ultra-processed products containing “questionable” ingredients
solutions for digestive problems during competition
The FODMAP diet
FODMAP is the acronym describing a category of carbohydrates that are difficult for some people to digest: Fermentable – Oligosaccharides – Disaccharides – Monosaccharides – And – Polyols
Initially designed to reduce digestive problems, this diet is increasingly used by endurance athletes to avoid cramps, bloating, and intestinal discomfort during exercise. It consists of temporarily limiting certain fermentable carbohydrates that are difficult to digest, particularly during the 24 to 48 hours before a race. This approach improves digestive tolerance when the digestive system is most vulnerable, as blood flow is directed toward the muscles rather than the intestines.
By choosing “low FODMAP” foods and products that are simple and natural, free from polyols and complex sugars, athletes considerably reduce the risk of gastrointestinal problems and optimize their comfort during a race. It is a targeted, effective strategy that is increasingly being adopted in endurance sports.
"Stomach training": Gut Training
One of the most important principles in sports nutrition is simple: never test a drink, gel, or nutrition strategy on competition day. That is the whole purpose of Gut Training: training your digestive system the way you train your muscles. During exercise, digestion is weakened 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 texture (gel, compote, drink), and suitable osmolarity. Gut Training therefore consists of reproducing the same intake during training as in competition, so that the intestine becomes accustomed to absorbing energy efficiently without causing digestive problems. This is the golden rule: anything that has not been tested under real conditions should never be used on the big day.
Rice Syrup vs Maltodextrin: Which sugar should you choose for digestion?
Mulebar went against the sports nutrition industry (historically manufactured by pharmaceutical laboratories) by betting on naturalness and digestibility as early as 2008. Here is why it has become the number 1 choice for sensitive stomachs.
Instead of maltodextrin, Mulebar uses brown rice syrup. It is made up of glucose (fast), maltose (medium), and maltotriose (slow). It does not use the fructose transporter, thus avoiding fermentation. It is the safest fuel for sensitive intestines.
100% natural & preservative-free
The recipes are simple, with as few ingredients as possible. No E211, no E202. Just fruit, seeds, rice syrup, and natural flavors. No chemical irritant therefore comes to aggravate your permeable intestinal lining.
The ultimate weapon for 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 (non-resealable sachets or tubes). This creates a severe osmotic shock in the stomach.
The Mulebar solution: The resealable cap lets you take a small sip every 2-3 km with a little water each time. You smooth out your sugar intake, never overload your stomach, and avoid nausea.
Why don't other brands offer this type of packaging? Quite simply because it is very expensive compared with an extremely inexpensive soft pouch.
Mulebar is compatible with a "low-FODMAP" approach
Which Mulebar product should I choose for my stomach?
For the ultra-sensitive: energy compotes
Mulebar energy compotes are the "game changer". A smooth fruit purée that is fruitier and easier to digest than a gel, although still a little 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. Its baby-food texture soothes the stomach when nothing else goes down.
For performance: energy gels
The Mulebar energy gels all contain fruit, caramel, or coffee concentrate, brown rice syrup, and Himalayan pink salt. It is the salt richest in trace elements in the world. Sodium helps water absorption for better rehydration.
The cherry and apple gels are antioxidants, so they are particularly suitable for the start of the race. The salted caramel gel, a real treat, will delight your taste buds midway through the race, while the lemon and coffee gels, with 50 mg and 100 mg of natural caffeine (guarana), respectively, will provide the final kick to get you to the finish line.
It is worth noting that, although this has nothing to do with the digestibility of Mulebar gels, they are the only ones with rounded edges to prevent them from piercing technical fabrics, and their tubes are reusable. Once rinsed with hot water, you can refill them with their eco-refills. An environmentally friendly and highly economical choice. A 12-gel eco-refill costs only as much as 8 gels, meaning 33% savings.
You can also fill small washable flasks (60 ml, equivalent to 2 gels, or 150 ml, equivalent to 6 gels) and make your own mixtures in them, such as half gel and half water for an ultra-liquid hydrogel-type mixture. This works better for those who are sensitive to excess sugar.
For the start of the race: energy bars
The range of 8 Mulebar energy bars contains only 100% natural recipes, 6 of them vegan and organic. Designed to support athletes during their efforts, whether in training or competition. Rich in slowly released carbohydrates and essential nutrients, they provide lasting energy, without spikes or slumps, to maintain your pace without weighing down your stomach. 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 may seem counterintuitive given the image of acidity associated with lemon. Ginger is clinically recognized as an anti-nausea remedy. Ideal to consume at the beginning of an event or just before the start to prevent stomach problems.
Hydration while running: which drink should you choose?
This is a question that comes up very frequently among the questions asked on the Mulebar chat. Here is some information to make things clearer.
To be properly absorbed during exercise, a drink must have an osmolality close to that of blood: it is then called an isotonic drink. This equivalence allows water and electrolytes to pass quickly into the body without disturbing digestive balance.
Conversely, a hypertonic drink (too concentrated in sugars or minerals) draws water into the stomach through osmosis: this is osmotic shock. The result: bloating, nausea, heaviness, slowed digestion… and reduced performance. This is why overly sugary drinks, fruit juices and some sodas are not recommended during exercise. A true isotonic drink contains a controlled concentration of carbohydrates (generally 6 to 8%), sodium and potassium, and is digested very quickly.
Mulebar’s endurance 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 quickly absorbed carbohydrates to maintain energy. Thanks to their natural and well-balanced formulation, they help prevent cramps, dehydration and fatigue, while promoting easy digestion. Whether for trail running, cycling, running or endurance sports, these drinks optimize endurance, recovery and comfort. A real asset for staying at your best from start to finish.
For more information, see the guides:
For racing (trail running, cycling) or after the race: protein bars
After exercise, your body deserves effective recovery. The Mulebar protein bars are designed for this. Rich in high-quality plant proteins, good carbohydrates, and essential nutrients, they support muscle repair, replenish your glycogen stores, and promote fast, lasting recovery. Thanks to their digestible, natural formula, they limit discomfort after exercise and provide the balance needed between energy and recovery.
"Zero Gastric Issues" Protocols
Marathon Protocol (3h30 - 4h30 target)
Before the start: 45 minutes beforehand, eat an energy bar for comfort and stress relief, and because it will take a good twenty minutes to digest and provide energy for the first few kilometers. 10 minutes beforehand, take a small sip of Banana compote to line your stomach.
Km 5 to 30: One gel every 45 min. Take it in 3 or 4 portions (using the cap) and drink plain water immediately afterward.
Km 30+ (The Wall): Switch to caffeinated lemon or coffee gels. If your stomach gets blocked, dilute the gel in your bottle!
Ultra-Trail Protocol
0h - 4h: Eat solid foods, including lemon-ginger bars. Chew well and drink regularly.
4h - 10h: Alternate compotes and gels with hydration
Night / End of the race: Liquids 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.
Stop letting your stomach decide your result. By switching to natural, preservative-free nutrition, you eliminate the main causes of digestive issues. Ready to try? Discover the mulebir.compote Discovery Pack and rediscover the pleasure of running pain-free.
This article is not medical advice. In case of chronic conditions, consult a sports physician.
