In the world of endurance sports (marathons, triathlons, ultratrails, cycling), an uncomfortable truth circulates through the pack: it is not always the legs that give out first, but the stomach. The statistics are telling: between 30% and 50% of distance runners experience gastrointestinal (GI) problems during competition.
For athletes, the question “What nutrition is best for a sensitive stomach?” is not simply a dietary request; it is a search for a solution to save months of training. The answer does not lie in a magic formula, but in a nuanced 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 commercial sports nutrition often fails, and demonstrate why Mulebar’s natural approach is the most advanced solution for athletes with sensitive stomachs.
Why does running make your stomach hurt?
To solve the problem, you need to understand why a healthy digestive system at rest becomes impaired 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 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 stagnates in the stomach (sloshing, the 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)
When oxygen is lacking, the intestinal wall becomes permeable. It allows toxins to pass into the bloodstream, triggering nausea and inflammation.
The mechanical shocks specific to running
With every stride, your internal organs are subjected to a shock wave. This constant “shaking” disrupts digestion and promotes 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 problems with certain energy gels? Well, it is often because of ingredients that are unsuitable for a stomach pushed to its limits.
Excess fructose
Many brands use inexpensive glucose-fructose syrups. Fructose uses a specific transporter (GLUT5) that becomes saturated quickly. Unabsorbed fructose then ferments in the colon, creating gas, bloating, and pain.
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, just 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
Originally designed to reduce digestive problems, this diet is increasingly used by endurance athletes to prevent cramps, bloating, and intestinal discomfort during exercise. It involves temporarily limiting certain hard-to-digest, fermentable carbohydrates, particularly during the 24 to 48 hours before a race. This approach improves digestive tolerance when the digestive system is most vulnerable, because blood flow is directed toward the muscles rather than the intestines.
By choosing simple, natural “low-FODMAP” foods and products free from polyols and complex sugars, athletes significantly reduce the risk of gastrointestinal problems and optimize their comfort while running. 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: you should never test a drink, gel, or nutrition strategy on competition day. That is the whole purpose of Gut Training: training your digestive system just as 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 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 a golden rule: anything that has not been tested under real-world conditions should never be used on race day.
Rice Syrup vs. Maltodextrin: Which Sugar Should You Choose for Digestion?
Mulebar took the opposite approach to the sports nutrition industry (historically manufactured by pharmaceutical laboratories), betting on naturalness and digestibility from 2008 onward. Here is why it has become the number-one choice for sensitive stomachs.
Instead of maltodextrin, Mulebar uses brown rice syrup. It consists 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 irritants can therefore attack your permeable intestinal lining.
The ultimate weapon for energy gels and compote: 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 30 g of sugar all at once (in 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 feeling sick.
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 slightly thicker, and much easier to swallow than a bar. Chewing while running is very difficult.
The star: organic banana compote. Banana is a natural antacid rich in potassium. Its “baby food” texture soothes the stomach when nothing else goes down.
For performance: Energy gels
Mulebar energy gels all contain fruit concentrate, caramel or coffee, brown rice syrup, and Himalayan pink salt. It is the salt richest in trace minerals in the world. Sodium helps the absorption of water for better rehydration.
The cherry and apple gels are antioxidants, making them 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.
Note that this has nothing to do with the digestibility of Mulebar gels, which are the only ones with rounded edges to prevent them from piercing technical fabrics, and whose tubes are reusable. Once rinsed with hot water, you can refill them with their eco-refills. An ecological and very economical choice. A 12-gel eco-refill costs only as much as 8 gels, saving 33%.
You can also fill small washable flasks (60 ml, or 2 gels, or 150 ml, or 6 gels) and make your own mixtures, such as half gel and half water for an ultra-liquid hydrogel-style 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 includes only 100% natural recipes, 6 of which are vegan and organic. Designed to support athletes during their efforts, whether in training or competition, they are rich in slowly released carbohydrates and essential nutrients, providing lasting energy without spikes or crashes, so you can 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 sensitive athletes, even though this may seem counterintuitive given the acidity associated with lemon. Ginger is clinically recognized as an anti-nausea remedy. Ideal to consume at the start of an event or just before setting off to prevent gastric problems.
Hydration during a race: 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: this is known as an isotonic drink. This equivalence allows water and electrolytes to pass quickly into the body without disrupting 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. That 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 sports and hydration drinks are designed to support your performance, especially during prolonged or intense efforts. They help you rehydrate effectively, replace electrolytes lost through sweat, and provide quickly absorbed carbohydrates to maintain energy. Thanks to their natural, well-balanced formulation, they help prevent cramps, dehydration and fatigue, while ensuring 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 post-race: protein bars
After exercise, your body deserves effective recovery. 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 Problems” Protocols
Marathon Protocol (3:30–4:30 Target)
Before the start: 45 minutes beforehand, have an energy bar for comfort and stress relief, as 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 coat your stomach.
Km 5 to 30: One gel every 45 minutes. 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 struggles, 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 500 ml flask of water. This is your isotonic “magic potion” that requires no digestive effort.
Stop letting your stomach determine your result. By switching to natural, preservative-free nutrition, you eliminate the main causes of digestive problems. Ready to try? Discover the Mulebar Compote Discovery Pack and rediscover the pleasure of running without pain.
This article is not medical advice. In case of chronic problems, consult a sports physician.
