The Marathoner's Delicate Gut

Some runners have cast-iron digestive systems that can handle any food and any kind of physical activity without a wobble. Others, and there are plenty of them, have sensitive stomachs and intestines that can make their lives hell, especially during long efforts. Are digestive problems simply part and parcel of distance running?

Nausea, flatulence, burping, vomiting, diarrhoea… the list of gastrointestinal symptoms is as unpleasant and endless as a marathon run with a stomach ache. Runners who have never experienced the slightest digestive discomfort during exercise can count themselves lucky, because the vast majority of distance runners regularly suffer the whims of their guts. Months of preparation can be wiped out on race day by “simple” plumbing problems. Why is the digestive system so sensitive to physical exertion, particularly when it lasts for several hours? How can gastrointestinal symptoms be prevented and managed?

“I can’t digest, I’m running”

During physical exertion, the body sets priorities. It cannot afford to send energy to secondary tasks when the muscles need to keep working above all else! Blood flow is therefore directed first towards the muscles, at the expense of the stomach and intestines. According to Guillaume Millet, a researcher at the University of Saint-Etienne, gastrointestinal blood flow is reduced by around 80% during exercise at 70% of VO2max. With such a restricted supply, it is easy to see why the digestive system may run into trouble!

Running itself also involves factors that can affect digestion. Repeated impacts with the ground, especially on a hard surface such as asphalt, cause vibrations in the abdominal wall and movement of the organs. This shockwave affects the digestive system and can even cause bleeding, something many marathon runners know all too well. Put under strain by exercise, the gastrointestinal system reduces the digestive tract’s absorptive capacity and increases intestinal permeability, allowing bacteria to cross the intestinal wall and enter the bloodstream. Toxic substances known as endotoxins are then released, encouraging the appearance of certain inflammatory markers, diarrhoea and nausea.

Bowel function is also influenced by stress, weather conditions, and the use of anti-inflammatories, aspirin or even coffee. Failing to allow enough time for the stomach to empty, or eating an unsuitable meal before the start of a race, also increases the risk of digestive discomfort. Meals that are too high in fat, too acidic or too high in fibre are particularly problematic. Once the first symptoms appear, the ability to drink and eat during exercise drops sharply or may even disappear altogether. Intestinal distress worsens, and a vicious circle begins. An unwanted DNF may not be far away!

Better safe than sorry… or sick

Preventing digestive problems starts several weeks before race day. For example, supplementing with prebiotics, glutamine, vitamin E and omega-3s for several months before race day may be beneficial. Paying close attention to acid-base balance may also improve digestive tolerance during exercise, as can getting the body used to taking in food while exercising. It is essential to practise fuelling during training with both fluids and solids, so you can observe how your body reacts, identify which products you tolerate and enjoy, and avoid worsening intestinal permeability.

In the few days before the race, the aim is to:

Store enough fuel, in other words glycogen, so that your body can cope with the physical effort on race day
Avoid any digestive irritation to maximise gastrointestinal comfort during the race.

✓ How can you achieve both objectives?

Four to two days before the race

Eat starchy foods at breakfast, lunch and dinner.
Add a mid-morning or mid-afternoon snack to properly top up glycogen stores: fresh or dried fruit, dairy or plant-based milk, bread or cereal, jam or honey.
In general, choose cooked vegetables and fruit, which are easier to digest, and limit gluten intake.
Drink sparkling water (Quézac, Rozana, St Yorre) or still water rich in minerals (1 to 1.5 litres per day).

The day before and race day

Eat starchy foods at breakfast, lunch and dinner, without overdoing it.
A morning or afternoon snack is optional: keep it in if there is a long gap between meals, to avoid drawing on glycogen stores. Choose a banana that is not too ripe, or even slightly green, stewed fruit, fermented dairy such as yoghurt or fromage blanc, rice pudding, gluten-free biscuits…
Avoid raw vegetables, raw fruit, fruit juice, especially citrus, and foods that are difficult to digest, such as cabbage, cheese and red meat.
Drink sparkling water (Quézac, Rozana, St Yorre) or still water rich in minerals (1 to 1.5 litres per day).

During the race, it is crucial to watch your sugar intake. Most gastric problems and episodes of vomiting during exercise are linked to sugar! Remember that sugar is a secondary energy source during long efforts, with fat metabolism quickly taking over, and that refined sugar strongly acidifies the body, including the digestive system. Choose unrefined sugars and low-glycaemic-index options such as agave syrup or acacia honey, and limit total intake, from both liquids and solids, to a maximum of 80 g of sugar per hour of running.

Hydration is also one of the keys to preventing digestive problems during exercise. Although the recommended intake is 500 to 800 ml per hour of running, and never more than a litre, each drink should be limited to 100 to 150 ml. Your drink should not be too cold or over-concentrated with sports mix. Hypertonic drinks such as Coca-Cola should be diluted, otherwise they may cause dumping syndrome, in which water is retained in the intestines and can trigger diarrhoea and an urgent need to defecate. Water should be supplemented with an isotonic drink mix and/or mineral salts, including sodium and potassium. At an outside temperature of 10°C, sodium requirements rise to 0.4 g per hour, equivalent to 1 g of table salt. If an energy bar containing 0.2 g of sodium is eaten during an hour of running, the water should contain an additional 0.2 g of sodium, or 0.5 g of salt, just a tiny pinch and no more!

Since stress can cause or worsen digestive problems, emotional management should be an integral part of training. Mental preparation, along with different relaxation methods such as sophrology and meditation, gives runners useful tools to avoid being overwhelmed by anxiety, a lack of confidence and other negative emotions that can have a damaging effect on the gastrointestinal system.

Exercise intensity is another factor that can contribute to digestive discomfort: starting at a moderate pace and keeping your effort relatively even, without pronounced surges, helps limit the risk. If nausea appears, reduce the intensity gradually to slowly restore blood flow to the gastrointestinal system. A sudden, prolonged stop causes a large rush of blood back to the stomach and encourages vomiting. It is therefore better to keep stops at aid stations short and eat while walking, chewing carefully because digestion starts in the mouth!

4 classic mistakes

✓ Mistake no. 1: sticking to the famous “pasta party”

Old habits die hard! Who does not see pasta as the athlete’s ultimate ally? Yet it is far from a miracle food. Packed with gluten, it can irritate the gut, and it turns into a fast-release sugar when overcooked. It is better to choose pulses such as coral lentils and dried beans, gluten-free grains such as quinoa and rice, or potatoes.

✓ Mistake no. 2: eating a meal immediately before the start

Do not wolf down a solid breakfast a few minutes before the starting gun! Depending on the individual and the meal eaten, the gap between the last food intake and the race should be two to three hours. Only certain specialised products, such as some sports cakes or breakfast creams, can be consumed shortly before the start.

✓ Mistake no. 3: eating too many fast sugars, fibres or acidic foods

It is best to leave foods that are difficult to digest or irritating to the stomach and intestines out of your pre-race meal, including raw vegetables, citrus fruit, meat and overly fatty dishes.

✓ Mistake no. 4: eating and drinking too much during the race

All excesses should be avoided! Keep food and fluid intake reasonable. Do not overload a body that is focused on physical exertion rather than digestion.

✓ Mistake no. 5: trying new things during the race

If you want unpleasant surprises during the race, go ahead and choose foods or drinks you have never tried in training! To reduce the risk of digestive discomfort, stick to tried-and-tested options that you know your body tolerates.

| Marathon calendar in France, here.