GI (gastrointestinal) issues are prevalent amongst runners and endurance athletes and can quickly turn a race upside down, ruining months of training.

There is no one reason for GI issues and the symptoms along with the causes vary between runners, which we discussed in a previous post Common GI Issues and Conditions Amongst Runners and Athletes.

However, there appears to be a number of ways to reduce the risk of GI issues when running or exercising. Not all of these suggestions will work for everyone, but hopefully, every sufferer can find one or two options from the list that will work for them.

The seven guidelines below are based on limited research, but anecdotally these guidelines should help you prevent GI issues on your next run.

1. Avoid high fibre foods

Avoid high fibre foods the day or even the days before a race or competition. For a runner or athlete in training, a diet with adequate fibre will help to keep the bowel regular. Fibre before race day is different. By definition, fibre is not digestible, so any that is eaten essentially passes through the intestinal tract. Increased bowel movements during exercise are not desirable, which can accelerate fluid loss and may result in unnecessary gas production, resulting in cramping.

In particular, runners who are prone to GI issues should adhere to a strict low fibre diet a for a few days before a race or competition. Choose processed white foods, like regular pasta, white rice, and plain bagels instead of whole grain bread, high fibre cereals, oats and brown rice. Check the food labels for fibre content. Most fruits and vegetables are high in fibre but there are a few exceptions: zucchini, tomatoes, olives, grapes, and grapefruit all have less than one gram of fibre per serving.

Check the food labels for fibre content. Most fruits and vegetables are high in fibre but there are a few exceptions: courgettes, tomatoes, olives, grapes, and grapefruit all have less than one gram of fibre per serving.

2. Avoid aspirin and non-steroidal anti-inflammatory drugs (NSAIDs)

Avoid aspirin and non-steroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen. Both aspirin and NSAIDs are commonly have been shown to increase intestinal permeability and may increase the incidence of GI complaints. The use of NSAIDs in the pre-race period should be discouraged.

3. Avoid milk products

Avoid milk products that contain lactose as even mild lactose intolerance can cause problems when exercising or running. You can even avoid milk completely with the array of milk free choices on offer today or get try lactose-free milk. Milk alternatives such as soy, rice, and almond milk generally don’t contain lactose.

4. Avoid dehydration

Since dehydration can exacerbate GI issues, it is important to avoid dehydration at all costs, especially when competing in an endurance event such as a marathon –  start the race well hydrated.

5. Practice new nutrition strategies

Make sure to experiment with your pre-race and race-day nutrition plan many times prior to race day. This will allow you to figure out what does and does not work for you, reducing the chances of GI issues ruining your race day.

6. Avoid fructose-only foods

Avoid high fructose foods (in particular drinks that have exclusively fructose). Fructose is not only found in fruit, but also in most processed sweets; candy, cookies etc., in the form of high fructose corn syrup. Some fruit juices are almost exclusively fructose. Fructose is absorbed by the intestines more slowly the tolerance of fructose is much less than glucose (may lead to cramping, loose stool and diarrhoea). Having said that, I have discussed before that fructose in combination with glucose may not cause problems and may even be better tolerated.

7. Train your gut

Training the gut is another practice. If your gut is adapted to the foods you consume during a race, you are less likely to get stomach problems. If you are avoiding carbohydrate in daily life, your intestines will respond by reducing intestinal transporter numbers so your ability to absorb carbohydrate is reduced.

On race day you may not be able to absorb all of the ingested and this may cause GI issues. The advice is therefore not to restrict carbohydrate intake and regularly consume carbohydrate during training.


About the author:
Asker Jeukendrup is a Professor of Exercise Biochemistry and Director of Mysportscience. He has published extensively on the topic of sports nutrition and acts as a consultant to many elite and not so elite athletes all over the world. He is also a keen runner and triathlete. You can contact Asker on twitter or