Post-exercise GI distress and dehydration

Over Thanksgiving weekend, my best friend ran a half marathon as training for his second marathon that he’ll be running in December. Even if it was just a training run and he had had a cold the week prior to the race, preventing him to run, James still finished in 4^th place with a time of 1:30:03! Not too shabby!

However, about an hour after the race, James started to experience incapacitating waves of stomach cramps that lasted over 24 hours. What caused this gastrointestinal distress?

The most likely explanation: dehydration.

Suzanne Girard Eberle RD explains in her book, Endurance Sports Nutrition, that during prolonged exercise, your body diverts blood from your colon to your active muscles and skin that need it more. When you’re dehydrated, this situation is exacerbated since dehydration causes a decrease in blood volume, so there’s even less blood available to the large intestine. This causes a whole array of gastrointestinal problems. It slows gastric emptying, meaning that food and fluids sit in your stomach rather than passing through your intestine, causing abdominal discomfort, bloating and cramping. The lack of blood flow can also seriously damage the walls of your colon, potentially causing ‘athletic colitis’, aka ischaemic colitis. Symptoms include diarrhea during exercise, but not at other times, extreme abdominal pain and bloody stools. If you experience these symptoms, you should go to the emergency room as it is a serious condition. There are 3 documented cases of elite triathletes that have had to have parts of their colons surgically removed as a result of athletic colitis. Luckily, early recognition and appropriate management, that includes intravenous rehydration and antibiotics, lead to complete resolution of symptoms and no long term damage.

Another indication that James was probably dehydrated is his performance. Although 1h30 is a great time for a half marathon, believe it or not, James is capable of running even faster! Studies have shown that for every 1% loss of body weight, athletes slow down by 2%. A 150-lbs runner that loses just 3lbs water (2% body weight loss), will slow down by 4%. At 8min/mile, that’s 20 seconds/mile. That’s a lot. Moreover, a loss of 2lbs water can increase your heart rate by 8 beats/minute- that’ll tire you out quickly.

Apart from dehydration, other possible factors that could have contributed to James’ stomach cramps include:

The intestinal jostling caused by running,

Exercise-induced changes in GI hormones,

Mental stress (he did want to impress me...!) that can actually slow down gastric emptying,

NSAIDs (Non-steroidal anti-inflammatory drugs). James took an Advil before the race. Many athletes take anti-inflammatory drugs (ie.s NSAIDs like Advil, Nuprin, Aleve, Actron) before or during long races to minimize pain. However, NSAIDS can cause stomach irritation. Moreover, taking NSAIDs when it’s very hot or when you’re dehydrated can potentially be dangerous as it has been shown to possibly contribute to hyponatremia (low sodium concentration in the blood) and kidney problems. When taking these drugs, be very aware of your fluid intake to prevent dehydration.

Other possible factors that can cause GI distress during exercise (but that were not factors for James) include:

Training status- the more trained you are, the less likely you are to experience GI problems.

Age- younger athletes are more at risk for GI problems- most likely because they may be less trained and have less nutrition knowledge and experience.

Intensity- The more intense the exercise, the more blood is shifted from the intestine to the active muscles, resulting in delayed gastric emptying and GI distress.

Caffeine- Can cause diarrhea and upset stomach.

Gels and too-concentrated liquid solutions- When highly concentrated solutions like gels, juice or soda pop are not diluted with water, they may cause stomach distress. Sports drinks, like Gatorade or Powerade, are diluted to the perfect concentration.

How James can avoid this next time:

Start out hydrated: Adults need about 8-10 cups non-alcoholic fluids a day in addition to drinking before and after exercise.

Stay hydrated: During exercise, drink early and drink often. Aim for 2-3 cups fluid (16-24oz)/hour. (A helpful tip: for the average adult, 1 gulp=1oz; for younger athlete, 2 gulps=1oz). Follow a pre-determined drinking schedule rather than relying on your thirst. Once you feel thirsty, you’ve likely lost 3% of your weight as fluid, resulting in a 10% loss of performance. Plain water is acceptable for activities under an hour (although a sports drink could give you the extra boost you need) but rely on a sports drink when exercising >1 hour.

Replenish your fluids: Weigh yourself before and after exercise- that weight lost is water, not fat! Drink 3 cups non-alcoholic fluids for every pound lost to replenish your fluids.

Congratulations James!