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As of September 2017, new Sweat Science columns are being published at www.outsideonline.com/sweatscience. Check out my bestselling new book on the science of endurance, ENDURE: Mind, Body, and the Curiously Elastic Limits of Human Performance, published in February 2018 with a foreword by Malcolm Gladwell.

- Alex Hutchinson (@sweatscience)

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You may want to skip this post if you’re about to eat…

Swedish scientists just published a delightful and highly detailed study comparing the gastrointestinal characteristics of a group of 15 elite orienteering during a week of heavy training, and again during a week of rest. (I don’t generally keep up to speed on the Scandinavian Journal of Gastroenterology, so thanks to Amby Burfoot for the tip-off!) Most people won’t be surprised to learn that, during the heavy training week, subjects had more bowel movements (1.5 per day versus 1.3) and looser stools (4.2 versus 3.9 on the Bristol Stool Form Scale, which rates stools from a hard, nut-like 1 to a watery 7).

The study also delved into much greater detail, using “radiopaque markers” whose progress could be followed through the digestive system using fluoroscopic imaging (essentially a real-time X-ray). Throughout the week, the subjects ate 10 little ring-shaped markers with breakfast each day; then on the final day, they ate 20 little spherical markers with breakfast, and were imaged every half-hour for the next eight hours. The spheres were used to track progress out of the stomach and through the small intestine (where most nutrient absorption occurs), which is measured in hours. The number of ring-shaped markers still in the body allowed the researchers to calculate how long food was taking to travel through the colon, which is measured in days.

The key results: gastric emptying (how quickly food left the stomach) was not significantly different during the two weeks (average of 1.8 hours during the resting week, 2.4 hours during the exercise week). Transit through the small intestine was significantly quicker during the training week (3.7 hours versus 6.9 hours on average). Transit through the colon wasn’t significantly different (1.2 days during training, 1.4 during rest).

So what does it all mean? Well to me, this is one of those “Am I normal?” studies: without going into excessive detail, it’s nice to see that my personal observations match up with typical patterns. One of the questions that remains unanswered is: are athletes in training less efficient at absorbing nutrients from their food since they’re forcing the food through their small intestine more quickly?