Exercise and poop


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- Alex Hutchinson (@sweatscience)


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?

4 Replies to “Exercise and poop”

  1. Thanks for writing up on this. I’ve always been interested in my own bowel movements and how training affects them, and in turn, myself. I currently run 70 to 80km per week and since my return to running after a 7 year hiatus, I’ve taken a keen interest in how my body reacts to training. The biggest challenge I’ve had to face is how my GI system reacts to the increased load I’m subjecting my body to, especially evident on days in which I increase the intensity.

    I’ve noticed that foods are processed much quicker by my body on days in which I have a heavy load (be it volume or intensity), and my stools are considerably softer on those days too. I’m not sure why yet, but I’d love nothing more than to try and smooth out the ‘peak’ and the subsequent ‘low’ the following day.

  2. Very interesting. I’ve always wondered about a variation on your last question… does fibre do the same thing and would that help for weight loss?

  3. Andre: I hear you, and wish I knew how to, er, smooth things out… 🙂

    barnee: Interesting question (and indeed related to my last question). I did a little digging — just enough to realize that it’s a pretty complex topic with no simple conclusions. My general sense is that fibre plays a bigger role in the colon (which comes after the small intestine, where most absorption takes place).

    There’s a good discussion in this article (http://www.ncbi.nlm.nih.gov/pubmed/20388134 – send me an e-mail if you’d like the full text). They review the very bewildering array of studies looking at fibre consumption and weight loss, which overall offer pretty equivocal evidence. Then they talk about the mechanisms:

    “It has been proposed that DFs increase fecal energy loss and reduce spontaneous energy intake, effects mainly attributed to the properties of viscous DF. Viscous DFs induce thickening when mixed with liquids, and viscous DFs include many soluble forms, such as gums, pectins, and ?-glucans, whereas insoluble DFs are the main fiber source in cereal grains… Viscosity is mainly linked to physiological effects in the small intestine, where increased viscosity slows gastric emptying (GE), and diminishes or delays nutrient absorption due to reduced mixing of the chyme and to thickening of the unstirred water layer, which presents a greater barrier to absorption…

    Various cereal DFs, with varying viscous DF contents, have been found to increase fecal energy and/or fat excretion by 2–10%, whereas others report no effect. Isolated viscous DFs, such as gum tragacanth, gum arabic, and psyllium, have been found to decrease apparent fat and energy digestibility.”

    So it sounds like increased dietary fibre intake CAN reduce the amount of nutrients you absorb from a given amount of food. But it’s not so much related to how fast it moves through the system as to how it affects the mixing and gelling of material in the small intestine. Overall, I think fibre is good to consider when assessing the overall mix of your diet, but not necessarily something you want to be explicitly supplementing. (When I’m choosing whole grain options in the supermarket, I’m thinking more about avoiding overly processed foods than about the fibre content per se. But then again, as indicated in the post above, I’m doing enough running that I don’t need any help, digestively speaking, from extra fibre!)

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