“You have to wait 45 minutes after your lunch before you can get back in the pool!” – I think we all heard something similar to this when we were kids.
Our parents cautioned us against exercising right after we had eaten. They said we would get cramps if we exercised after eating. Is it true? How does exercise affect digestion? First, let’s talk a bit about the organs responsible for digestion and how they are controlled.
The Gastrointestinal Tract and the Autonomic Nervous System.
The Gastrointestinal (GI) tract is essentially a long tube that runs through our bodies, from the mouth to the anus. It includes the esophagus, stomach, and small and large intestines – all of which are large muscles of a type called smooth muscle. Smooth muscles work a little differently than the typical types of muscle (skeletal muscle) that we think about. For one, we don’t typically think about it at all – as the GI tract is controlled by our body’s autopilot: the autonomic nervous system.
The smooth muscle of the GI tract does most of its work when the rest of our bodies are more relaxed. When we are relaxed, the parasympathetic branch of our autonomic nervous system is most active, which keeps our heart rate and metabolism low but increases GI tract activity. The GI tract rhythmically contracts to push food through it and allow for digestion in a process called peristalsis. Parasympathetic nervous activity increases the speed of peristalsis, or gut motility (movement).
What happens when we exercise?
When we become more active, we see a shift in our nervous system activity, with parasympathetic activity decreasing and sympathetic activity increasing. Sympathetic nervous system activity increases heart rate and metabolism so we can move (fight-or-flight response), but it slows peristalsis. Also, sympathetic nervous activity reroutes blood flow within your body, increasing flow to the heart and active skeletal muscles and reducing flow to the GI tract.
Mom and Dad were right to an extent: hard exercise right after you have eaten a heavy meal can cause discomfort due to a temporary reduction in peristalsis and blood flow to the GI tract. As a consequence, food can “sit heavy” in your gut, and you may feel a stomachache. It is not a given, though.
Whether you feel discomfort when you exercise right after eating depends largely on how hard you exercise and what/how much you ate. Harder exercise means more sympathetic nervous activity, so a greater reduction of blood flow/peristalsis, which = a greater risk of a stomachache. Coupling this with greater amounts of food, particularly slower-to-digest foods like fat, fiber, and protein, will also increase the odds of stomach discomfort.
It also bears mentioning that there is considerable variation among people – some folks seem immune to stomach discomfort during exercise. For example, back when I was competing in weightlifting, I trained with a guy who ate pepperoni pizza during his squat workout! He does a set of squats (like 5 reps at 400-ish pounds), then eats a slice, then does another set. Wrapping up his workout and polishing off a large pie at the same time. It was weirdly fascinating (and mildly disgusting) to watch.
The good news: Long-term training and digestion
Each time we exercise, we shift from a parasympathetic to a sympathetic-dominant nervous tone. Then, when we stop each exercise session, we increase parasympathetic activity to help us recover from the exercise session.
With regular exercise training, flipping back and forth between sympathetic and parasympathetic over many exercise bouts, as they become fitter, people typically adapt to a lower level of sympathetic activity during exercise and a stronger level of parasympathetic activity at rest. This translates into faster gut motility/reduced risk of constipation, which is generally a good thing and one of many positive health benefits of regular exercise.
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Comments
Carolyn Tonidandel
07/06/2022 at 11:00 AM
Great!
Good information
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