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The gut during exercise
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In one pass The heart can pump out only so much blood in a minute.
Educational content, not medical advice. See a clinician about your own case.
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Chapter 1
Blood goes to muscle and skin
The heart can pump out only so much blood in a minute. Once you start running, the working muscles need blood, and as you heat up, the skin needs blood to shed heat. Together those two demands can only be met by squeezing out the share the gut gets at rest.
The body does not take a little from every organ equally; it tightens by priority. The splanchnic circulation, the pool of blood shared by the stomach, small intestine, liver, and spleen, is the first to be turned down a notch by signals that narrow blood vessels. A review by van Wijck 2012 describes this as splanchnic hypoperfusion during exercise (less blood reaching the gut): the gut wall gets less oxygen and fewer nutrients, its barrier becomes less tight, and you may feel nauseated, crampy, or ready to throw up.
So the churning stomach late in a long run may be exactly this tissue having its blood taken away, not necessarily something bad you ate at lunch. The Water & Electrolytes story covers this once from the angle of falling plasma volume; the point here is that even before you are short of water, intensity alone moves blood away from the gut.
The body does not take a little from every organ equally; it tightens by priority. The splanchnic circulation, the pool of blood shared by the stomach, small intestine, liver, and spleen, is the first to be turned down a notch by signals that narrow blood vessels. A review by van Wijck 2012 describes this as splanchnic hypoperfusion during exercise (less blood reaching the gut): the gut wall gets less oxygen and fewer nutrients, its barrier becomes less tight, and you may feel nauseated, crampy, or ready to throw up.
So the churning stomach late in a long run may be exactly this tissue having its blood taken away, not necessarily something bad you ate at lunch. The Water & Electrolytes story covers this once from the angle of falling plasma volume; the point here is that even before you are short of water, intensity alone moves blood away from the gut.
Mechanism · Why the gut is first to lose blood
The amount of blood the heart pumps each minute (cardiac output) equals the amount pumped per beat multiplied by heart rate. Both rise during exercise, but almost all of the extra is claimed by muscle and skin. The blood vessels of the gut respond to vessel-narrowing signals from the sympathetic nervous system far more readily than those of the brain or heart, so the list of cuts starts with the gut.Once blood supply falls, the intestinal epithelium (the innermost, very fast-renewing layer of cells in the gut wall) is the first to run short. van Wijck describes low blood supply plus possible ischemia-reperfusion (damage caused when blood returns in a rush after a spell of too little): when intensity drops, blood surges back, and injury can show up only at that moment. So discomfort does not necessarily peak in the hardest minutes; it can also arrive just after you slow down.
This step and a lack of water are two lines that can stack on each other, not the same line. While blood volume is still adequate, intensity on its own is already taking the gut's share.
Chapter 2
Why the stomach empties slower
Sugar has to leave the stomach before it can reach the blood. Stomach emptying is the first valve: however fast the transporters further down in the small intestine are, they cannot take up what is still sitting in the stomach.
Costill and Saltin 1974 had people drink a sugar-and-electrolyte solution, then drew what was left back out of the stomach through a tube and weighed it. At moderate intensity, the rate of emptying stayed close to its resting level; push the intensity higher and the same drink left the stomach clearly more slowly. The other hand on the valve is the drink itself: raise its glucose concentration and emptying slows both at rest and during exercise.
The two brakes are separate. Intensity works through the blood supply to the gut; concentration works through the stomach's response to osmotic pressure and energy density. Putting the valve down to "I throw up whenever I drink during exercise" merges two hands into one complaint.
Costill and Saltin 1974 had people drink a sugar-and-electrolyte solution, then drew what was left back out of the stomach through a tube and weighed it. At moderate intensity, the rate of emptying stayed close to its resting level; push the intensity higher and the same drink left the stomach clearly more slowly. The other hand on the valve is the drink itself: raise its glucose concentration and emptying slows both at rest and during exercise.
The two brakes are separate. Intensity works through the blood supply to the gut; concentration works through the stomach's response to osmotic pressure and energy density. Putting the valve down to "I throw up whenever I drink during exercise" merges two hands into one complaint.
Mechanism · The two brakes on stomach emptying
In Costill's experiment, each cycling bout lasted about a quarter of an hour. Emptying dropped clearly only when intensity was high; it did not stop the moment movement began. So the stomach keeps working at moderate intensity, and blaming every gut complaint during exercise on stomach emptying would hide the other line, the one about blood being taken away from the gut.The concentration hand works even when you are sitting still: a more concentrated glucose solution empties more slowly. Exercise simply adds a second hand. With both turning the valve down, the least flows through to the entrance of the small intestine, and however wide the two sugar doors in the small intestine are, they cannot take sugar that is still in the stomach.
This is not a lesson in how dilute to mix your drink. It only separates the two hands on the valve, so that the absorption ceiling of the small intestine and the emptying ceiling of the stomach are not merged into one number.
Chapter 3
The gut can absorb only so much sugar
Once out of the stomach, glucose has to pass through the sodium-glucose cotransporter (SGLT1) in the membrane of small-intestine cells that faces the gut interior. It pulls glucose into the cell together with sodium, and water follows the osmotic gradient. The oral rehydration solution described in the Digestive System story relies on this same door.
This door can fill up. A review by Jeukendrup 2010 places the ceiling on burning exogenous carbohydrate (sugar eaten during exercise) at the absorption step. Sugar that can only use SGLT1 is burned at a rate that levels off around 1 gram per minute; add more of the same sugar and the extra does not become fuel the muscles can burn but stays in the gut. Fructose mostly uses a different door, GLUT5. With both doors open, measured burning of exogenous carbohydrate can rise to about 1.75 grams per minute.
So drinking more does not mean the muscles get more. Once the door is full, the extra is still sitting in the gut.
This door can fill up. A review by Jeukendrup 2010 places the ceiling on burning exogenous carbohydrate (sugar eaten during exercise) at the absorption step. Sugar that can only use SGLT1 is burned at a rate that levels off around 1 gram per minute; add more of the same sugar and the extra does not become fuel the muscles can burn but stays in the gut. Fructose mostly uses a different door, GLUT5. With both doors open, measured burning of exogenous carbohydrate can rise to about 1.75 grams per minute.
So drinking more does not mean the muscles get more. Once the door is full, the extra is still sitting in the gut.
Mechanism · What happens once the first door is full
SGLT1 carries glucose and galactose and barely carries fructose (Wright 2011); GLUT5 is the route fructose takes. Jeukendrup 2017 puts the point where SGLT1 starts to saturate at about 60-70 grams per hour of a single type of carbohydrate. Beyond that point, keeping on with glucose alone is where gut problems become more common. Adding fructose uses the other door, and only then can the burning of exogenous carbohydrate climb further.These numbers describe the shape of a ceiling, not a race-day feeding schedule. Jeukendrup 2010 himself limits the high-rate combination to endurance exercise lasting about 3 hours or more. In shorter or easier exercise, the first door is often not full.
The same 2017 review also suggests that repeatedly eating a lot of carbohydrate during training might increase the number of SGLT1 transporters. It then states, straight away, that the human evidence is limited. Do not read this section as a program for training your gut wider.
Chapter 4
Unabsorbed sugar pulls in water
Once the door is full, the extra sugar is still sitting inside the gut. With more dissolved material in the gut, osmosis pulls water into it. This is not the body flushing out toxins; the two sides of the gut wall are simply evening out which side is more concentrated. So bloating, loose stools, and that feeling of something sloshing around in your belly are, like the blood being taken away and the stomach emptying more slowly, a third blockage on the same pipe.
This story stops here. It does not answer which bottle to drink during a race, how many grams, or how often. Those are fueling plans, and once they move away from these two doors and this pool of diverted blood, they are no longer about what happens inside the body.
What can be drawn is one sentence: muscle and skin take blood first, the stomach turns down the valve, and the doors of the small intestine have a ceiling. Whatever is left over is still in the gut.
This story stops here. It does not answer which bottle to drink during a race, how many grams, or how often. Those are fueling plans, and once they move away from these two doors and this pool of diverted blood, they are no longer about what happens inside the body.
What can be drawn is one sentence: muscle and skin take blood first, the stomach turns down the valve, and the doors of the small intestine have a ceiling. Whatever is left over is still in the gut.
Myth · Gut trouble is plumbing, not willpower
Calling gut trouble during exercise a weak gut or poorly practiced fueling squeezes three mechanisms into a judgment of character. Blood being taken away, slower emptying, and a saturated door can each make you feel bad on their own; the extra layer that heat and dehydration add on top is covered in the Water & Electrolytes story.In the other direction, do not read the two doors as a reason to buy an energy gel that contains fructose. The figures in Jeukendrup 2010 are a ceiling on burning carbohydrate, not a product label. If your first door is not full, the second door has no work to do.
The Glycogen supercompensation story is about the enzyme in muscle that stores sugar; this story is about the gut, the stretch before the sugar ever reaches the muscle. Do not merge the two into a single "just eat more carbohydrate."
References · 5
- van Wijck, K., Lenaerts, K., Grootjans, J., Wijnands, K. A. P., Poeze, M., van Loon, L. J. C., Dejong, C. H. C., & Buurman, W. A. (2012). Physiology and pathophysiology of splanchnic hypoperfusion and intestinal injury during exercise: strategies for evaluation and prevention. American Journal of Physiology-Gastrointestinal and Liver Physiology, 303(2), G155-G168. 10.1152/ajpgi.00066.2012
- Costill, D. L., & Saltin, B. (1974). Factors limiting gastric emptying during rest and exercise. Journal of Applied Physiology, 37(5), 679-683. 10.1152/jappl.1974.37.5.679
- Wright, E. M., Loo, D. D. F., & Hirayama, B. A. (2011). Biology of human sodium glucose transporters. Physiological Reviews, 91(2), 733-794. 10.1152/physrev.00055.2009
- Jeukendrup, A. E. (2010). Carbohydrate and exercise performance: the role of multiple transportable carbohydrates. Current Opinion in Clinical Nutrition and Metabolic Care, 13(4), 452-457. 10.1097/MCO.0b013e328339de9f
- Jeukendrup, A. E. (2017). Training the gut for athletes. Sports Medicine, 47(Suppl 1), 101-110. 10.1007/s40279-017-0690-6
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