Place · Level 3 · Movement
The gut during exercise
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In one pass The heart can send out only so much blood in a minute. Educational content, not medical advice — consult a clinician.
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Chapter 1
Blood goes to muscle and skin
血先给肌肉和皮肤
The heart can send out only so much blood in a minute. Once you run, working muscle wants blood, and heat sends blood to the skin to dump heat. Those two flows together are more than the resting gut ever received.
The body does not shave a little off every organ. It clamps by priority. The splanchnic bed — stomach, small intestine, liver, spleen — is turned down first. van Wijck 2012 writes this as splanchnic hypoperfusion during exercise: the gut wall gets less oxygen and less substrate, the barrier gets brittle, and the person feels sick, crampy, ready to vomit.
That late-race wave of nausea is often this tissue having its blood stolen, not lunch gone bad. The water-and-electrolytes island already marked the line where plasma volume itself is falling. This island is the prior step: intensity alone, before thirst, already moves blood off the gut.
The body does not shave a little off every organ. It clamps by priority. The splanchnic bed — stomach, small intestine, liver, spleen — is turned down first. van Wijck 2012 writes this as splanchnic hypoperfusion during exercise: the gut wall gets less oxygen and less substrate, the barrier gets brittle, and the person feels sick, crampy, ready to vomit.
That late-race wave of nausea is often this tissue having its blood stolen, not lunch gone bad. The water-and-electrolytes island already marked the line where plasma volume itself is falling. This island is the prior step: intensity alone, before thirst, already moves blood off the gut.
Mechanism · one pump, two extra customers
Cardiac output is stroke volume times heart rate. Both rise in exercise, but almost all of the extra is claimed by muscle and skin. Splanchnic vessels answer sympathetic constriction far more readily than brain or heart, so the gut is first on the list.Once perfusion falls, the gut's fast-turnover epithelium is the first layer to run short. van Wijck writes hypoperfusion plus a possible ischaemia-reperfusion: intensity drops, blood rushes back, and injury can show in that return. Discomfort need not peak in the hardest minutes. It can arrive just after you ease off.
This step and a water shortage are two lines that can stack. They are not the same line. While blood volume is still adequate, intensity itself is already taking the gut's share.
Chapter 2
The stomach turns the valve down
胃先把阀门拧小
Sugar has to leave the stomach before it can reach blood. Gastric emptying is the first valve. Intestinal transporters, however fast, cannot take what is still sitting in the stomach.
Costill and Saltin in 1974 sampled meals back out through a tube. At moderate intensity, emptying stayed near the resting rate; push intensity higher and the same meal left more slowly. The other hand on the valve is the meal itself: raise the glucose concentration and emptying slows at rest and during exercise.
The two brakes are separate. Intensity is stealing splanchnic blood. Concentration is the stomach answering osmolarity and energy density. Writing the valve as one sip and you throw up collapses two hands into a complaint.
Costill and Saltin in 1974 sampled meals back out through a tube. At moderate intensity, emptying stayed near the resting rate; push intensity higher and the same meal left more slowly. The other hand on the valve is the meal itself: raise the glucose concentration and emptying slows at rest and during exercise.
The two brakes are separate. Intensity is stealing splanchnic blood. Concentration is the stomach answering osmolarity and energy density. Writing the valve as one sip and you throw up collapses two hands into a complaint.
Mechanism · intensity one brake, concentration the other
Costill's bouts were about a quarter-hour of cycling. Emptying dropped clearly only when intensity was high, not the moment movement started. So the stomach is still working at moderate intensity, and blaming every gut complaint in sport on emptying would hide the ischaemia line in the first scene.The concentration hand works even sitting down: a more concentrated glucose meal empties more slowly. Exercise adds a second hand. With both turned, flow at the small-intestine door is lowest — the two sugar transporters in the next scene cannot take sugar that is still in the stomach.
This page is not teaching you how thin to mix a bottle. It only separates the two hands on the valve, so the next scene does not merge an absorption ceiling and an emptying ceiling into one number.
Chapter 3
Sugar has two doors, each with a ceiling
糖有两扇门, 各有上限
Once out of the stomach, glucose crosses the small-intestine apical membrane through the sodium-glucose cotransporter (SGLT1). It pulls one glucose in with sodium; water follows the osmotic gradient — the same door the digestive-system island uses for oral rehydration salts.
The door fills. Jeukendrup 2010 puts the ceiling on exogenous carbohydrate oxidation at absorption: sugar that only uses SGLT1 oxidises at about 1 gram per minute; more of the same sugar in the mouth does not become more sugar the muscle can burn. It stays in the lumen. Fructose mostly uses a second door, GLUT5. Open both at once and measured exogenous oxidation can rise to about 1.75 grams per minute.
So more in the bottle is not more in the muscle. When the door is full, the extra portion is still in the gut.
The door fills. Jeukendrup 2010 puts the ceiling on exogenous carbohydrate oxidation at absorption: sugar that only uses SGLT1 oxidises at about 1 gram per minute; more of the same sugar in the mouth does not become more sugar the muscle can burn. It stays in the lumen. Fructose mostly uses a second door, GLUT5. Open both at once and measured exogenous oxidation can rise to about 1.75 grams per minute.
So more in the bottle is not more in the muscle. When the door is full, the extra portion is still in the gut.
Mechanism · the second door after the first is full
SGLT1 takes glucose and galactose and barely takes fructose (Wright 2011). GLUT5 is the fructose road. Jeukendrup 2017 puts the start of SGLT1 saturation around 60-70 grams per hour of a single carbohydrate; past that point, pouring only glucose is where gut trouble is more common. Stacking fructose uses the other door, and only then can exogenous oxidation climb further.The numbers are the shape of a ceiling, not a race timetable. Jeukendrup 2010 himself limits the high-rate combination to endurance work of about three hours or more. Shorter, easier work often never fills the first door.
The same 2017 paper also says repeated high carbohydrate in training might raise SGLT1 number. It then says the human evidence for that is limited. Do not read this scene as a syllabus for widening the gut.
Chapter 4
What stays in the lumen pulls water
剩在肠腔里的会拽水
Once the door is full, leftover sugar still sits in the lumen. More solute in the lumen pulls water back by osmosis — not the body detoxing, the two ends of a tube comparing concentration. Bloating, loose stool, that something sloshing feeling, sit on the same pipe as the ischaemia in scene one and the emptying valve in scene two. Three kinds of blockage, one tube.
The island stops here. It does not answer which bottle to drink on a course, how many grams, how often. Those are a menu. A menu that leaves these two doors and this stolen blood is no longer something that happens inside the body.
What you can draw is one sentence: muscle and skin take blood first, the stomach turns the valve down, the small-intestine doors have a ceiling. What is left is still in the gut.
The island stops here. It does not answer which bottle to drink on a course, how many grams, how often. Those are a menu. A menu that leaves these two doors and this stolen blood is no longer something that happens inside the body.
What you can draw is one sentence: muscle and skin take blood first, the stomach turns the valve down, the small-intestine doors have a ceiling. What is left is still in the gut.
Judgement · discomfort is plumbing, not will
Writing gut trouble in sport as a weak gut or untrained feeding collapses three mechanisms into a character judgement. Ischaemia, slower emptying, and a saturated door can each hurt on their own. Heat and dehydration stacking on top are already on the water-and-electrolytes island.The other direction is also wrong: do not write the two doors as so buy a fructose gel. Jeukendrup 2010's numbers are an oxidation ceiling, not a shelf label. If the first door is not full, the second door has no job.
The glycogen-supercompensation island is synthase inside the muscle. This island is the gut, before the sugar has entered the muscle. Do not merge the two into 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. Human gastric-aspiration study: emptying of a carbohydrate-electrolyte meal held near the resting rate until exercise intensity was high, then slowed; raising the glucose concentration of the meal slowed emptying at rest and during exercise. The two brakes — intensity and concentration — are independent. Short bouts (~15 min cycling); not a field feeding protocol. 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. Narrative review: the ceiling on exogenous carbohydrate oxidation sits in intestinal absorption, most likely saturation of carbohydrate transporters. Glucose alone had been taken as peaking near 1 g/min; combining carbohydrates that use different intestinal transporters (glucose plus fructose) raised measured exogenous oxidation to 1.75 g/min. The paper's own limit: this combination, ingested at high rates, is discussed for endurance work lasting 3 hours or more — not a universal feeding table. 10.1097/MCO.0b013e328339de9f
- Jeukendrup, A. E. (2017). Training the gut for athletes. Sports Medicine, 47(Suppl 1), 101-110. Names the two apical doors: SGLT1 for glucose/galactose saturates around 60-70 g/h of a single carbohydrate; fructose largely uses GLUT5. Adding fructose once SGLT1 is full raises exogenous oxidation; ingesting a single carbohydrate above that rate is linked with gastrointestinal problems. The paper proposes that repeated high carbohydrate intake during training might increase SGLT1 number or activity, then states the human evidence for that upregulation is limited. Do not read this record as a proven 'train the gut' protocol. 10.1007/s40279-017-0690-6