How the gut absorbs water, salt, and carbs: the physiology of endurance nutrition

You can drink a perfectly formulated beverage, but if your gut can't absorb it in time, it won't do you any good: the fluid will just slosh around in your stomach instead of heading to your working muscles. Absorption isn't "stomach magic" — it's a very concrete piece of physiology that anyone running longer than an hour would do well to understand. Let's unpack how sodium, water, and carbohydrates are connected, and why almost every rule of sports nutrition grows out of that link.

Sodium drags glucose and water along with it

The main workhorse of the small intestine is a transporter protein called SGLT1 (the sodium-glucose cotransporter). It grabs a molecule of glucose (or galactose) only together with two sodium ions and hauls them into the enterocyte cell all at once. This is what's known as secondary active transport: SGLT1 itself spends no energy — it "rides" the sodium gradient.

And that gradient is created by another pump — the sodium-potassium ATPase on the opposite (basolateral) side of the cell. It pumps sodium out, keeping it in short supply inside, and it's precisely this difference in concentration that makes SGLT1's work possible.

Here's where the key to hydration is hiding. When sodium and glucose enter the cell, water follows them along the osmotic gradient. Put simply: water follows salt. Plain water is absorbed slowly and reluctantly; water together with sodium and glucose is absorbed fast and efficiently. That's why rehydration solutions always contain both salt and sugar: it's not marketing, it's a direct exploitation of the SGLT1 mechanism.

Why "glucose + fructose" works better

Fructose is a separate story in this scheme. It's absorbed not through SGLT1 but through its own transporter, GLUT5, by facilitated diffusion, without any involvement of sodium. On the way out of the cell into the blood, both glucose and fructose use a shared transporter, GLUT2.

The practical takeaway is huge. Every transporter has a limit to its throughput — a "bottleneck at the entrance." If you load up on glucose alone, you hit the SGLT1 ceiling, and the excess carbohydrate simply stays in the gut lumen. But glucose and fructose travel different roads (SGLT1 and GLUT5, respectively) and don't get in each other's way. By putting both routes to work in parallel, the body absorbs more total carbohydrate per hour than it can on glucose alone. This is the whole idea behind "multi-transportable" gels and blends of glucose (or maltodextrin) plus fructose.

Where stomach trouble comes from

Most nutrients are absorbed in the jejunum — this is the main site of absorption. The duodenum is responsible more for iron, calcium, magnesium, and fat-soluble vitamins; the ileum handles vitamin B12 and bile acids. But carbohydrates and water are the jejunum's business, and its capacity is not unlimited.

What goes wrong during a race:

Too concentrated a solution. If your drink is hypertonic (lots of sugar in little water), osmosis works against you: water leaves the body and moves into the gut lumen to dilute the contents. The result — heaviness, gurgling, diarrhea.
Overloading a single transporter. Too much pure glucose at once, and SGLT1 chokes; the unabsorbed carbohydrate ferments under the action of bacteria, producing gas and cramps.
An untrained gut. Absorption is an adaptable function. If you've never eaten during training, your system simply isn't ready for the load on race day.

How to apply this in practice

Don't forget sodium. Salt in your drink isn't only about replacing what you lose in sweat — it's about the very mechanism of absorbing water and glucose. Water without sodium is absorbed less well.
Mix your carbohydrate sources. Look for products with a glucose/maltodextrin + fructose combination rather than a single type of sugar — that's how you get around the SGLT1 ceiling.
Watch the concentration. Keep your solution closer to isotonic (a rule of thumb is a carbohydrate concentration < ~8%). Wash down a gel that's too "thick" with water.
Train your gut (gut training). Regularly eat and drink on your long training sessions in the same way you plan to on race day. The gut adapts — both the number of transporters and your tolerance increase.
Test in advance. All products go through training only. Race day is no place for experiments.

The bottom line

SGLT1 pulls glucose into the cell together with two sodium ions; the energy for this comes from the sodium-potassium ATPase.
Water follows sodium and glucose osmotically — which is why salt + sugar are absorbed and retain fluid better than plain water.
Glucose and fructose take different paths (SGLT1 and GLUT5), so a mix of the two is absorbed better than glucose on its own.
The main site of absorption is the jejunum, and its throughput is limited.
GI trouble most often comes from nutrition that's too concentrated, overload, and an untrained gut — all of which are manageable.

Source: NCBI Bookshelf — Physiology, Nutrient Absorption. https://www.ncbi.nlm.nih.gov/books/NBK597379/