Water is one of the few substances that the stomach is capable of absorbing – the stomach is primarily a processing station, and few compounds actually make it across the stomach lining into the bloodstream.
Water is not actively transported across the intestinal lining. In other words, there are no cells or transporters that carry water into your bloodstream- it just sort of flows by itself. Why is this so important? Because it means water absorption is heavily dependent on osmotic gradients- if the gut is filed with large quantities of mineral ions(particularly sodium), free glucose, etc., water will remain in the gut to serve as a buffer. If you’ve ever been on an exceedingly long run and taken in a massive quantity of Gatorade, you’ll understand exactly what this feels like. It sloshes around, flows back up into your esophagus, and can inspire truly astounding-looking Technicolor yawns.
Many compounds are co-transported, for example sodium cannot be efficiently absorbed without some sugars (remember this), as sodium uptake is coupled with glucose uptake, and absorption of both accelerates water uptake (as it forces water into the cells to buffer the two). Almost as importantly, amino acid uptake is also coupled with sodium uptake.
If your gut is filled with water much saltier than your blood, the water will stay in your gut to buffer the salt. This, incidentally, is why drinking seawater can kill you- not only is the water poorly absorbed due to the high salt concentration (meaning water stays in the gut until the salt is absorbed), but once the salt is in your system, your body needs to buffr it in your blood, then excrete it, which means a huge amount of water ends up being removed from your cells (to maintain equilibrium between your bloodstream and your cells) then dumped into your kidneys (to maintain the gradient between your urine and your blood).
Now, granted, this is simplified – there are a number of ways in which the body can continue to absorb water against an osmotic gradient (look up “three compartment model” if you’re truly curious), but the end lesson still holds – if the gut is too full of solutes, water absorption is slowed. The body needs to absorb these solutes in order to optimally absorb the water, which means the more electrolytes, sugars, and other molecules in the gut, the longer this will take. It also means that, in order for optimal absorption, there needs to be good blood flow to the cells in the gut to keep “sweeping” absorbed molecules away… which in turn lets more molecules be absorbed (again, simplified but fundamentally accurate).
So if keeping the gut free of sodium is the best way to facilitate passive water uptake, why not just drink plain water? Well, simply put, for short exercise duration this is indeed often the best option – plain old water is absolutely adequate to hydrate during a typical weight lifting session, short set of sprints, or short distance run in moderate temperatures.
During longer duration activity, however, it becomes important to take in compounds other than water that are lost, which includes the aforementioned sodium, as well as carbohydrates to fuel continued activity.
The purpose of all the background information was to drive home a point pure water alone can be detrimental to long term performance, as athletes require other nutrients to function, while too
much sugar or salt in the digestive system will slow water absorption and result in a similar performance decrease, along with severe GI distress.
So what is ideal? Sports drinks tend to become increasingly too high in electrolyte and (mainly) sugar concentrations as activity increases, as the uptake of these nutrients slows and they begin to accumulate in the gut. The ideal glucose concentration used in Oral Rehydration Therapy (ORT – for patients with cholera or other diseases affecting the GI system whose lives depend on absorbing water and electrolytes, but who may not have access to an IV)- about 7 grams of carbohydrates per 8 ounces of fluid.
