HYDRATION
Proper hydration during any sort of physical activity is an absolute necessity - there are few more self-evident statements in the entire field of sports nutrition. When the body begins to run low on water, the list of systems that become compromised is so extensive that it’s almost not worth listing - anybody who has become dehydrated or severely dehydrated while engaged in any sort of physical activity knows the impact, I’m one of them.
HOW IS WATER ABSORBED BY THE BODY?
Water is one of the few substances that the stomach is capable of absorbing - the stomach is primarily a processing station, and few compounds make it across the stomach lining into the bloodstream. It is not, however, the primary organ for water absorption, and why this is important will become more relevant later. Most water is absorbed in the small intestine, and what is most critical is the understanding that water is not actively transported across the intestinal lining. In other words, there are no cells or transporters that carry water into your bloodstream. Why is this so important? Because it means water absorption is heavily dependent on osmotic gradients- if the gut is filled 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 a sugary sports drink, you’ll understand exactly what this feels like. It sloshes around, flows back up into your esophagus, and can end up on the floor and you along with it.
Most other nutrients, on the other hand, are more actively transported - there are certain receptors lining those intestinal cells (cells called enterocytes, if anybody cares) that pull salts, sugars, amino acids, etc. through the intestinal lining into the cells in exchange for other compounds (e.g. they’ll pull in a hydrogen ion at the same time as an amino acid, then exchange the new hydrogen atom for a sodium molecule later.) One interesting fact here is that 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, an interesting fact that will soon become relevant.
If you are new to the concept of osmotic gradients, or have deliberately blocked out all basic chemistry like any other traumatic experience, just understand that if you have a semi-permeable membrane (that water can pass through, but nothing larger) with fluid on both sides, and ions or small molecules dissolved on in the solution on one side that cannot penetrate the membrane, the fluid will always flow to the side with the higher concentration of dissolved molecules.
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 buffer 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).
There are several 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 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, 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 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 sodium, as well as carbohydrates to fuel continued activity.
If you take in nothing but water, sodium and potassium loss through sweat can eventually be exceedingly detrimental to performance, as these compounds are vital to muscle contraction and, indeed, life in general. (Note that this will not cause cramping - cramps are not caused by low sodium or potassium. I would encourage you to repeat this to everybody who will listen until this myth dies. Cramping is caused by under-conditioning; this will be the focus of a later). Replacing lost carbohydrates is also critical, as glycogen losses will eventually result in the body losing its ability to perform under anaerobic conditions, which means even the most efficient runner will be reduced to a slow stagger. Hence, the advent of sports drinks - fluids that contain electrolytes that your body needs, as well as glucose to continue fueling activity (and aid water/sodium uptake). During moderate duration exercise, these sports drinks may seem like a godsend - they taste good (your body craves the salt and sugar), they go down easily, and they taste better than lukewarm brackish bottle water. The electrolytes replace those being lost through sweat, and the glucose is both useful for extending performance and for assisting in electrolyte uptake.
There is one major issue with these drinks, however - they are almost all calibrated to be ideally absorbed by a body at rest. Once the body begins to exert itself, blood concentrations of certain electrolytes and carbohydrates can decrease, and most importantly, circulation to the gut can decrease. This last point bears explaining - when you exercise, the body shunts blood away from momentarily less essential systems (like the digestive system) and towards more essential systems (heart, lungs, muscles). If less blood is getting to your digestive tract, what does this mean for nutrient absorption? Remember how water is passively diffused and relies on the bloodstream to keep those molecules flowing away from the gut? If that slows down, suddenly the body can no longer establish a strong gradient (since those molecules that were previously being actively carried into the bloodstream then swept away are now just sitting there), and water just ends up sitting in the gut as a buffer.
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.
The ideal Sports drinks, as mentioned earlier, 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 250ml.