Hydration is not only a matter of the balance between fluid lost and fluid ingested, its complexity encompasses concentration ratios between fluid and solute , and in a more complex way the proportions between the different salts and other macronutrients such as sugars and carbohydrates that have been ingested while we hydrated.
So much so that in a correct hydration strategy during sports practice, we must consider different options depending on our state and nutritional strategy, which is why, apart from water as a universal fluid, we must take into account isotonic drinks and hypotonic drinks, knowing when and why to use each one of them.
Let's understand the reason for isotonicity
The difference between them is the osmolarity -measured in moles of solute per unit of liquid-, being the osmolarity of isotonic drinks is approximately 330mmols/l and that of hypotonic drinks is approximately 220mmols/l, meaning that the concentration of the isotonic drink is higher than that of the hypotonic drink.
Isotony is the language in which our cells understand the delicate concentration balances between their interior and the extracellular matrix, their transmembrane part. The cell membrane is a border where there are different proteins and lipids prepared to make water enter or leave the cell as well as allow different molecules to enter or leave (solute ). Cellular osmotic regulation fulfills the final function of recovering homeostasis, but while this is not possible, that is, during physical activity, this regulation will be dynamic and continuous, trying to search for two factors in an elementary way:
- The first, equality of concentration between the interior and exterior of the cell.
- The second search for an isotonic concentration both inside and outside the cell.
When we perform physical activity, thermoregulation is managed through sweat to reduce system temperature, which causes us to loss of fluids and salts through our skin, this loss comes largely from the extracellular environment and imbalances start there. From that initial isotony, where our cells can perform at 100% physiological, it begins to unbalance, losing that ideal concentration from the outside of the cell. At that moment, our cells will decide if they should draw water from inside to match the concentration with the outside of the membrane and at that moment its function is already compromised.
That is why we must try to drink the optimal drink and at the earliest possible time to avoid this mismatch.
In this way, it is easily deducible that if we rehydrate ourselves with an isotonic drink 15 minutes after starting a physical activity, it would be ideal to restore that equality of isotonicity between the interior and exterior of the cells, and it is so. For this reason, the osmolarity of isotonic drinks are approximately equal to those of our cells. And this is also why in high intensity and short duration activities, up to an hour or so, in which we are not going to eat anything, the most correct solution is the isotonic drink.
Hypotonic drinks, with less amount of solutes
The drink hypotonic therefore it is to drink it in activities in which we are going to eat, since the food does not stop being solute that will go to our extracellular matrix, so if we drink isotonic at the same time that we eat, we would generate a concentration above the ideal, hypertonia and our performance would be compromised.
Always taking particularities into account, this would be the standard recommendation when choosing a rehydration drink.
At the intestinal level, other factors come into play that will improve or block the absorption of the electrolytes present in our hydration drink, such as the proportions between some specific sugars
The cells of our small intestine, the enterocytes, are glued together to form the border that separates our physiology from the outside world, these cells form the so-called tight junctions, in order to give to our intestinal wall its capacity of selective impermeability, choosing at each moment which micro or macro nutrients enter or not into our bloodstream.
Between this wall are integrated transporter protein channels that allow the entry of mainly glucose and galactose y sodium, the so-called SGLT channels and the channels of the GLUT family, specifically GLUT2 that works with a potassium pump and allows or not fructose input.
Both SGLT and GLUT2 require specific proportions not only between sodium and potassium but also between glucose and fructose, so that their transporter function is optimal. So much so, that the input of glucose and galactose that will later provide the muscles with energy, is subject to certain amounts of sodium, and at the same time the input of sodium as the main electrolyte can also be compromised by an excess of glucose and /o galactose.
In the same way, GLUT works with a potassium pump, which by pressure allows or not the entry of fructose into the blood.
If these sugars remain in the intestine and cannot penetrate due to salt imbalance, they will cause the subject gastrointestinal discomfort, but rather vomiting or diarrhea, and in turn the excessive presence of these will not allow the entry of sodium and potassium with what the osmotic balances will not be able to be restored and our performance will be well below our potential.
For this reason, quality rehydration drinks must have a 2:1 glucose: fructose ratio and balanced amounts of electrolytes so that they fulfill their function physiological in our body.
In the short term, poor hydration levels will undoubtedly compromise the athlete's performance, but in the long term they can affect health-related functions, such as blood pressure or insulin resistance due to accumulation of chronic low-grade inflammation significantly influenced by dehydration.
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Article written by Roger Pérez | Physiologist and scientific popularizer