What Does Science Say about Hydration?

Health Apr 05, 2021

Many people have faced drinking bans in training – from physical education at school to personal lessons in the gym. Is it reasonable to avoid drinking water during physical activity? Is it true that the “extra” fluid burdens the heart or causes swelling? And if you can drink, then how much and what is better to choose?

The basis of dietetics is not prohibitions and restrictions, but an adequate balanced diet, and it includes not only proteins, fats, carbohydrates, fiber, vitamins, but, of course, water. When I ask almost all of my clients, it turns out that people are not getting enough fluid. At the same time, many, including athletes, do it consciously – by themselves or according to the instructions of the coach.


The human body is about 70% water. Basically, it enters the body as part of solid food and liquids, and the remaining small percentage is formed as a result of various metabolic processes. Water is excreted mainly through the kidneys, but it also evaporates from the surface of the skin, is excreted in the form of sweat, and exits with exhaled air. These are large volumes: for example, by direct evaporation from the skin, a person loses about 300 ml of water per day and exhales half a liter with air. The amount of sweat can vary depending on the load and climate, but it is not less than 500 ml per day. Water is constantly used by the stomach and intestines to form digestive juices, the total amount of which reaches 8 liters per day. In general, it is vital to restore its reserves: WHO recommends that women and men get 2.7 and 3.7 liters of liquid per day from food and drinks, respectively.

Abstinence from drinking is often explained by the fact that excess fluid increases the volume of circulating blood and supposedly overloads the heart – but this is a misconception. On the contrary, difficulties arise with insufficient hydration, when part of the water to meet the needs of the body is removed from the blood – and the latter becomes more viscous. The myocardium has to do a lot more work trying to push blood into the smallest vessels. Even under drinking is often tried to explain the prevention of edema, but this is also incorrect. Water does not linger on its own – it is primarily bound by sodium (which we get from salt) and carbohydrates, especially sugar. Each gram of salt or sugar will hold approximately 3.78 grams of water – that is, 100 grams of sugar will hold nearly 400 milliliters. This water needs to go somewhere, and the most real contenders for the place of its storage are blood, the intracellular and intercellular environment – these are the reasons for the increase in blood pressure and edema. Together with sweat, mineral salts leave, which leads to disturbances in the conduction of electrical impulses.

And, of course, if a person goes in for sports, not just for pleasure, but sets certain goals, be it increasing muscle mass, losing weight, or increasing speed and endurance, it will be more difficult to achieve these goals if there is not enough fluid in the body. Without normal hydration, the activity of biochemical processes in cells decreases, protein synthesis is inhibited, strength, and efficiency decrease. Even worse, during training, fluid is rapidly lost, because a person sweats. Along with sweat, mineral salts also leave, and this leads to disturbances in the conduction of electrical impulses – in the short term, this can threaten with muscle spasms (for example, when the leg painfully reduces in a dream), and in the long term – disorders of the heart.


To determine how much water you lose when playing sports, do an experiment: do not drink during training, and weigh yourself before and after without clothes. The difference in weight will be roughly the amount of water you should drink before and during physical activity. If you love accuracy, you can try to do this calculation for different loads (for example, strength and cardio) – then it will become clear how much you need to drink in each case. In order not to feel discomfort, especially if you are not used to drinking during exercise, take often and little by little, literally a couple of sips – and start with a glass of water fifteen to twenty minutes before training.

Special sports drinks are also healthy, they help restore mineral balance and contain small amounts of carbohydrates (glucose, fructose, or ribose). They are called isotonic, but it must be understood that an isotonic is an aqueous solution of various substances with a concentration of no higher than 0.9%, that is, it is identical in density to blood plasma. More concentrated solutions are hypertonic, they are absorbed more slowly and are not suitable for drinking during exercise. Moreover, drinking such drinks can even accelerate dehydration: the body will give water to the intestines in order to dissolve the contents to a concentration necessary for absorption.

Ordinary bottled water, as a rule, is a hypotonic solution, that is, there are not enough minerals in it; it is absorbed very quickly, but under serious exertion (for example, if you are running cross-country in the hot sun) it does not compensate for the loss of minerals. If the workouts are not very long or intense, water will do, if more difficult – it is worth switching to isotonic. And, if you buy formula for preparing a drink in powder, follow the directions on the label so as not to get a hypertonic solution instead of an isotonic one.


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