Is it harmful to drink an energy drink before a run for several kilometers?

An energy drink does not add one iota of energy. There is no proven experience that shows benefits.

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First of all, it is worth answering the question whether you have contraindications to the use of energy drinks, namely: diseases of the cardiovascular system, arterial hypertension, atherosclerosis and glaucoma.

Now about how caffeine works : it stimulates the central nervous system and cardiac activity, accelerates the heart rate, causes vasodilation. A large influx of caffeine into the brain (also together with the influx of glucose into the blood) gives a strong boost of vivacity. Therefore, if you really need such a charge, it is more efficient to use an energy drink before training (or running) at high speed or during an exhausting many hours of ultramarathon . “Few Kilometers” are mid-range distances that hardly require such additional stimulation, as this is neither a high speed sprint nor an endurance marathon. Your heart rate will already rise due to running, so also caffeine will increase it, why is it needed - it is not clear.

It is also worth knowing that there is a lot of sugar in energy drinks, so they can cause stomach upset during exercise. If you have never consumed an energy drink before a race, you should not try it before a race, because the body's response can be unpredictable.