We all know we should stay away from dehydration. When I was young, I was not allowed to drink anything until all the training session was over. However, when sports science kicked in to the kendo world, kendo training has changed a lot. Yes, it is very important to keep ourselves hydrated; otherwise, we end up with severe life threatening emergency, especially in summer.
* I am not a doctor. I am writing this article thorough my experiences and what I learned from doctors I have seen. Please read this as your reference.
Some might think as long as you drink some fluid, you should be fine. But it is not very true. Do not consider fluids that contain caffeine such as coke or coffee since they act as a diuretic. You still can drink them but make sure that you drink water as well.
If you sweat a lot, it is not only water inside your body you are losing. You are losing electrolytes such as sodium as well. So you have to regain not only water but also these electrolytes.
Have sports drink (Gatorade is well-known in USA. Pocari Sweat in Japan) ready when training. For a long training such as training camp you should not forget to have fruits.
You should keep yourself hydrated so you might want to keep drinking sports drink during training especially in summer.
If you are an instructor or play an instructor role, it is a good idea to make your kendo mates to drink sports drinks when they can (after suburi or during a break).
Hope this helps.