My tae kwon do instructor always tells his students to avoid soft/energy drinks (like Pepsi, Coke or Red bull etc.) Do these kind of drinks harm your flexibility and stamina? What do experts say about it? PS scientific evaluation please.
From a metabolic point of view, when you ingest the simple sugars (primarily glucose) present in sodas you get a spike in insulin production. The insulin mediates the uptake of the glucose by your cells where it is converted (either aerobically or anaerobically) into energy plus various waste products (CO2, lactic acid). The problem is that your metabolism will work through the supply of glucose quickly, and with the spike in insulin will clearing you bloodstream of glucose you will become hypoglycemic... literally moving from feast to famine from the perspective of your cells.
Your best bet in the case of sugars is to modulate the rate that your body is taking in the glucose, stretching it out over a prolonged period of time. You could simply nursing your drink. A better approach is to take in the same amount of sugar as glycogen (a polymer of glucose), which takes longer to digest.
Similar effects are seen with pretty much any chemical that you introduce into your body, be it sugar or caffeine or nicotine or whatever.
From personal experience, I tried to stay away from soda and/or energy drinks because the sugar crash (for the stamina side of things, I know nothing of its effects on flexibility) seemed like a pretty big liability for me, especially if you still have fights to do after. I also found in my competitions that it's not unheard of to have to wait for 15-20 minutes in case of technical difficulties. I would be nervous that the sugar high I was counting on would burn out. It may work for some other players though. Some of my teammates eat a bite or two of a chocolate bar while in the holding area before they go out.
This is all from personal experience though.
I would nudge you toward eating correctly before your fight and specifically for taekwondo, to veggy-load.
Hope this helps!
Personally, I think they're fine; they're good for quick energy when you're in between rounds. Soft drinks and such in fact will harm your flexibility, but only because if you end up accumulating rolls of fat you'll have trouble stretching at times, so it's an overblown concern. (You would be surprised, even some fairly lean people complain about fat getting in the way of kicks/stretching)