In the 3rd episode, Yousuke asks Mikoto to hold his breath for 3 minutes by indulging his head into a bowl of water to let him join the bicycle club. After one day of training, Mikoto manages to hold his breath for 4 minutes. When Yousuke asks him how he achieved that, Mikoto answers that he replaced the water with soda to be able to do it.
I don't understand how using sode instead of water would help someone to hold his breath longer. What is the explanation for this?
I did not watch this Series, nor could I find a official source stating this. This is just a theory I deduced.
Considering he said "see, because it's harder to drink soda." He most likely figured: If you drink, you hold your breath. If you drink soda it's harder/takes longer. So I can hold my breath longer. Looking at the response of him and the people he says it to I find this to be one of the best explanations.
Then the actual physics behind it for him to actually manage to last longer due to this :
The air we breath is 21% oxygen. By inhaling 100% oxygen for a couple of minutes (enough to washout the nitrogen from the functional residual capacity in your lungs), you can pretty much hold your breath about 4-5 times longer than normal. Anesthesiologists routinely do this before putting patients out as a safety thing (hence the mask when you "go off to sleep") because in an emergency the patient can usually "hold" his or her breath for 5-10 minutes after breathing 100% oxygen. And this works because the patient is already unconscious and can't feel his/her CO2 level rising.
Doing this awake, however, is very difficult as you can feel that CO2 rise and even though your lungs are still full of oxygen your brain will be freaking out and demanding that you take a breath. It is possible to overcome that breath instinct, however, with some training or serious dedication. This is part of the reason that hyperventilation (really rapid deep breathing) before breath holding helps you hold your breath - you drive your CO2 level way down and it takes longer for your brain to reach the freakout point while you're breath-holding. This is also why it's dangerous to do so: while that CO2 level is rising again you may well burn through your oxygen reserve (and since your brain still feels fine with regards to CO2) you can lose consciousness from hypoxia. Fun, right? Source
Short Summary:
Due to him getting used to the CO2 level Rising, his brain does not panic him to take a breath. So due to training in CO2 he most likely reduced the point of forcefully needing to breath to 4 minutes. Which is also about the max we can hold in with 21% oxygen.
