Is a platoon sergeant authorized to prohibit SM from drinking sodas or anything besides water? What regulations apply?

I have told my Soldier that he is not to drink anything except water, unsweetened tea, and coffee black.
Background, he is fat and needs to cut calories. This alone cuts 500 calories a day from his diet. Does he understand why I did this, yes, because I explained it to him.

Good Grief- in a nutshell YES- may not find it in the regs at 1st. the PSG is responsible for the training, health, welfare of his troops. Is the SM perhaps out of shape, failing AFPT, or tape tests? Is the soda affecting his duties. If the answer to any of these is Yes, the PSG has 2 options- try to help the SM, watching his diet, extra PT, watching calorie intake- or just throw the SM out of the Army- which would you prefer?

No disrespect, but that's where the term Generation y comes from with me. Many here served long before me, and I retired 20 years ago. We rarely, if ever questioned authority. If someone said jump, we didn't ask why (y) or in which direction, or how high. We jumped. We trusted our leaders and their experience enough to know that it was probably for our own good. Not telling you to disregard illegal orders, just that we see several posts a week on RP on "can somebody do this to me?" Use your Chain of Command and find out, read the orders for yourself, and then decide. You will be a Sergeant someday and won't have RP, so start thinking and acting on your own. Or seek advice from your NCOs. We were told in Boot Camp that discipline was "instant and willing obedience to orders without hesitation". Don't take offense, but instead think about it. Do you want your orders followed, or are they up for debate too? It's a two way street.