How do you improve the nutrition centre as there is only room for 1 player? Thanks in advance. So far all the answers I've had have been useful, it's much appreciated.

There is a little + symbol in the upper right of the header for the nutrition center. If you click on that you can upgrage to the next level (the cost is $50,000 to upgrade). If you don't have enough money, the plus symbol is probably not available.

If you funds are low now, I would wait on nutrition center for a while. It doesn't have any effect in the current engine. It was added in preparation for the new engine and there's debate on when the new engine will be implemented.
At the moment, it's hard to say when would be a good time to purchase the nutrition center as we really won't understand it's full effect on the engine and until it's implemented. I would definitely upgrade to full VIP and exec seats first though.