Think about it from the perspective of your metabolism. If you would sit the whole day on your chair without any real movement your circulation will slow down, which will also influence the blood and as such substrate supply to your brain. The impact of a slowed circulation is a reduced metabolic rate and as such reduced energy needs. Whereas regular standing up and walking around can actually increase your metabolism somewhat, increase/improve your circulation and therefore not only help with weight maintenance but also with concentration.

As for liquid intake which is just as important as activity during a long office day try to download water aid onto your screen it will prompt regulalry to have a glass of water and should encourage you to achieve balance hydration levels and might make your stand up more often as well.