Sugar is burnt up as energy pretty much immediately, no matter what you're doing. Your body always burns fuel; if you intake lots of sugar, it burns that before it will burn fat or anything else. The problem there is that whenever you're burning fuel, you're burning away nutrients in your body, the good stuff as well. So you're burning sugar and nutrients instead of fat and nutrients, leaving fat just sitting there.
For those who are interested, caffiene is not an energy source. Caffiene is a toxin. The kick it provides comes because you ingest a toxin whihc rapidly spreads, and your body kick sinto overdrive briefly to fight it down, get rid of it. It's like a kick in the bum. But it's not an energy boost.