To eat or not to eat

For my weekday workouts, I have been working out in the AM before work lately. This has worked out really well for scheduling purposes, but the problem is - I just do not have as much strength in the AM as I do in the afternoon or night. Last week, I was extra weak from my liquid diet, but even today I wasn't the same as I was during my afternoon work outs over the weekend.

I have always heard that you shouldn't eat before working out because you'll just be burning off the food you ate and not your fat, but my MD says that you must eat before working out so that you have the strength to push yourself as much as you need to. I am going to try eating a low fat protein bar before my work out tomorrow AM to see if that makes a difference - I'll let you know!