Just a quick note to steer you in the right direction on working out on an empty vs. full tummy. Here  is a new study concluding the benefits of fasting before endurance exercise such as running. From the study, "Minimising carbohydrate (CHO) status in the peri-training period may accelerate the training adaptations normally observed."

That is quite amazing, since conventional wisdom has us believe that we must fuel up before exercise and that eating carbohydrates before exercise is the only way to obtain enough energy to get you through the session. Check out their conclusion.

The FAST group showed a significantly greater training-induced increase in VO(2max) and resting muscle glycogen concentration than FED (P=0.014 and P=0.047 respectively), but there was no gender interaction. In conclusion, these results suggest that (a) meal ingestion prior to daily exercise can modify some of the exercise training-induced adaptations normally seen with endurance training compared to when daily exercise is undertaken in the overnight-fasted state; and (b) the extent of these adaptations in skeletal muscle differ slightly between men and women.

Athletes got better results with fasting, but men even more so than women, most likely due to the higher testosterone a man carries.

Thanks Matt Metzgar