A nice idea, however you have too many variables for a valid experiment. As itdood mentioned eating anything, including sawdust, will cause an increased requirement for oxygen, and in general this would mean an increased heart rate.
Eating in itself will raise heart rate. Independent of the food, eating, having any food in your belly, will raise resting heart rate. This is to aid digestion, to make the most of food being there to absorb nutrients. This increases the amount of blood flowing through the intestines, again, to maximize absorption.