Glucose is what your cells oxidize ('burn') to make energy. Assuming you are reasonably healthy (namely, your liver and pancreas are functioning properly) your body will keep the amount of glucose in your blood around 80 to 110 mg/dL (4.4 to 6.1 mmol/L for those of you not in the US). It does this in a variety of ways... You body can produce more insulin to shift the glucose into the cells, and some cells (mostly in the liver) can chain lots of glucose molecules together into a big 'storage' molecule called glycogen. If your blood sugar drops too low, your body will cut back on insulin production and start releasing glucose from your glycogen stores.

Another side effect of your blood sugar getting a little low is that you get hungry, and being hungry is distracting. It not that your body is _actually_ low on glucose... A normal healthy adult has plenty of glycogen stores, and long before those are depleted your body will start breaking down stored lipids into their component fatty acids, which your cells can metabolize just as effectively as glucose.

http://en.wikipedia.org/wiki/Blood_sugar http://en.wikipedia.org/wiki/Glycolysis