Most of the food we take in is used to provide us with energy. This energy is used in three activities:
1) Processing of newly ingested food.
Max Kleiber's (1947) Basal metabolism expenditure rule:
kcal/day = 70 * weight^0.75
This rule works across species, but not within species. Food deprived animals have lower basal metabolism -- the body strives to keep its weight.
Large carbohydrates are broken into simple carbohydrates such as sugar to gives us energy; e.g., glucose. To store glucose, the liver combines excess glucose molecules (i.e., glycogen). Storing and using this energy is controlled by two protein hormones from the pancreas: glucagon and insulin.
Glucagon: Promotes breakdown of glycogen to glucose. Secreted from the
alpha cells of the islets of Langerhans within the pancreas.