While breathing supplies oxygen for metabolismic processes, the body requires much more to function. Oxygen merely serves as a vessel for electron transfers in many of the biochemical cylces that take place.
The power resource of life is called ATP (Adenosine triphosphate). This ATP is mainly acquired through glycolysis (spelling? I aint english native) in which various sugars like GLUCOSE, LACTOSE and FRUCTOSE are broken down to PYRUVATE. When breaking chemical bonds, energy emerges - this energy powers your body functions.
On the other hand, creating these bonds requires energy. PROTEINS, which are a vital part of nutrition, are used to create various AMINO ACIDS.
Although many of the essential AMINO ACIDS the body uses to digest, form tissue, and create certain compounds are created by the body itself, some of them have to be acquired through food we eat.
Biochemistry is very complex and I am just a biology student but just imagine many cycles of chemical reactions as the body's power plant and also waste ejection. These cycles have blank spots which have to be filled by nutrition. Lack of VITAMINS, PROTEINS or AMINO ACIDS prevents body functions from being executed correctly or working at all. This will cause weakness / illness or death eventually.
When deprived of food, the body will use GLUCONEOGENESIS to create sugars from your reserves - first fat, then your muscles are dissolved to keep the juices flowing. Eventually these are gone and starvation occurs.