As you are well aware, humans do not live on bread alone, much less by glucose alone. Nor does the typical diet contain exactly the required amounts of every possible nutrient, finely tuned to the activities and metabolic peculiarities of the individual. Accordingly, the cells of the human body seethe with biochemical reactions, synthesizing one amino acid from another, making fats from carbohydrates, and channeling surplus organic molecules of all types into energy storage or release.

     Glucose is one of the most common energy-rich molecules used in ATP production in plant and animal cells (and in fact is virtully the only substance used in some cases, such as in the human brain), and the metabolism of other types of molecules usually interconnect with the glucose pathways. Two examples of such molecular interconversions are the production of ATP from fats and proteins during fasting, and fat synthesis from sugars during surplus food intake. A typical fat molecule consists of three fatty acids connected to a glycerol backbone. Even the leanest people have some fat on their bodies, stored against potential food shortages. During fasting or starvation, the body mobilizes these energy reserves for ATP synthesis, since even the bare maintenance of life requires a continous supply of ATP, and seeking out new food sources demands even more energy expenditure. Metabolism of fats is very straightforward and feeds directly into cellular respiration.

     Some amino acids too are broken down to pyruvic acid, the end product of glycolysis. Just as fats can be funneled into the glucose metabolic pathway for energy production, so too the sugars and starches in corn flakes and candy bars can be converted into fats for long term storage.