Everything in this world can be rated by quality. This includes fuels: some burn efficiently and are considered high performance fuels while others come up a little short. The same holds true for the fuel for our biological machines. Not all food is created equal and our bodies burn high quality food differently than its lower quality versions.

For instance, there’s been a lot of hype about the glycemic index of foods and the GI diet. Basically, the GI ranks carbohydrate-containing foods and measures their effects on the body’s blood sugar levels. Foods with a high GI (or sugar content) get digested and metabolized faster by the body.
Protein, on the other hand, tends to be a high performance fuel. It takes longer for the body to process it and so it also satisfies our hunger long after consuming it. So comparably, carbohydrates are a low-grade fuel used for immediate energy sources.
