How do humans get the energy out of the food they eat?
Humans extract energy from food by breaking the chemical bonds in macronutrients (proteins, fats, and carbohydrates) during digestion. Carbohydrates are particularly important fuel molecules, and the human body breaks it down into carbon dioxide and water, releasing energy in the process. The energy is shuttled around the body in carrier molecules such as ATP and NADH. Food molecules are broken down in three stages to produce ATP:

  - **Stage 1:**  Digestion, which occurs either in the intestine outside cells, or in a specialized organelle within cells, the lysosome, breaks down large food molecules into simpler ones. Proteins --> Amino Acids, Carbohydrates --> Simple Sugars, Fats -> Glycerol.
  - **Stage 2:**  Glucose is broken down into an even simpler molecule, pyruvate, creating ATP and NADH. Pyruvate is then turned into acetyl CoA which is another energy carrying molecule. Fats can also be turned into acetyl CoA.
  - **Stage 3:**  Acetyl CoA is transported to the mitochondria, an organelle which is specialized for energy production. There, acetyl CoA is oxidized into CO2 and large amounts of NADH are produced. Finally, NADH is consumed in the mitochondria using molecular oxygen (O2) to make ATP. This final step releases the most of energy during metabolism.

In short, people get energy from food by 1) digesting complex nutrients to simpler ones, 2) breaking down simple molecules into even simpler forms, and 3) and then using the molecules from stage 2 to make NADH and ATP, the energy currency of the living things.
Ok, this explains how the body turns fats and carbohydrates into energy. Is it also possible for the body to get energy by digesting proteins?
Yes - great question! Protein are broken down into amino acids during the digestion in **Stage 1**. Some amino acids, like tryptophan, tyrosine, and phenylalanine, can be converted to into acetyl-CoA like fats and carbohydrates in **Stage 2**. From there on, the ATP and NADH production in **Stage 3** is identical to that of the fats and sugars.