Fat is energy-dense
Fat contains a lot of potential energy.
A fatty acid such as palmitate can regenerate approximately 106 molecules of ATP. If you have had a triglyceride with three palmitate fatty acids, you'd get 3 x 106 = 318 ATP molecules produced. Since glycerol also contributes to ATP regeneration, helping produce 19 more ATP molecules, the breakdown of each triglyceride generates a whopping 337 ATP molecules.
Compared with the 36 ATP molecules generated by one glucose, fat emerges as an ATP superstar.
Now, this might inspire you to think "hey, I should just use only fat for energy!", but there are a few issues here:
- Because fat us so energy-dense, a small volume of dietary fat adds up fast. We could easily meet our energy needs for the day with some handfuls of nuts and a few generous spoonful's of coconut oil. If you don't like eating and don't care about malnutrition, this is a possible option for meeting your basic energy, but if you're looking for a normal life and maybe some other vitamins and minerals, it's not necessarily an ideal choice.
- Fat oxidisation is efficient but slow. This is because it's aerobic (from the ancient Greek aero, or "air") and requires oxygen to work. We can only take in and transfer a certain amount of oxygen. So, if we were just lying around, we could survive without eating for quite a long time, perhaps even a couple months, living off our body fat stores.
- When we're active, we generally need more energy faster than that fat can provide. So, we stop using the fat oxidation pathway as our main fuel source, and use mostly glycolysis instead. This process is described as anaerobic, because we're not using oxygen.