Imagine a dual fuelled car. It has an electric motor, which provides powerful torque and powers the onboard computer. Then it has a petrol engine to keep it going for miles.
Now look at the human body. It has a big brain that likes carbohydrate for fuel, and needs glycogen for intense muscular contractions. However, it only has 2000 kcals of carbohydrate in reserve. In comparison, we have ample fat reserves, often in excess of 100,000 kcals.
Insulin is the storage hormone; when we eat it is released to put what we have eaten into our reserves. Some foods cause the release of more insulin than others. If we eat foods that spike insulin levels, we can’t burn fat for fuel because the hormone is telling our cells to store the fat.
Keeping protein out of the equation, because it is ideally only used for maintenance and repair, about a third of the fuel most people use in a day is carbohydrate, with the remaining two-thirds fat, either ingested or from stored body-fat. This is the ideal; the more we deviate from this, the greater the problems we have in trying to attain balance.