Energy is measured in calories. The three components of food that are responsible for most of the calories we consume are fats, carbohydrates, and proteins.
Fats have the most calories. Each gram of fat contains 9 food calories (a food calorie is the energy needed to raise a liter of water by one degree Celsius. Non-food calories (the kind chemists and physicists use) are 1/1000th of a food calorie). A pound of fat contains over 4,000 food calories, enough to support a man for two days.
Carbohydrates are sugars and things made from sugars, such as starches. Carbohydrates contain 4 calories per gram, or just over 1,800 calories per pound – about a day’s worth of calories.
Proteins also contain 4 calories per gram if they are burned in the body instead of used to make new proteins. If there is not enough energy in the other foods we eat, the protein will be burned for fuel. But much of the protein in a normal diet goes into building enzymes, skin, hair, muscle, and fingernails, some of which is lost every day and must be replaced.
Getting enough energy in our food is not the same as feeling energetic. We might feel most energetic in the morning before breakfast, when we have been fasting for 8 hours or more. Or we may be addicted to the caffeine in our morning coffee, and not feel energetic at all until after a cup or more of the stimulant.
So-called energy drinks take advantage of this confusion, and contain both stimulants and lots of sugar. The stimulants wake you up, and make your muscles overactive (jittery) which is often confused with being energetic.