The energy or calories in the food we eat comes from three macronutrients: proteins, fats, and carbohydrates. Macro means large, and these nutrients are needed in large quantities to sustain our growth, metabolism, and other bodily functions.
Our bodies require other nutrients, too, including vitamins and minerals. However, these nutrients are required in smaller quantities and are therefore called micronutrients. While critical to our health, micronutrients do not provide us with energy or calories.
Protein is the main component of our organs, muscles, all our living cells, and almost all our body fluids. Proteins are chains of amino acids linked together in complex formations.
There are 20 different amino acids, and all of them must be present in order for our bodies to build, maintain, and repair themselves. Nine of the 20 amino acids are considered essential because they cannot be manufactured by your body; they must come from food sources.
Proteins that contain all 20 amino acids are called complete proteins, and they are found in animal sources: meat (poultry, fish and other meats) and dairy (eggs and milk products). Proteins that come from plant sources are considered incomplete because they do not contain all 20 amino acids, though you can combine different plant sources to obtain all of them.
It is a common misconception that you must eat animal products in order to supply your body with adequate protein. In fact, if you compare meat and dairy to dark green vegetables, soybeans, and other plant sources, you will find that the plant foods often contain more protein -- based on an equal number of calories -- than their animal counterparts.
And speaking of animal counterparts, gorillas become huge and muscular without eating meat or dairy.
While consumption of unhealthy fats should be kept to a minimum, fat, like protein, is necessary to maintain a healthy body. It is a vital component for building body tissue and cells, and it aids in the absorption of some vitamins and other nutrients. And just as there are essential amino acids, there are essential fatty acids which must come from food sources.
Many people eat too much of the bad fats, but also eat too little of the good fats required for optimal health. In our topic on fats we discuss saturated fats, unsaturated fats (monosaturated and polysaturated), trans fats, the essential fatty acids (primarily omega-3 and omega-6), and cholesterol.
It's not just the fat we eat that can become fat on our bodies. Any macronutrient not immediately needed by our bodies is stored in our energy reserve of body fat. When needed, it can be broken down and used for energy. Though all too often it is just left to sit there.
Carbohydrates are chains of small, simple sugars, and are the body's main source of fuel. They are broken down and enter the bloodstream as glucose. Excess glucose is stored in the form of glycogen in the liver and, in limited quantities, the muscles.