The human body is exceptionally clever machine that will make do with whatever type of foods are available to it. The mere fact we can survive on a variety of diets has been a vital tool for a species that evolved under conditions where food sources were scarce and unpredictable. Imagine a world where you had to depend on successfully hunting a woolly mammoth, discovering the carcass of a previous predators kill or finding berry bush for survive and you have the world our ancestors evolved in!
Today, for many of us, calories are mostly cheap and plentiful, and in reality probably too easily available. Understanding what the basic macronutrients have to offer can help us make better choices when it comes to structuring our own diets.
With every bite of food we eat, each portion of nutrition starts to be broken down for use by the body and so our bodies metabolism gets to work. A series of chemical reactions begins, that transforms food into components that can be used for the body's basic processes. These being Proteins, carbohydrates, and fats travel through various sets of metabolic routes that are specific to each major nutrient.
Basically if all three nutrients are available in your diet, then carbohydrates and fats will be used primarily for energy while proteins provide the raw materials for making hormones, muscle, and other essential biological equipment.
Think carbs and fats = fuel, protein = bodies building blocks.
Proteins in food are broken down into pieces (called amino acids) that are then used to build new proteins with specific functions, such as allowing communication between different cells, or transporting biological molecules from here to there. When there is a shortage of fats or carbohydrates, proteins can also yield energy. It really is the bodies swiss army knife that can do it all.
Fats typically provide more than half of the body's energy needs. Fat from food is broken down into fatty acids, which can travel in the blood and be captured by hungry cells. Fatty acids that aren't needed right away are packaged in bundles called triglycerides and stored in fat cells, which have unlimited capacity.
Carbohydrates, on the other hand, can only be stored in limited quantities, so the body is eager to use them for energy. We can only store a day or two of carbs, with in the muscles before they are stored as fats. The carbohydrates in food are digested into small molecules known as glucose or a sugar that is easily converted to glucose, (like fructose or lactose), that can be absorbed through the small intestine's walls. After a quick stop in the liver, glucose enters the circulatory system, causing blood glucose levels to rise. The body's cells hoover up this mealtime glut of glucose more readily than fat.
Once the cells are full of glucose, the liver stores some of the excess for distribution between meals should blood glucose levels start to fall below a certain level. If there is leftover glucose beyond what the liver can hold, it can be turned into fat for long-term storage so none is wasted (fat). When carbohydrates are scarce, the body runs mainly on fats. If energy needs exceed those provided by fats in the diet, the body must liquidate some of its fat tissue for energy (achieved through creating caloric deficit).
While these fats are a welcome source of energy for most of the body, a few types of cells, such as brain cells, have special needs. These cells could easily run on glucose from the diet, but they can't run on fatty acids directly. So under low-carbohydrate conditions, these cells need the body to make fat-like molecules called ketones. This is why a very low carbohydrate diet is often referred to as a "ketogenic." Ketone bodies could on their own provide enough energy for the parts of the body that can't metabolize fatty acids, but some tissues still require at least some glucose, which isn't normally made from fat. Instead, glucose can be made in the liver and kidneys using protein from elsewhere in the body. But take care: If not enough protein is provided by the diet, the body starts chewing on muscle cells.