How the Body Uses Carbohydrates, Proteins, and Fats
The human body is remarkably adept at making do with whatever type of food is available. Our ability to survive on a variety of diets has been a vital adaptation for a species that evolved under conditions where food sources were scarce and unpredictable. Imagine if you had to depend on successfully hunting a woolly mammoth or stumbling upon a berry bush for sustenance!
Today, calories are mostly cheap and plentiful—perhaps too much so. Understanding what the basic macronutrients have to offer can help us make better choices when it comes to our own diets.
From the moment a bite of food enters the mouth, each morsel of nutrition within starts to be broken down for use by the body. So begins the process of metabolism, the series of chemical reactions that transform food into components that can be used for the body's basic processes. Proteins, carbohydrates, and fats move along intersecting sets of metabolic pathways that are unique to each major nutrient. Fundamentally—if all three nutrients are abundant in the diet—carbohydrates and fats will be used primarily for energy while proteins provide the raw materials for making hormones, muscle, and other essential biological equipment.
Proteins in food are broken down into pieces (called amino acids) that are then used to build new proteins with specific functions, such as catalyzing chemical reactions, facilitating 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.
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. "We are really good at storing fat," says Judith Wylie-Rosett, EdD, RD, a professor of behavioral and nutritional research at Albert Einstein College of Medicine.
Carbohydrates, on the other hand, can only be stored in limited quantities, so the body is eager to use them for energy. "We think of carbs as the [nutrient] that's used first," says Eric Westman, MD, MHS, director of the Lifestyle Medicine Clinic at Duke University Medical Center. "We can only store a day or two of carbs." The carbohydrates in food are digested into small pieces—either glucose or a sugar that is easily converted to glucose—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 gobble up this mealtime bounty of glucose more readily than fat, says Wylie-Rosett.
Once the cells have had their fill of glucose, the liver stores some of the excess for distribution between meals should blood glucose levels fall below a certain threshold. 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. 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.
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 finicky cells need the body to make fat-like molecules called ketone bodies. This is why a very-low-carbohydrate diet is sometimes called "ketogenic." (Ketone bodies are also related to a dangerous diabetic complication called ketoacidosis, which can occur if insulin levels are far too low.) Ketone bodies could alone 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.