Carbohydrates, fats and proteins are macronutrients. We require them in relatively large amounts for normal function and good health. These are also energy-yielding nutrients, meaning these nutrients provide calories. Carbohydrates, protein and fats are macronutrients, meaning the body requires them in relatively large amounts for normal functioning. The Recommended Dietary Allowance (RDA) for carbohydrates for children and adults is 130 grams and is based on the average minimum amount of glucose used by the brain. The Acceptable Macronutrient Distribution Range (AMDR) for carbohydrates is 45–65%. If, for instance, you ate 1600 kcals per day, the acceptable carbohydrate intake ranges from 180 grams to 260 grams.

Carbs are fuel. Glucose is the primary fuel for most of your cells and is the preferred energy for the brain and nervous system, the red blood cells and the placenta and fetus. Once glucose enters the cell, a series of metabolic reactions convert it to carbon dioxide, water and ATP (Adenosine Tri-Phosphate), the energy currency of the cell. If you have more available glucose than your body needs for energy, you will store glucose as glycogen (glycogenesis) in your liver and skeletal muscle. When your blood glucose drops, as it does when you’re sleeping or fasting, the liver will break down glycogen (glycogenolysis) and release glucose into your blood. Muscle glycogen fuels your activity. The body can store just a limited amount of glucose, so when the glycogen stores are full, extra glucose is stored as fat and can be used as energy when needed.

Carbs spare protein. If you go without eating for an extended period or simply consume too little carbohydrate, your glycogen stores will quickly deplete. Your body will grab protein from your diet (if available), skeletal muscles and organs and convert its amino acids into glucose (gluconeogenesis) for energy and to maintain normal blood glucose levels.