Carbohydrates, fats and proteins are the three macronutrients the body uses for fuel and building material. Each one is digested into small units, absorbed and then used or stored.
Carbohydrates
Carbohydrates are broken down to glucose, the brain's main fuel. Extra glucose is stored as glycogen in the liver and muscle, and the rest is turned into fat.
Fats
Fats (lipids) are the densest energy store, with about 9 kcal per gram compared with 4 kcal for carbohydrates and proteins. They also form cell membranes, carry fat-soluble vitamins and are used to make steroid hormones.
Proteins
Proteins are built from amino acids and form muscle, enzymes, antibodies and carriers such as albumin and hemoglobin. The body has no true protein store, so long illness or starvation breaks down muscle.
Fasting and starvation
Between meals the liver releases glucose from glycogen, a supply that lasts about a day. After that the body burns fat, makes ketones and breaks down muscle protein to make new glucose.
| Nutrient | Broken down to | Stored as |
|---|---|---|
| Carbohydrate | Glucose | Glycogen in liver and muscle |
| Fat | Fatty acids and glycerol | Fat tissue |
| Protein | Amino acids | No true store (muscle is used) |
At the bedside
Weight loss and muscle wasting point to poor nutrition, which slows wound healing and recovery. Blood glucose and lipid tests show how the body is handling carbohydrates and fats.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).