The macronutrients carbohydrate, protein and fat are all broken down in the digestive system so the body can use them for energy. After they’re digested, the energy is sent throughout the body wherever needed.
The way macronutrients are broken down and converted into energy is a complicated process, but here are the basic steps that outline how your body turns carbohydrates into energy.
Carbohydrates are found in fruit, vegetables, grains and other starches. Digestion for carbohydrates actually starts in the mouth. Enzymes in saliva begin to break down carbohydrates. Carbohydrates travel through the esophagus, stomach and enter the small intestine.
In the small intestine, carbohydrates get further broken down into single carbohydrate units called monosaccharide. These single molecules get absorbed across the intestine wall and are sent through the blood stream. Carbohydrate in the blood is in the form of a monosaccharide called glucose.
The more carbohydrate eaten at one time, the more glucose is going to be released into the blood after digestion.
In the blood
When glucose levels in the blood are high, like after a meal, the body sends a message to the pancreas to release the hormone insulin. Insulin acts like a gate opener to the body’s cells. Without insulin, the glucose can’t enter cells to be used for energy.
When cells don’t respond to insulin or if the pancreas stops making insulin with diabetes, the body’s cells can’t get glucose in the cell. Therefore, the cells won’t have any energy.
In the muscle cell
When glucose is now in the cell, thanks to insulin, the glucose can go through metabolic pathways to be broken down for energy or stored for later use. In order for the cell to use glucose for energy, it has to be converted to adenosine tri-phosphate (ATP). ATP is like the money “currency” for the cell to use as energy.
Glucose has 2 different metabolic pathways it can go through, depending on the needs of the cell. For example, during high intense, short exercise glucose can give energy to muscle cells through anaerobic glycolysis (break down of glucose). The pathway provides ATP from glucose fast and also makes lactic acid as a byproduct.
For longer duration, lower intensity exercise, glucose can be broken down via aerobic glycolysis. This process gives more ATP production for the cell, but takes longer.
If energy is not immediately needed for the muscle, the cell can store the glucose. Glucose molecules stored in muscle or liver tissue is called glycogen. As the muscle cell needs energy, the glycogen will be broken down and go through anaerobic or aerobic glycolysis.
Muscle cells rely on carbohydrate to energy, but other cells in the body can use carbohydrates for energy with these same metabolic pathways.
To the liver
After a meal, glucose is sent to the liver. The liver can also store glucose molecules as glycogen, like the muscle cells. The liver can use glucose for energy through glycolysis. Glucose can also be stored as fat if glucose is not needed for energy.
What happens when glucose levels are low?
Carbohydrate’s main role in the body is for energy, and blood levels of glucose are tightly regulated by the body. When blood glucose levels get too low, the hormone glucagon gets released to raise blood glucose levels.
When blood glucose levels get too low, that is a signal that stored carbohydrate, glycogen, needs to get broken down to raise blood glucose levels to a normalized level.
Glucagon will act on glycogen in muscles and the liver to break off glucose molecules. These molecules enter the blood and can travel to where they are needed for energy in the body.
When blood glucose levels are low, that is also a physiological signal to eat. Many people can feel when they have low blood sugar, and it is the body’s way of trying to replenish energy stores.
When you go long periods without eating, or are following a low carbohydrate diet, the body can make carbohydrates out of protein or fat. This process is called ketosis and can have unpleasant side effects such as weakness, fatigue, bad breath or headaches.
The brain and central nervous system use carbohydrates as the only energy source. Therefore, it is important the body can make carbohydrates when there are not enough in the diet or long periods of semi-starvation. However, long term ketosis could have potential negative side effects.
Carbohydrates are a main source of energy for the body; they can be metabolized to ATP fast or slow depending on the body’s needs. Insulin brings glucose from the blood into the cells to be used for energy. The cell can either use it right away for energy or store it to use later.
When blood sugar gets too low, the hormone glucagon regulates blood sugar levels by stimulating breakdown of glycogen. Blood sugar is tightly regulated, and when there are factors affecting glucose regulation, like diabetes, energy levels can suffer.