The Structure of Carbohydrates
Carbohydrates is a general term for sugar, starch, and fiber.
- Monosaccharides – also known as simple carbohydrates. They consist of a single
sugar unit. Examples include glucose, fructose, and galactose.
Disaccharides – consist of two linked sugar units. For example, sucrose (
, or white sugar) consists of 1 glucose + 1 fructose, and lactose (milk sugar) consists of 1 glucose + 1 galactose.- Polysaccharides – consist of a large number of linked sugar units, typically 20–30 units
, and are commonly referred to as starch.
The Functions of Carbohydrates
Carbohydrates are not essential or vital for us. We can produce them ourselves
through the breakdown of protein and fat.
- Energy source
The primary function of carbohydrates is to provide energy, especially during high-intensity
activities, when carbohydrates are the main source of energy.
In response to starvation, depleted glycogen stores, and insufficient carbohydrate intake,
the body begins to break down fat and protein in greater quantities in order to produce glucose from
amino acids and glycerol.- Provides energy to nerves and red blood cells.
For these cells, carbohydrates are normally the only source of energy.
Recommended daily intake: About 50% of the energy in the diet. At least 120–200 g per day.
Digestion, Absorption, and Metabolism of Carbohydrates
Digestion and absorption
- The breakdown of carbohydrates begins in the mouth with the help of the enzyme amylase in saliva.
- Enzymatic breakdown stops in the stomach.
- Most of the breakdown occurs in the small intestine with the help of amylase.
- The monosaccharides are then absorbed into the intestinal lining. The indigestible fibers then continue on to the large intestine, where they are broken down by bacteria and other microorganisms.
- Glucose is the primary breakdown product of carbohydrates. The hormone insulin plays a role in the uptake of glucose into cells.
- In adipose tissue or in the liver, glucose can be converted into glycerol to form triglycerides.
- Glucose can also be converted into glycogen in muscles.
Metabolism
The body uses carbohydrates as an energy source during high-intensity exercise.
Intensity: The higher the intensity, the less fat is available as an energy source, and the body switches to using its carbohydrate reserves.
Duration: Carbohydrate stores are depleted relatively quickly. After one hour of high-intensity exercise, 50% of the liver’s glycogen is depleted, and after about two hours of high-intensity exercise, it is nearly depleted. Fat then becomes the primary source of energy.
Fitness level: Fit people are better able to utilize their fat stores. This conserves carbohydrate reserves and allows them to maintain a high intensity during exercise.