The Basics

Digestion, absorption and transport of carbohydrates

    • Carbohydrates are broken down to provide glucose for energy
    • Digestion predominantly occurs via enzymes lining the wall of the small intestine
    • Once absorbed, galactose and fructose are metabolised further by the liver to produce glucose and minimal amounts of other metabolites 

___________________

The metabolism of carbohydrates is the process of getting the carbohydrates in the foods we eat into the right format to provide fuel to our body's cells. This process involves digestion, absorption and transportation. Most commonly, carbohydrate metabolism results in the production of glucose molecules which are the most efficient source of energy (ATP) for our muscles and our brains. Energy or fuel from our food is used for cell growth, repair and normal cell functioning.

Digestion

Carbohydrates are most commonly consumed as polysaccharides (e.g. starch, fibre or cellulose) or disaccharides (e.g. lactose, sucrose, galactose) and therefore need to be broken down into their simpler monosaccharide forms which the body can utilise.

The digestion process of polysaccharides such as starch will begin in the mouth where it is hydrolysed by salivary amylase. The amount of starch hydrolysed in this environment is often quite small as most food does not stay in the mouth long. Once the food bolus reaches the stomach the salivary enzymes are denatured. As a result, digestion predominantly occurs in the small intestine with pancreatic amylase hydrolysing the starch to dextrin and maltose.

Enzymes classed as glucosidases on the brush border of the small intestine break down the dextrin and maltase, lactase and sucrase convert the other disaccharides into their two monosaccharide units.

Absorption & transport

The monosaccharide units, glucose, galactose and fructose are transported through the wall of the small intestine into the portal vein which then takes them straight to the liver. The mode of transport varies between the three monosaccharides and is described in brief below. Both glucose and fructose are absorbed relatively quickly, depending on what other nutrients are eaten at the same time. For example a meal or food containing protein and fat causes the sugars to be absorbed slower than when consumed on their own.

  • Glucose, at low concentrations is transported through the mucosal lining into the epithelial cells of the intestine by active transport, via a sodium dependant transporter. At higher concentrations, a second facilitative transporter becomes involved. From the epithelial cells glucose is moved into the surrounding capillaries by facilitated diffusion.
  • Galactose is transported in the same way as glucose, utilising the same transporters. As galactose is not found as a monosaccharide in nature, absorbed galactose primarily comes from the breakdown of lactose.
  • Fructose moves entirely via facilitated diffusion. The process utilises a different transporter to glucose when entering the enterocytes, however both fructose and glucose utilise the same transporter to exit the enterocyte into the capillaries. The absorption of fructose is much slower than that of glucose and is quantitatively limited. Consumption of large amounts of fructose has been shown to produce a level of fructose malabsorption in almost all cases. Co-ingestion of glucose with fructose has been shown to facilitate fructose absorption. The exact mechanisms for this are still unknown.

Once in the liver galactose and fructose are removed from the blood and converted into other metabolites. When eaten in moderate quantities, most fructose is taken up by the liver and converted to glucose, glycogen and lactate. A fraction may also be oxidised or converted into fatty acids and uric acid. Only a small amount of fructose reaches the bloodstream, so blood fructose concentrations are always low. Galactose is primarily converted into glucose and stored as glycogen.

On the other hand most of the glucose derived from food is transported via the blood stream to the peripheral tissues where, under normal circumstances, the hormone insulin enables it to be taken up by the cells and used as an energy source via the glycolysis pathway. As glucose is the most important fuel source for the body and in particular the brain the body attempts to keep a basal circulating blood glucose of around 4-5mmol/L. This homeostasis mechanism is predominantly controlled by the actions of glycogen and insulin.

Storage

Surplus glucose is initially stored as glycogen in the liver or muscles. The liver can store approximately 100g of glycogen which is used to maintain basal blood glucose levels between meals, whilst the muscles typically store 400-500g often used during movement. Once these reserves are saturated, excess glucose is converted to fat for longer term storage.

Our bodies need energy from carbohydrates, fats and proteins for normal functioning. Consuming more energy than we need from any of these sources results in storage of excess energy as body fat. So it's important to consider the energy we get from all sources in achieving a balanced diet.

Exceptions

The most notable exception to the carbohydrate metabolism explained above is dietary fibre. Dietary fibre, a type of polysaccharide, can be classed as either soluble (dissolves in water) or insoluble (cannot be dissolved in water). The body cannot digest or absorb dietary fibre like other carbohydrates. Instead, a portion is fermented by colonic gut bacteria. As a result, it passes relatively untouched through the digestive system and is removed in stools. 

NEXT: Sources and types of carbohydrates and sugar

MORE ABOUT SUGAR

resources.jpg

Resources

Latest resources, fact sheets and scientific studies.

appleIco.jpg

Sugar and health

View and download articles, reports and fact sheets containing current findings about the role of sugar in our diets.

askQuestionIco.jpg

Frequently asked questions

Explore the most common questions asked about sugar, and read up on some of the prevailing misconceptions.