Digestion, absorption and transport of carbohydrates
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.
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.
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.
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.
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.
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.