Digestion, absorption and transport of carbohydrates
Whenever we eat something, not only are we eating a delicious snack or meal, we’re also ingesting the molecular compounds and elements that make up those foods. As our food makes its way through our bodies it goes through a series of changes so we’re able to digest it more effectively and extract the nutrients and fuel needed to nourish our body’s cells.
Many of the foods we eat contain carbohydrates - which includes both sugars and starches, and our bodies will metabolise them in three main ways:-
When our body metabolises carbohydrates it results in the production of glucose molecules which are the most efficient source of energy for our muscles and our brains. Everything we eat contributes to cell growth, repair and normal cell functioning, or if too much food (energy) is consumed, we store this excess in various places throughout our bodies.
When we eat food it’s typically made up of many different nutrients and elements that when combined can make up a healthy meal. Most of the food and drink we eat can be broken down into three major parts –proteins, fats and carbohydrates. The others are – vitamins, minerals and water.
What are Polysaccharides? (eg. starch, fibre or cellulose)
What are Disaccharides? (eg. lactose, maltose, sucrose)
What are Monosaccharide? (eg. glucose, galactose, fructose)
All carbohydrates are made from small building blocks called simple sugars or monosaccharides.
When two building blocks or monosaccharides join together they form a disaccharide. The most common disaccharide that we are all familiar with is what we know as sucrose or table sugar.
Apart from sugars, other types of carbohydrates are made up of long chains of monosaccharides or disaccharides, all joined together in different combinations – that can often be very complex – these are called polysaccharides. They usually contain from 10 up to several thousand monosaccharides arranged in chains. The main types of polysaccharides you have probably heard of already are: Starch, cellulose, pectin, gums and fibre.
There are three main monosaccharides that combine to form many of the different types of sugars or disaccharides found naturally in foods.
Glucose – this is one of the most important forms of sugar used by the body for energy. All other carbohydrates (including other sugars) are converted into glucose during the digestion of food. Glucose is naturally found in some fruits and vegetables and the nectar or sap of plants.
Fructose – is also known as fruit sugar, and is the main sugar found in fruits, berries, honey, root vegetables and some grains.
Galactose – this monosaccharide is mostly found in milk and yoghurt.
Sucrose – this is the most common form of sugar and is usually obtained from sugar cane or sugar beet. It can also be found in some fruits and vegetables.
Sucrose = 1 Glucose + 1 Fructose
Lactose – this is what we normally call milk sugar, because it is found in all mammals' milk and dairy products.
Lactose = 1 Glucose + 1 Galactose
Maltose – is found in germinating grains such as barley, as well as in malt or malted foods and beverages. It is often called malt sugar
Maltose = 1 Glucose + 1 Glucose
Starch is probably the most common of the polysaccharides and it is made up of long chains of glucose. Starch is made by plants during photosynthesis. It is present in cereal grains [wheat, oats, rye, barley, buckwheat, rice etc] potatoes and legumes [beans, peas, lentils].
Cellulose – is another long chain polysaccharide made from many glucose building blocks. We often talk about cellulose as dietary fibre or what we used to call "roughage" as the human body is unable to break it down during digestion.
Pectin – is a type of fibre, found mainly in plant walls, which gives fruit its structure and firmness. If you've ever tried to make jam or fruit jelly you will know about pectin! Pectin is found naturally in fruit and vegetables, but in varying amounts, which is why some jams set without added pectin and others do not. Fruits high in pectin include apples and most citrus fruits. Lower amounts are found in berries, stone fruits, figs and rhubarb.
Gums - ever wondered what agar agar, guar gum or xanthan gum are? You may see these written on some food labels. Well they are what we call vegetable gums that are also polysaccharides and they are used primarily as thickeners in food. Some seaweeds are also excellent sources of gums and are commonly called carrageenan and alginates. They are often used as a vegetable substitute for gelatin.
Glycogen – is the stored from of glucose in the human body. The body stores enough glycogen in the liver, muscles and brain to last for 24 hours.
When we eat these foods that contain carbohydrates the body needs to break these down into simple monosaccharides for the body to utilise.
The digestion process of polysaccharides such as starch will begin in the mouth where it is broken down or 'hydrolysed' by salivary amylase [an enzyme in your saliva that helps to break down starches]. The amount of starch hydrolysed in your mouth is often quite small as most food doesn’t stay in your mouth for very long.
Once you've swallowed your carbohydrate food and it reaches the stomach the salivary enzymes that help with digestion are either altered or destroyed so won’t work as effectively. As a result, digestion predominantly occurs in the small intestine with another enzyme, pancreatic amylase, transforming or breaking down [technically hydrolysing] the starch to more manageable molecules of dextrin and maltose.
Further to this, enzymes classed as glucosidases on the brush border wall of the small intestine [a section of the small intestine that helps with the absorption of the digested nutrients] break down the dextrin to maltose and then further onto glucose. The other disaccharides are broken down and converted into their two monosaccharide units.
The monosaccharide units, glucose, galactose and fructose are transported through the wall of the small intestine and then into the portal vein which then takes these elements 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 more slowly 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 of these elements 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 typically 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 bloodstream 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 a metabolic pathway known as 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.
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 then used to maintain basal blood glucose levels between meals, whilst the muscles typically store 400-500g often used during movement.
Once these reserves hit a peak or are saturated, excess glucose is converted to fat for longer-term storage. Consuming more energy than we need from any of these sources results in the storage of excess energy as body fat.
Whilst our bodies need energy from carbohydrates, fats and proteins for normal functioning, it's important to consider the energy we get from all sources so we can achieve 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.
Clemens RA et al. Functionality of Sugars in Foods and Health. Comprehensive Reviews in Food Science and Food Safety 2016: Vol 15, 433-470 https://doi.org/10.1111/1541-4337.12194