Carbohydrates and Sugars
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As the human body’s preferred energy source, carbohydrates are simple and quickly broken down and absorbed. When broken down, carbohydrates form sugars.
There are multiple types of carbohydrates, but they all contain carbon, hydrogen and oxygen molecules.
For most people, carbohydrates form the bulk of an individual’s overall caloric intake compared to proteins and fats.
This article will cover the functions, structure and metabolism of carbohydrates.
Most cells in the body prefer glucose as their primary energy source over other sources (e.g. fatty acids). Cells such as red blood cells and the neurons in the brain can only use glucose for energy. Therefore a regular supply of glucose from their most readily available form, carbohydrates, is required.
Excess glucose is stored as glycogen. In times of low glucose, it can be broken down and used for cellular energy. Glycogen is stored in muscle and liver, but is limited in supply, meaning that prolonged exercise can deplete these stores, and cause a switch to the less preferable sources of glucose – lipids and proteins.
Glucose is a key building block in forming macromolecules such as DNA, RNA and ATP.
When inadequate glucose is available in the body, it is formed from amino acids, mainly from breaking down muscle tissue. Adequate glucose prevents this muscle breakdown from occurring.
Adequate glucose levels prevent the body from using lipids as an energy source, thus preventing ketosis.
From a chemical viewpoint, carbohydrates can be divided into simple carbohydrates known as sugars, and complex carbohydrates, known as starches and fibre.
Simple carbohydrates consist of monosaccharides (single sugars) and disaccharides (pairs of single sugars).
Three monosaccharides are found in food, each of which has a chemical formula of C6H12O6 but different structures.
The three monosaccharides are glucose, galactose and fructose. Glucose and galactose are hexagonal in shape, whereas fructose is pentagonal. Other sugars are deoxyribose and ribose.
There are three disaccharides in food. Each contains glucose paired with a second glucose, a galactose or a fructose molecule.
Disaccharides are linked together via a condensation reaction and are broken apart via hydrolysis. The three disaccharides are sucrose, maltose and lactose.
Compared to simple carbohydrates, complex carbohydrates contain many glucose units in long chains, called polysaccharides.
In nutrition, there are three key structures of polysaccharides:
- Glycogen: found in limited amounts in meats, and not in plants. Glycogen is hence not a significant source of carbohydrates. However, carbohydrates in the body are stored as glycogen.
- Starches: plants store carbohydrates as starches. Starches are hydrolysed during digestion, and thousands of glucose molecules are released.
- Fibres: fibres are the structural parts of plants. Whilst made up of polysaccharides, the bonds between their monosaccharides are unable to be broken down by human digestive enzymes, meaning they contribute little in the way of monosaccharides and hence, energy. Fibre is excellent for bowel health.
The goal of carbohydrate digestion is to break down sugars and starches so they can be absorbed and used for energy.
Large molecules (e.g. starch) require significant breakdown, whereas disaccharides require just one bond to be broken, and monosaccharides require none.
There are several stages and places where carbohydrates are digested.
Mechanical digestion occurs via chewing, allowing for the physical breakdown of food into smaller chunks, increasing surface area.
Chewing also stimulates the production of saliva, which contains salivary amylases. These amylases start to hydrolyse starch shorter polysaccharides and disaccharides. Only a small amount of digestion occurs in the mouth.
The swallowed food bolus enters the stomach and mixes with stomach acid and digestive enzymes, inactivating the salivary enzymes.
The food then enters the small intestine, where the bulk of digestion occurs. Pancreatic amylase is released and enters the duodenum via the pancreatic duct, and acts to break down the polysaccharides into maltose and short glucose chains.
Once only disaccharides remain, specific enzymes are used to form monosaccharides:
- Maltase: breaks down maltose into two glucose molecules
- Lactase: breaks down lactose into one glucose and one galactose molecule
- Sucrase: breaks down sucrose into one glucose and one fructose molecule
At this point, the absorption of monosaccharides occurs.
Once absorption has occurred, dietary fibre remains. In the large intestine, dietary fibre attracts water which helps soften stool for easy passage. Bacteria within the colon ferment some of this fibre to produce gas, water and short-chain fatty acids.
In most individuals, the enzyme lactase is produced sufficiently to break down lactose and enable digestion and absorption.
In some individuals, when greater amounts of lactose are consumed than the available lactase can manage, some lactose remains undigested.
The undigested lactose molecules continue through the intestines. These attract water, causing the bloating, diarrhoea and abdominal pain synonymous with lactose intolerance. Upon entering the large intestine, lactose is used as food for the bacteria present, causing gas and further discomfort.
Lactose intolerance is most common in South East Asian and least common in Scandinavia and those of Northern European descent.
Treatment for lactose intolerance is dietary avoidance or reduction in dairy products to levels that do not produce symptoms.
Glucose, as a monosaccharide, can be absorbed through the mucous membranes of the mouth. However, most carbohydrates are absorbed in the small intestine in the jejunum.
There are two types of absorption used for monosaccharides – active transport and facilitated diffusion.
Glucose and galactose are absorbed via active transport through the jejunal lining.
Their transport into cells is via the Na+/K+ATPase pump, enabling the “up-hill” entry of these monosaccharides.
Fructose is absorbed by facilitated diffusion in the jejunum, specifically via the GLUT 2 and GLUT 5 transporters embedded in the cellular plasma membrane.
Compared to glucose and galactose, this is passive transport as it does not require energy.
For every gram of carbohydrates consumed, 16.7 kilojoules (4.2 kilocalories) are released.
Carbohydrates are sourced from plant foods, including fruits, vegetables (particularly tubers and roots), grains, flour, and legumes such as lentils.
As dietary fibre passes through the body without being broken down, it contributes very little energy and essentially no kilojoules.
The ketogenic diet is a very low carbohydrate diet, with moderate protein consumption and high fat intake. Forcing the body into ketosis leads to the body making ketone bodies instead of glucose for fuel due to the lack of glucose available.
Current scientific evidence suggests that a ketogenic diet, under medical supervision, benefits children with epilepsy and some other neurological conditions. However, it is growing in popularity as a mode of rapid weight loss, despite limited and poor-quality evidence.
Common side effects include fatigue, dehydration, headaches, constipation and dizziness.
Despite its popularity, the ketogenic diet is not currently evidence-based for managing obesity.
Dr Chris Jefferies
- Medical Libretexts (2020) 3.3: Digestion and absorption of carbohydrates, Medicine LibreTexts. Libretexts. Available from: [LINK]
- Whitney E, Rolfes SR, Crowe T. Understanding Nutrition. 5th ed: Cengage Learning Australia; 2022.
- Martin K, Jackson CF, Levy RG, Cooper PN. Ketogenic diet and other dietary treatments for epilepsy. Cochrane Database Syst Rev. 2016;2:CD001903. Published 2016 Feb 9. doi:10.1002/14651858.CD001903.pub3
- Ting R, Dugré N, Allan GM, Lindblad AJ. Ketogenic diet for weight loss. Can Fam Physician. 2018;64(12):906.
- Figure 1. OpenStax College. 217 Five Important Monosaccharides. License: [CC BY]
- Figure 2. OpenStax College. Figure 03 02 05. License: [CC BY]
- Figure 3. OpenStax College. Mechanical and chemical digestion. License: [CC BY]