Monosaccharides – Simple Sugars
Monosaccharides (Gk. mono- = single, saccharide = sugar) have only single sugar unit in their structure. They are called simple sugars, since they cannot be split into substances that would still have characteristics of a sugar. Monosaccharide units can combine together to form disaccharides (containing two sugar units) or polysaccharides as starch (containing several sugar units).
Monosaccharides of main importance in the human body are glucose, ribose and deoxyribose. Other monosaccharides, used by human mainly as nutrients are fructose, galactose, mannose, and tagatose.
Monosaccharides are made of carbon, hydrogen, and oxygen and are arranged in groups according to the number of carbon atoms in their molecules such as trioses containing three carbon atoms, tetroses four, pentoses (e.g. ribose, deoxyribose) five, and hexoses (e.g. glucose, fructose) six carbon atoms.
Detailed List of Monosaccharides
Glucose (Picture 1) is the most important monosaccharide found in human body and is the prime energy source for humans and animals. It is also known as grape sugar, corn sugar, starch sugar and blood sugar. It is a six-carbon sugar (hexose). The empirical formula of glucose is C6H12O6.
Picture 1. Glucose, structural formula
Synthesis of Glucose in Plants
Plants with the help of chlorophyll (a green pigment) produce sugar. Plants and some organisms having a chlorophyll are the only living things which can produce glucose from simple elements (carbon, hydrogen, and oxygen) utilizing solar energy through a process known as photosynthesis. This sugar is stored in plants in the form of monosaccharides, disaccharides (e.g sucrose, maltose), or complex carbohydrates such as starch or cellulose. Animals and human consume starch (animals like cattle also consume cellulose) and through its enzymatic breakdown or hydrolysis obtain glucose that is absorbed from their intestines into the blood and then mainly used as a source of energy.
Natural Sources of Glucose
Glucose, that can be used by human, is generally found in fruits, vegetables, honey, and starch. Table sugar or sucrose is made of glucose and fructose molecules. Glucose is also a component of other major disaccharides such as lactose and maltose.
Artificial Production of Glucose
Glucose is commercially produced from hydrolysis of starch- corn starch, rice, wheat, potato, cassava, arrowroot, sago, and so on.
Glucose, Created In the Human Body
Glucose in the human body can be synthesized also from non-carbohydrates such as fats, carboxylic acids and amino acids by first converting into acetyl CoA or pyruvic acid during the process known as gluconeogenesis. This mostly occurs in liver and to some extent in renal cortex.
Glucose is a natural component in human blood.
Absorption of Glucose From the Gut
Glucose is absorbed from the human small intestine into the blood by the help of two transporters, SGLT-1 and GLUT-2. The sodium-dependent transporter SGLT-1 is responsible for the active transport of glucose with an equimolar amount of sodium against a concentration gradient through the food side of epithelial lining into the small intestinal cell. From there glucose is pumped out onto the blood side of intestinal cells by the transporter GLUT-2 and then diffuse down to capillary blood.
Other transporters involved in glucose absorption are:
- GLUT1- in muscle, brain, kidney, and colon
- GLUT2-in liver and pancreatic beta cells
- GLUT3- in brain
- GLUT4- in skeletal muscles and adipose tissue
Absorption of Simple Sugars – Monosaccharides
Glucose Absorption – Detailed
Absorption of glucose may decrease after surgical removal of the part of the stomach or small intestine. Sugar absorption increasesin hypothermia (decreased body temperature), hyperphagia (“over-eating”), pregnancy, lactation (milk production in women) and chronic consumption of high-carbohydrate diets. Absorption of glucose may be different at different daily hours. Read about factors influencing intestinal sugar absorption (research article).
Once released into circulation, the glucose molecules are taken up by body cells with the help of a pancreatic hormone insulin. Insulin therefore decreases the blood level of glucose. Other hormones influencing glucose level in serum are :
- Glucagon is activated by low level of glucose in the blood (during exercise, between meals) and causes breakdown of glycogen and glucose production in liver – therefore increasing blood glucose level.
- Epinephrine (adrenalin- a stress hormone) increases serum level of glucose rapidly.
- Cortisol (another stress hormone) promotes breakdown of proteins that yield amino acids that can be used in glucose synthesis (gluconeogenesis) – thus increasing blood glucose level.
- Growth hormone also increases serum glucose level.
Normal glucose levels in blood lie between 4.5 and 5.5mmol/l (80-100mg per 100mL). When it increases above this range glucose is converted into glycogen and stored in the liver and muscles. Excess glucose is converted to glycogen that is broken down to glucose (glycogenolysis) again when required for energy.
Once inside the cell, glucose is burnt to release heat and ATP – the energy molecules. Glucose may be broken down in the presence of oxygen (aerobic process) or without oxygen (anaerobic process).
Glucose, when oxidized (aerobically metabolized) during cell respiration, liberates energy with production of carbon dioxide and water at the rate of 3.9 kcal per g, or 686 kcal per gram-mol, or 16 kilojoules per gram.
If no glucose is available for energy production as in starvation, body synthesizes glucose from carboxylic acids, fats, and amino acids by a process known as gluconeogenesis.
Glucose is converted to glycogen and stored in liver, which can be converted to glucose when needed.
Excess glucose remaining in circulation after glycogen saturation limit, the surplus is converted into fatty acids and glycerol which are stored as triglycerides (fat) in fat tissue, muscles, and liver.
Glycemic Index (GI)
GI is a measure of the relative blood glucose response to foods containing carbohydrate.
- High GI value – 70 or more.
- Medium GI value – 56 to 69 inclusive.
- Low GI value – 55 or less.
International Table of Glycemic Index/Load Values – 2008
Glycemic Index of Foods for Diabetics
Medical Uses of Glucose
Glucose as a medically given nutrient can be given:
- for treating hypoglycemia, diabetic coma, and shock – where it is given intravenously
- for oral rehydration therapy.
- for parenteral nutrition- postoperatively and during recovering
Disorders Related to Glucose
- Diabetes mellitus type I and II, gestational diabetes, and pre-diabetes.
- Hyperglycemia unrelated to diabetes mellitus – due to endocrine disorders such as thyrotoxicosis, acromegaly, Cushing’s syndrome, etc
- Dental caries
- Glucose intolerance as a genetic disorder. Read more here (link doesn’t work)
Another important natural monosaccharide found in human nutrition is fructose. It is a ketohexose sugar commonly known as fruit sugar or levulose. The empirical formula is C6H12O6.
Natural Sources of Fructose
Fructose is available naturally from honey, fruits such as apples, pears, grapes, peach, banana, apricot, berries, dried fruits and vegetables such as melons, beets, sweet potatoes, sweet corn, carrot, red pepper, onion, yam, sugar cane, sugar beet, sucrose, and so on
- More hygroscopic and 1.73 times sweeter than sucrose.
Commercial production of fructose is from corn starch by enzymatic conversion of glucose or by enzymatic hydrolysis of sucrose.
Fructose Creation in the Human Body
Body can produce fructose by metabolizing sucrose.
Absorption of Fructose From the Gut
Humans can absorb only a limited quantity of fructose. Even normal person can absorb only 25-50 g/serving and the threshold is even lesser in those with fructose malabsorption to as little as 1g per serving.
Presence of glucose in equimolar ratio or with more glucose enhances absorption of fructose from the small intestine. This is important especially for people with fructose malabsorption.
Fructose does not require any enzyme to be absorbed into the body. It is transported into the small intestinal cells with the help of transport protein GLUT-5 and from there transported into the blood through small intestinal basal membrane with the help of transporter GLUT-2.
Fructose is rapidly metabolized in liver. The enzymes involved are fructokinase and aldolase b. Major product of fructose metabolism are triglycerides. In contrary to the glucose, fructose can enter any cell in the human body without assistance of hormone insulin.
Disorders Related to Fructose
Fructose malabsorption is inability of absorption of at least 25 g of fructose per serving due to deficiency of hexose transporters GLUT-5 or GLUT-2. Disorder may run in families, but exact cause is not known.
Hereditary fructose intolerance (HFI) is a strong body reaction to even small amounts of fructose. This rare genetic disorder is due to lack of an enzyme in liver- aldolase b.
its major use is as a sweetener in confectionery and baking. It increases the shelf life of baked products and improves aroma and has browning effect.
Galactose is another monosaccharide important for humans and it is a basic building block of life. The empirical formula of galactose is C6H12O6 and it is an epimer of glucose.
It is an important monosaccharide in antigens of different blood groups also. Galactose is also called brain sugar. Galactose is less sweet than glucose and has a lower glycemic index 20-24.
Natural Sources of Galactose
Galactose is a part of milk sugar or lactose in combination with glucose. It is naturally seen in milk and other dairy products, mucilage, and sugar beet and also in human body.
Artificial Sources of Galactose
Galactose is manufactured by enzymatic hydrolysis from lactose or hemicelluloses with the help of enzyme beta galactosidase.
Galactose Creation in The Human Body
During lactation, body converts glucose to galactose in mammary glands to produce lactose in mother’s milk.
Absorption of Galactose In the Gut
Galactose is absorbed from the small intestine with the help of hexose transporter GLUT-2.
Galactose is metabolized in liver. It is converted into glucose to be utilized for energy production. This is achieved with the help of galactokinase enzyme and galactose-1-phosphate uridyltransferase.
Disorders Related to Galactose
- Galactose intolerance
Mannose is a monosaccharide with the same empirical formula as glucose.
Natural Sources of Mannose
Mannose is naturally found in the secretions of some trees and shrubs. It is present in cranberry juice also. D-mannose is present in Aloe Vera, fenugreek, black or red currants, gooseberries, green beans, capsicum (cayenne pepper), cabbage, eggplant, tomatoes, turnips, shiitake mushrooms, and kelp.
Manose is manufactured by oxidation of mannitol.
Role of Mannose In the Body
Mannose can be found in most body cells and it important for nerve myelination.
Mannose is first converted into fructose and then glucose to be metabolized for energy production in liver.
Disorders Related to Mannose
MBL deficiency (Mannose-Binding Lectin deficiency)
Tagatose is a simple sugar with a structure similar to fructose and texture similar to sucrose. It has a very low glycemic index, 1.5 kcal/g. that qualifies it to be used in diabetic diet and low-carb diet.
Sources of Tagatose
Tagatose is naturally seen in very small amounts in dairy products such as milk, cheese, and yoghurt. It is commercially manufactured from lactose.
Absorption of Tagatose in the Gut
Body absorbs only 15-20 percent of the consumed tagatose from small intestine and the rest is fermented in the colon. Metabolism of tagatose is similar to fructose.
- Potential to be used in antidiabetic drugs/food supplements due low calorie value
- Potential to be used as antiobesity drug/food supplement as it has ability to increase HDL cholesterol.
- Prospect of being used in toothpastes as it is tooth friendly.
F. Ribose and Deoxyribose
Ribose and deoxyribose are building blocks of nucleic acids (RNA and DNA) present in cells.
- Ribose is a pentose sugar (contains 5 carbon atoms). It is the sugar unit in RNA or ribose nucleic acid, which is important for protein production and also plays an important role in transferring genetic codes.
- Ribose sugar combines with purine or pyramidine bases and phosphoric acid to form RNA.
- It produces an important energy molecule in combination with adenine base to form ATP or adenosine triphosphate.
- Ribose sugar is a component of several coenzymes and vitamins.
Deoxyribose sugar is the base unit of DNA or deoxyribose nucleic acid, which is the carrier of genetic information in chromosomes. The empirical formula of this monosaccharide is C5H10O4. This is also a pentose sugar. This sugar is present in the cytoplasm and nucleus of cells.
Glycemic Index of Some Sugars
|Glucose consumed with15-20grams of fiber||57-85|
|Glucose consumed with protein and fat||56|
|Sucrose (granulated table sugar)||68|
Table 1. Glycemic index of main monosaccharides and disaccharides
Sugars Compared by Sweetness
Picture 3. Sugars compared by sweetness
|SUGAR||RELATIVE SWEETNESS||OTHER NAME|
Table 2. Comparison of sweetness of various sugars (source: healthyeatingclub.com)
Glycemic Index of Some Sugars
|Glucose consumed with15-20grams of fiber||57-85|
|Glucose consumed with protein and fat||56|
|Sucrose(granulated table sugar)||68|
Detailed List of Sugars and Sweeteners
Carbohydrate Food Charts
- Nutrition Guide for Fructose Malabsorption
- Low Fructose Diet in Fructose Malabsorption
- Foods High in Fructose, Sorbitol and FODMAPs
- Low-FODMAP Diet
Article reviewed by Dr. Greg. Last updated on January 17, 2013