Monosaccharides

Monosaccharides are the simplest sugars, made of a single sugar unit. The three that matter in food and the body are glucose, fructose and galactose. Every other digestible carbohydrate is broken down into these before it is absorbed.

What is it?

"Mono" means one: a monosaccharide cannot be broken down into smaller sugars. Glucose, fructose and galactose all have the same chemical formula (C6H12O6) but are arranged differently, which gives them different sweetness and different fates in the body. Together with the disaccharides, they make up the sugars in food.

Glucose, fructose and galactose rarely occur on their own in large amounts. Most are released when you digest starch and disaccharides such as sucrose and lactose. See Glucose, Fructose and Galactose.

What it does

1. Provide quick energy, at about 4 kilocalories per gram
2. Glucose fuels the brain, red blood cells and muscles
3. Fructose is used mainly by the liver and is the sweetest natural sugar
4. Galactose is used to build some brain and cell-surface molecules, and is converted to glucose

Food sources

Free monosaccharides are found in fruit (fructose and glucose), honey, and ripe vegetables, and are used in syrups, sports drinks and sweets. Most monosaccharides in the diet are released during digestion of starch, sucrose, lactose and maltose.

Singapore food examples

1. Ripe fruit — banana, mango, durian, grapes and dates (glucose and fructose)
2. Honey and fruit syrups
3. Fruit juices and sugarcane juice
4. Sports drinks and glucose sweets
5. Rice and noodles, which break down to glucose during digestion

How much do I need?

There is no recommended intake for this sugar on its own, and Singapore's RDA table does not list it. Singapore's dietary guidelines advise that added sugar provide no more than 10 percent of daily energy — about 10 teaspoons (around 50 g) on a 2,000-kilocalorie diet, with about 5 teaspoons (25 g) even better — see Singapore's dietary guidelines for adults (HealthHub).

Low intake and deficiency

There is no deficiency of monosaccharides as a group. The body can make glucose from other carbohydrate, protein and, to a smaller extent, fat. Very low blood glucose (hypoglycaemia) occurs mainly with diabetes medicines, and is covered under Glucose.

Too much and safety

Eating more sugar than the body needs adds energy and, in the form of sweet drinks, is linked to weight gain, tooth decay, fatty liver and type 2 diabetes. Natural monosaccharides in whole fruit come with fibre and are much less of a concern than free sugars in drinks, syrups and sweets.

Absorption and bioavailability

Monosaccharides are absorbed directly through the wall of the small intestine without further digestion. Glucose and galactose share one transporter (SGLT1), which also pulls in sodium and water — the principle behind oral rehydration drinks — while fructose uses a different, slower transporter (GLUT5).

Connection to daily life

1. Energy and exercise: glucose is the main fuel for hard exercise — see Exercise
2. Blood sugar: fibre, protein and fat in a meal slow absorption and flatten blood-sugar peaks
3. Dental health: frequent sugary snacks and drinks matter more for teeth than total sugar
4. Related: see Carbohydrates and Fibre

Sources: 1. HealthHub Singapore — Dietary guidelines for adults: https://www.healthhub.sg/well-being-and-lifestyle/food-diet-and-nutrition/dietary_guidelines_adults 2. HealthHub Singapore — Recommended Dietary Allowances: https://www.healthhub.sg/well-being-and-lifestyle/food-diet-and-nutrition/recommended_dietary_allowances 3. Health Promotion Board — Singapore Food Insights Database (SG FoodID): https://www.hpb.gov.sg/healthy-living/food-and-beverage/sgfoodid/ 4. FAO/INFOODS — Food composition tables and databases: https://www.fao.org/food-composition/tables-and-databases/en/ 5. National Academies of Sciences, Engineering, and Medicine — Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (2005)