Dietary carbohydrate is an important source of food energy, and has a wide range of chemical, physical and physiological properties. Carbohydrate is classified according to chemical composition, but these groupings are not always helpful when describing physical and nutrition functions. As a result, a number of terms have emerged to group carbohydrates based on physiological properties and to help focus on specific health benefits.

The World Health Organization and Food and Agriculture Organization (WHO/FAO) have recommended that terminology be standardized primarily based on its chemical make-up (i.e., number of sugar units, type of linkage between units). Key terminology used to describe carbohydrate summarized below.

Carbohydrates

Carbohydrates contain carbon, hydrogen, and oxygen atoms; in approximately a 1:2:1 molar ratio, giving the general formula CxH2xOx. The ratio of hydrogen to oxygen is always 2:1. General names for carbohydrates include sugars, starches, saccharides, and polysaccharides. The term saccharide is derived from the Latin word "sacchararum" from the sweet taste of sugars. The simplest carbohydrates are monosaccharides (single sugar units) such as glucose, fructose and galactose. Other carbohydrates are composed of monosaccharide units, and break down under hydrolysis. These may be classified as disaccharides, oligosaccharides, or polysaccharides, depending on whether they have two, several, or many monosaccharide units.

Monosaccharides contain one sugar unit and are the building blocks of disaccharides, oligosaccharides and polysaccharides. Principle monosaccharides are glucose, fructose and galactose.

Disaccharides contain two sugar units. In almost all cases one of the sugars is glucose, with the other sugar being galactose, fructose, or another glucose. Common disaccharides are sucrose, lactose and maltose.

Oligosaccharides contain a range of three to nine sugar units, and are often the products of the breakdown of polysaccharides. Common oligosaccharides include raffinose and stachyose.

Polysaccharides contain many sugar units (≥10) in long polymer chains of many repeating units. The most common sugar unit is glucose. Common polysaccharides are starch, glycogen, and cellulose.

A useful classification system recommended by the WHO/FAO groups carbohydrates according to their degree of polymerization (DP)*, as shown in the table below.

Class (DP*) Sub-Group Components
Sugars (1-2) Monosaccharides Glucose, galactose, fructose
Disaccharides Sucrose, lactose, maltose, trehalose
Polyols (sugar alcohols) Sorbitol, mannitol, lactitol, xylitol, erythritol, isomalt, maltitol.
Oligosaccharides (3-9) Malto-oligosaccharides (α-glucans) Maltodextrins
Non α-glucan oligosaccharides Raffinose, stachyose, fructo and galacto oligosaccharides, polydextrose, inulin
Polysaccharides (≥10) Starch Amylose, amylopectin, modified starches
Non-starch polysaccharides (NSPs) Cellulose, hemicellulose, pectin, arabinoxylans, β-glucan, glucomannans, plant gums and mucilages, hydrocolloids

*DP = Degree of polymerization (i.e., number of single sugar units bonding in chain)

Despite this classification, controversy still exists over how sugars and carbohydrates are defined. Inconsistency in terminology presents a challenge when comparing and analyzing data that could be used to determine actual intakes or to measure health effects. For example, the WHO/FAO report recommends that the terms "simple" and "complex" carbohydrates should no longer be used; however, this has not been universally accepted.

Sugars

The term "sugars" refers to all monosaccharides and disaccharides naturally occurring or added to foods. Examples of monosaccharides are glucose and fructose and examples of disaccharides include sucrose, lactose and maltose. Sugars occur naturally in foods such as milk (lactose) and fruits and vegetables (glucose, fructose and sucrose). Sugars such as sucrose, honey, glucose and many others are also added to foods to provide sweetness and other important functional properties. The source does not affect the chemical composition of sugars nor their effect on health. For example, the sucrose extracted from sugar cane or sugar beet is the same as the sucrose found in maple syrup, and all fruits and vegetables. These sucrose molecules are chemically identical and treated the same in the body. While WHO/FAO categorizes polyols as "sugars" (see table above), the definition of "sugars" in Canada's Food and Drug Regulations does not include polyols.

Naturally Occurring vs. Added Sugars

‘Naturally occurring sugars’ (such as those found in fruit) are often viewed as healthier than ‘added sugars’ (such as those added to cereals or yogurt). However, from a nutritional perspective all sugars, whether they are added or naturally-occurring, are equivalent. The various sugars that are added to foods have the same chemical make-up as the sugars as they occur in nature. They contain the same number of calories and are metabolized by the body in the same way. As an example, the sucrose found naturally in a banana is the same sucrose added to flavoured yogurt. The banana is packaged with micronutrients including potassium, magnesium and vitamin C, and the flavoured yogurt is packaged with micronutrients including calcium, magnesium and phosphorus.

Sugars Content of Various Fruits and Vegetables

It is commonly believed that fructose is primarily found in fruits and that sucrose is mainly added to foods during manufacturing. In fact, almost all fruits and vegetables naturally contain sucrose, as well as glucose and fructose, in varying amounts. For example, bananas, sweet peas and peaches contain primarily sucrose, whereas glucose and fructose are the main sugars in pears and tomatoes. While sucrose is found in almost all plants, sugar cane and sugar beets store sucrose in the greatest quantities and thus are harvested as the commercial sources of sugar. The following chart summarizes the sugars content of various fruits and vegetables.

Sugars Content of Fruits and Vegetables: 100 grams, edible portion
tomatoes 1.1 1.4 0
sweet peas 0 0 4.3
sweet corn 0.8 0.6 3.4
carrots 1 1 3.6
peaches 1.1 1.3 5.6
oranges 2.2 2.5 4.2
watermelon 1.6 3.3 3.6
pears 1.9 6.4 1.8
canned pears 4.8 5.1 1.1
apples 2.3 7.6 3.3
mangos 0.7 2.9 9.9
bananas 4.2 2.7 6.5
glucose fructose sucrose  

Source: USDA Nutrient Laboratory, Sugar Content of Selected Foods

Sugar

Sugar is sucrose, a carbohydrate found in every fruit and vegetable and is the major "disaccharide" (see below) in most diets. All green plants manufacture sucrose through photosynthesis, the process by which plants transform sunlight into their food and energy supply. Both sucrose and starch are major carbohydrate end-products of photosynthesis; however sucrose is the major carbohydrate that is transported throughout the plant. Some plants, such as sugar cane and sugar beet, accumulate high levels of sucrose and as such are used as commercial sources of sugar. A stalk of the cane plant contains about 14% sugar and sugar beets contain about 19%.

Sucrose is a disaccharide, made up of the two sugar units, glucose and fructose (monosaccharides). It is present in honey, maple syrup, fruits, berries, and vegetables. It is also added to food products as crystalline sucrose (granulated, brown sugars) or in liquid form.

In Canada, the Food and Drug Regulations prescribe the standard for sugar as the "food chemically known as sucrose". It has a specific compositional standard; i.e. must "contain not less than 99.8% sucrose". For food labelling purposes, sugar is considered the "common name" of the food and must meet this standard. When listed in the ingredient list of a food, different types of sugar can be listed; for example, sugar, liquid sugar, invert sugar or liquid invert sugar. Compositional standards for other types of sugar are also defined in the Regulations, including: icing sugar, brown sugar, fancy molasses, etc. All of these types of sugar are derived from sugar cane or sugar beet.

Other Terminology

In addition to the terminology described above, a number of other chemical and physiological terms are used to describe carbohydrate. A wide range of terms have been reviewed and were considered useful if they were: 1) measurable by a laboratory analyst; 2) understandable to the consumer; and 3) indicative of the properties of the carbohydrates rather than the food itself. (Please see chart below) It was also noted that grouping carbohydrate by its physical properties or nutritional characteristics is more difficult than classifying by chemical composition because the physiological effects of a particular carbohydrate can vary between individuals. In addition, classification in this manner involves ongoing revision due to the evolving scientific evidence in the area of food science and metabolism.

  Chemical Physiological/Botanical
Useful Monosaccharides
Disaccharides
Polyols
Total sugars
Short-chain carbohydrates
Oligosaccharides
Polysaccharides
Starch
Non-starch polysaccharides
Total carbohydrate
Prebiotic
Resistant starch
Dietary fibre(a)
Glycaemic
Less Useful Sugars
Sugar
Free sugars
Refined sugars
Added sugars
Extrinsic and intrinsic sugars
Non-digestible oligosaccharides
Soluble and insoluble fibre
Available and unavailable carbohydrate
Complex carbohydrate
(a) Intrinsic plant cell wall polysaccharides.
Source: Cummings and Stephen, EJCN, 2007

References

Food and Drug Regulations, Department of Justice Canada

Carbohydrate Terminology and Classification. Cumming JH and Stephen AM. European Journal of Clinical Nutrition, 2007, 61:Suppl1: S5-S18

Carbohydrates in Human Nutrition, Report of a Joint FAO/WHO Consultation, 1997