- The two types of carbohydrate that provide dietary energy are sugars and starch. 1 gram provides 3.75 kcal (16 kJ)
- Dietary fibre is also a type of carbohydrate found almost exclusively in plants. Unlike other carbohydrates, it is not absorbed in the small intestine to provide energy, although some metabolism occurs in the large bowel and it is now recognised that it contributes 2 kcal (8.4kJ) per gram. Dietary fibre has a number of functions including keeping the digestive system healthy
- At least half the energy in our diets should come from carbohydrate, mostly as starchy carbohydrates
- Frequent consumption of food and drinks containing fermentable carbohydrates can increase risk of tooth decay, especially if regular tooth-brushing with fluoride toothpaste is not practiced.
Carbohydrates are key components in the diet, comprising sugars, starchy carbohydrates and dietary fibre. Starchy carbohydrates provide an important source of energy, and fibre is important for digestive health. There is also evidence to show that the type of carbohydrate consumed can affect risk of certain diseases including heart disease.
Classification of carbohydrates
All carbohydrates are compounds of carbon, hydrogen and oxygen. They can be classified in many different ways. One common way is according to their structure. They can be divided into three main groups, according to the size of the molecule:
These are the simplest carbohydrate molecules. The most commonly occurring monosaccharides in food are glucose, fructose and galactose. The formula for glucose (C6H1206) is shown in Figure 1.
These sugars are formed when two monosaccharide molecules join together with the removal of one molecule of water. They have the general formula C12H22011. Examples of disaccharides are sucrose (glucose and fructose), lactose (glucose and galactose) and maltose (2 molecules of glucose).
Oligosaccharides are carbohydrate molecules that have longer carbon chain lengths than sugars (mono and disaccharides) but are shorter than polysaccharides. They generally have carbon chains of between 3 and 10 carbon molecules. Generally humans do not have enzymes that digest oligosaccharides so that they pass through the digestive tract and may be metabolised by gut bacteria.
Polysaccharides are made up of many monosaccharide molecules (usually glucose), joined together. They have the general formula (C6H1005)n where `n´ is a large number. Examples of polysaccharides are starch, glycogen (the form in which glucose is stored in the body), and cellulose, beta glucan and pectin (components classed as dietary fibre).
For dietary purposes, carbohydrates have also been described in the following way:
• Intrinsic sugars, which are incorporated into the cellular structure of foods, e.g. sugars in whole fruits and vegetables.
• Extrinsic sugars, which are not bound into a cellular structure, e.g. the lactose (milk sugar) in dairy products. Honey, fruit juices, table sugar and confectionery are also examples of foods containing extrinsic sugars, referred to as non-milk extrinsic sugars (NMES).
- Complex carbohydrates
• Starch, found in potatoes, bread, rice and pasta
• Dietary fibre - carbohydrate polymers with three or more monomeric units (to exclude mono- and disaccharides, simple sugars of one or two molecules). These polymers are neither digested nor absorbed in the small intestine. The term as defined by the European commission includes:
- Edible carbohydrate polymers naturally occurring in the food as consumed;
- Carbohydrate polymers that have been obtained from food raw material by physical, enzymatic or chemical means and which have a beneficial physiological effect demonstrated by generally accepted scientific evidence;
- Synthetic carbohydrate polymers which have a beneficial physiological effect demonstrated by generally accepted scientific evidence.
Carbohydrate as a nutrient
Carbohydrate has many important functions as a nutrient:
• It provides energy. One gram of carbohydrate in the form of starch or sugars provides 3.75kcal (16kJ). For the purposes of food labelling, a conversion factor of 4kcal (17kJ) is used. It has recently been agreed by the European Commission that dietary fibre also makes a small contribution to energy as it is digested in the large bowel by the resident bacteria. An energy value of 2kcal/g (8.4kJ) has been attributed to dietary fibre.
• The body’s tissues require a constant supply of glucose, which is used as a fuel. The main source of glucose is dietary carbohydrate but it can also be synthesised from protein. If the diet is low in carbohydrate, a greater percentage of dietary protein is used to provide glucose, which means less is available for the growth and repair of body tissues. Thus, carbohydrate in the diet has a protein-sparing effect.
• Most foods contain some carbohydrate. Table 1 shows a selection of foods that contain carbohydrate, listing their sugars, starch and fibre (measured as NSP) content.
Table 1. Carbohydrate content of selected foods (per 100g)
|Food (per 100g)||Energy value Kcal/KJ||Carbohydrate (g)||Sugars (g)||Starch (g)||Fibre (NSP) (g)|
|Brown rice, boiled||141/597||32.1||0.5||31.6||0.8|
|Spaghetti, white, cooked||104/442||22.2||0.5||21.7||1.2|
|Spaghetti, wholemeal, cooked||113/485||23.2||1.3||21.9||3.5|
|Lentils, red, cooked||100/424||17.5||0.8||16.2||1.9|
Carbohydrates in the diet
The National Diet and Nutrition survey of UK adults found the average daily intake of carbohydrate was 252g for men and 198g for women, representing 47.5% and 48.3% of food energy intake for men and women, respectively. As shown in Figure 2, cereal and cereal products were the main source of carbohydrate for adults in Britain (including 20% of carbohydrate in the form of bread).
The contribution of different foods to the average daily non-milk extrinsic sugars (NMES) intake in British adults is shown in Table 2. Intake of NMES exceeded the dietary reference value of 11% of dietary energy in all age groups and was highest in children and in elderly people in institutions. The average NMES intake in men was 12.8% of food energy, while in women NMES contributed 11.8% of food energy. Among children, NMES contributed, on average, 15.1% and 14.7% of food energy for boys and girls, respectively.
Table 2: Percentage contribution of food types to average daily non-milk extrinsic sugar intake of British adults.
|Food/drink type||% contribution to average daily NMES intake|
|Sugars, preserves and confectionery||27%|
|Cereal and cereal products||20%|
|Milk and milk products||6%|
Carbohydrate and dental health
Dental caries is the progressive destruction (demineralisation) of the teeth by acid produced by bacteria acting on sugars and other fermentable carbohydrates on the tooth’s surfaces. Dental caries arise when several factors occur simultaneously, in particular a susceptible tooth surface, acid producing bacteria present in the mouth, and a source of carbohydrate (e.g. sugars and starches) that can be broken down by bacteria. It occurs over a period of time in situations when the process of demineralisation of the tooth enamel is greater than remineralisation The most important way to protect against dental caries is regular (twice daily) brushing of teeth with a fluoride containing toothpaste. Fluoride strengthens the tooth enamel, providing resistance to decay.. Children, adolescents and older people are at particular risk of dental caries.
Although all fermentable carbohydrates (including sugar and starch) have the potential to cause dental decay, the main dietary factor is frequency of sugars consumption, this being more important than the total amount consumed. The more frequently sugars are consumed, the greater the time during which the tooth is exposed to the low pH (acid) levels at which demineralisation occurs.. Sucrose (table sugar) is most commonly associated with caries, although glucose, fructose and maltose seem equally cariogenic (likely to cause caries). Other carbohydrates, especially cooked starch (e.g. present in savoury snacks), which can be broken down to component sugars by enzymes in saliva, may also damage teeth, although to a lesser degree. This is because starch has to be broken down to sugars before fermentation can occur.
At higher (alkaline) pH levels, remineralisation can occur, utilising calcium and other minerals present in saliva. It is important, therefore, to try to encourage less frequent consumption of food and drinks containing sugar and other fermentable carbohydrates so that teeth have a chance to repair themselves. It has been suggested that limiting sugar-containing foods and drinks and other fermentable carbohydrates (e.g. savoury snacks) to meal times (e.g. 3 meals plus a couple of snacks) is one way to reduce the incidence of caries (the presence of other foods limits the drop in pH). Chewing sugar-free gum, which promotes saliva production, is also thought to be helpful in restoring optimal pH at the tooth surface. A common artificial sweetener in sugar-free gum (Xylitol) has also been found to reduce the risk of caries.
Another factor which affects the risk of developing caries is the retentiveness (stickiness) of the carbohydrate. Foods such as dried fruit, crips or toffees may stick to the teeth and so reduce the pH in the mouth for a longer time than would occur with less sticky food.
Although diet is associated with dental health, the most important strategy for reducing the risk of dental caries is regular brushing of teeth. It is important that teeth are brushed twice each day, preferably with a fluoride toothpaste, to remove any food sticking to the teeth or trapped between the teeth. Regular tooth brushing, and also the use of dental floss, is needed to remove the dental plaque coating the tooth surface and gum margins; this constrains the bacteria responsible for dental decay. Regular visits to the dentist are important to ensure that dental health is maintained.
Dental erosion is also a concern for dental health, but this is caused by acidic foods and drinks rather than fermentation of dietary carbohydrate. More information about dental erosion can be found on our section about healthy hydration.
Carbohydrates and diabetes
The level of glucose in the blood is carefully monitored and kept within narrow limits by the action of the hormone insulin.
In diabetes, the body either does not produce enough insulin, or the insulin it does produce is ineffective. If untreated, the glucose level in the blood becomes too high and this is harmful for the individual. Raised blood glucose, known as hyperglycaemia, can adversely affect brain function and a lack of functional insulin means that the cells of the body cannot take up glucose and use it as a fuel, resulting in fatigue and weight loss. Despite popular belief, there is no evidence to suggest that a high consumption of sugars causes diabetes.
People with diabetes need to modify their diet and may also need to receive injections of insulin, or take tablets that help the insulin in their body to function more effectively.
The diet recommended for individuals with diabetes should be high in complex carbohydrates and low in fat, i.e. similar to that recommended for the general population. As such, large intakes of sugars should be avoided, although they do not need to be omitted from the diet and it is not necessary to choose so-called 'diabetic foods'. Individuals who take insulin to control their diabetes need to ensure that adequate amounts of carbohydrate foods are eaten at regular intervals. This type of diet will help individuals with diabetes to control their blood glucose levels and at the same time may help reduce the risk of developing coronary heart disease and some other conditions to which people with diabetes are prone.
The most common type of diabetes, type 2 diabetes, is typically associated with being overweight; it has become more common because obesity is on the rise in the UK population. Although this type of diabetes used to be known as ‘adult onset diabetes’, it is also now being diagnosed among obese children. People with type 2 diabetes often need to restrict their energy intake so as to lose weight, in addition to following general advice for healthy eating.
The glycaemic index (GI) of a food is a measure of how quickly glucose is released into the bloodstream after eating. Low GI foods, such as brown pasta, porridge, beans and lentils, breakdown slowly during digestion and therefore release glucose slowly into the blood stream. Although some studies have suggested that a low GI, high fibre diet may help to reduce the risk of type 2 diabetes, there is not sufficient consistent evidence to recommend a low GI diet for prevention. However, many low GI foods that are rich in fibre are an important component of a healthy varied diet.
Evidence suggests that current recommendations for physical activity for adults of 150 minutes of moderate intensity physical activity per week, can be effective in reducing the risk of type 2 diabetes in the general population. Keeping physically active is also likely to help those with diabetes manage their condition. The optimum level of activity for those with diabetes may be higher than current recommendations (e.g. if overweight), but any level of activity is better than none. Those with diabetes should talk to their doctor before starting new activities.
For more information on diet and diabetes, visit :
Carbohydrate and cardiovascular disease (CVD)
A high blood cholesterol level is associated with an increased risk of CVD. Some forms of complex carbohydrates, such as pectin in fruit and beta glucan in oats, may slightly reduce the level of cholesterol in blood. These are forms of soluble fibre. There is also some evidence that resistant starch may have a slight, beneficial effect on CVD risk. Regular consumption of wholegrain foods has been linked with heart health, possibly as a result of the breakdown products (e.g. short chain fatty acids such as butyrate) formed and absorbed in the large bowel.
Carbohydrate and cancer
Some studies suggest that populations with a high intake of fibre-rich foods experience a lower incidence of large bowel cancer than populations with low intakes of these foods.
Carbohydrate and obesity
Too much energy in the diet can lead to excess weight gain irrespective of the type of food eaten, but contrary to popular belief, gram for gram carbohydrate provides less energy than fat or alcohol. Fibre-rich foods are often recommended in weight-reducing diets. They contain less energy, gram for gram, than foods with a high fat content, and give a feeling of fullness and satiety.
Last reviewed 5 October 2012. Next review due 5 October 2015