Nutritionists agree that sugar has a place in the balanced diet. Find out where this carbohydrate fits into the daily energy supply, how it is digested, classified, and more:
Sugar: a carbohydrate
There are three so-called "macronutrients" in the diet: proteins, fats and carbohydrates. Carbohydrates and fats supply the body with the energy vital for existence.
Sugars, along with starches and dietary fibres, fall into the carbohydrate group. When broken down in the body, sugars and starches provide 16kJ of energy per gram. Most dietary fibres aren't absorbed by the body and, therefore, supply only small quantities of energy (approximately 8kJ per gram).
Carbohydrates are formed in plants through the process of photosynthesis, which requires exposure to sun, water and carbon dioxide.
Plant foods, such as cereals and grains (maize, wheat, rice) and fruits and vegetables, are the primary sources of carbohydrates in the diet. Plant carbohydrates vary widely in sweetness, texture, rate of digestion, and degree to which they are absorbed in the body.
Carbohydrates can be categorised as:
monosaccharides (glucose, fructose and galactose);
di- and oligosaccharides (e.g. sucrose, lactose, maltose);
polysaccharides (starch and fibres).
The prefix "mono" refers to one sugar, "di" indicates that two sugars are combined, and "poly" means that the carbohydrate consists of many sugars.
Different types of sugar
Dietary sugar comes in many guises, but fructose ("fruit sugar") and sucrose ("table sugar") are probably the best-known forms. Sucrose is the most commonly used sweetener in South Africa.
Both these sugars are similar in nutritional terms.
Whether one adds a teaspoon of honey (mainly fructose) or a sugar cube (sucrose) to tea, both sugars provide more or less the same amount of energy. However, sucrose, in contrast to fructose, must also be broken down into "smaller pieces" before it can be absorbed.
This breakdown is necessary since the body can only absorb three types of sugar: glucose, fructose and galactose (i.e. the monosaccharides).
For sucrose, a disaccharide, to be absorbed, it must first be split up into its "building blocks": glucose and fructose.
Another common form of sugar in the diet is lactose, or "milk sugar". To be absorbed, lactose from milk must be broken down to glucose and galactose.
A common misconception is that fructose is the only sugar present in fruit. Although fruits contain between 1% and 7% fructose, they may also contain glucose and sucrose. Sugar cane and sugar beet have the highest concentration of sucrose.
Approximately 40% of the sugar in mature honey is fructose.
In recent years, high-fructose corn syrup has been labelled a contributor to the worldwide obesity problem. Corn syrup, because it is cheaper to produce, has increasingly come to replace sugar and other sweeteners in products like soft drinks in countries such as the USA.
Researchers have blamed this on the lower prices, and subsequent increased sales, of sweet snacks and drinks. In America, the consumption of corn syrup has increased dramatically in the last two decades.
How carbohydrates are digested
The digestion of sugar and starch begins in the mouth, where enzymes in the saliva start to break down carbohydrates into their simpler forms.
However, this breakdown is minimal and is halted when the food reaches the stomach. The enzymes can't continue their work in the acid environment of the stomach.
Carbohydrates usually leave the stomach rapidly, from where they move on to the small intestine – the primary site for digestion.
An enzyme from the pancreas (amylase) is secreted to break down starch. Most of the starch is digested to oligosaccharides and disaccharides. Enzymes produced in the intestine further reduce the molecules to monosaccharides before they are absorbed – a process that also takes place in the small intestine.
The monosaccharides (glucose, fructose and galactose) are absorbed into the bloodstream and carried to the liver. Here, final digestion (i.e. conversion of fructose and galactose to glucose) takes place.
Lastly, glucose is transported to the tissues. However, some glucose gets stored as glycogen in the liver and muscles, from where it can be utilised at a later stage.
A small amount of fructose may be converted to glucose before it passes into the blood, but most is transported as fructose to the liver where, like galactose, it is converted to glucose.
How much sugar in the diet?
A benchmark for sugar intake has been set by dietary authorities. It is recommended that sugar intake should be no more than 10% of total daily energy intake. This value should, however, only be regarded as a rough guideline.
Sugar will supply energy in the diet, but no other nutrients. The amount of sugar that is acceptable in the diet will depend on:
whether the individual is getting enough nutrients from other foods; and
whether or not the rest of the diet is high in kilojoules, or not.
Think of food intake as a pie: the largest part of the pie should be made up of core foods, such as fruits, veggies, starches, fish, meat etc. Only a small part of the pie should be made up of "discretionary" foods, i.e. foods that contain sugar, fat or alcohol, and which should only form a limited part of the diet.
If the diet includes a lot of fat, an individual will have less room for sugar.
In general, younger, more active people can get away with a slightly higher energy intake than older, less active people.
Sugar and diabetes
Small amounts of sugar can be tolerated by diabetics. For example, about 25g of fructose can be used for baking purposes, as this will improve the colour of baked products. However, most people can only eat a limited amount of fructose before they start to develop gastric discomfort (e.g. diarrhoea and cramps).
Diabetics may also eat sucrose in small quantities, but only if their diets have been worked out specifically for their circumstances by a dietician.
Instead of concentrating on sugar, diabetics should shift their focus to the so-called glycaemic index (GI) – a tool that classifies carbohydrate foods according to their effect on blood sugar levels.
For the diabetic, a diet rich in low-GI foods is generally ideal.
Sugar and oral health
When it comes to dental health, sugar isn't considered to be the only culprit anymore.
Scientists now know that all fermentable carbohydrates – which include all types of sugar and starch, both in liquid and solid form – should be taken into consideration.
Microorganisms ferment carbohydrates to form acids. These acids then eat away the tooth enamel and set the scene for tooth decay.
Take note of the different effects of different types of foods:
Foods that are bad for the teeth
Foods that are good for the teeth
- Crackers, pretzels, sweets, cookies, pasta, rice, bread, cereals and porridge.
- Any form of sugar, including brown sugar and honey.
- All fresh, dried and canned fruit.
- All juices and cooldrinks, and any drink that contains sugar.
- Protein foods such as eggs, fish, meat and chicken.
- Sugar-free gum.
- Cheeses such as aged cheddar and Swiss cheese.
- Gum containing xylitol.
Carbohydrates should form part of the healthy diet, so the aim should not be to eliminate these foods. Instead, one should try to combine "bad" foods with "good" or "neutral" foods whenever possible.
It also helps to finish off a carbohydrate-rich meal/snack with a small piece of cheese (just keep the added calories in mind) or sugar-free gum.
Mahan, K.L. Escott-Stump, S. (2000) Krause's Food, Nutrition & Diet Therapy: 10th Edition. Saunders.
- (Carine van Rooyen, Health24, updated April 2011)