What are the types of sugar?

Complex carbohydrates are composed of multiple simple sugars, joined together by chemical bonds. The more chains and branches of simple sugars, the more complex a carbohydrate is and in turn, the longer it takes to be broken down by the body and the less impact it has on blood sugar levels. Examples of complex carbohydrates include wholegrains such as jumbo oats, brown rice, spelt, rye and barley.

Simple carbohydrates are either monosaccharides (one sugar molecule) or disaccharides (two sugar molecules). They are digested quickly and release sugars rapidly into the bloodstream. The two main monosaccharides are glucose and fructose. The two major disaccharides are sucrose (composed of glucose and fructose) and lactose (which is made up of galactose and glucose).

Simple carbs are either naturally occurring, like lactose found in milk, or added like the table sugar you add to your tea or the refined, added sugar found in processed foods.

What is glucose?

Glucose is the primary source of energy your body uses and every cell relies on it to function. When we talk about blood sugar, we are referring to glucose in the blood. When we eat carbohydrates, our body breaks them down into units of glucose. When blood glucose levels rise, cells in the pancreas release insulin, signalling cells to take up glucose from the blood. As the cells absorb sugar from the blood, levels start to drop. Glucose alone does not taste particularly sweet compared to fructose and sucrose.

Nutritional profile of glucose

Per 100g

• 385kcal / 1611kj
• 0.1g fat
• 98g carbs
• 0.1g protein

The glycemic index is a ranking of how quickly foods make your blood sugar levels rise after eating them. Glucose is the defining standard and has a GI value of 100. High GI foods are easily broken down into glucose.

How does glucose affect your body?

Research suggests that, as glucose stimulates insulin release from the pancreas, it also results in the release of two other hormones, leptin and ghrelin. Leptin is known as the appetite suppressor and ghrelin, the appetite increaser. It is thought that lower GI foods (such as wholegrains and protein) suppress ghrelin, so leave you feeling full and satisfied.

What is fructose?

Fructose or fruit sugar, is a simple sugar naturally occurring in fruit, honey, sucrose and high-fructose corn syrup. Fructose is very sweet, roughly one and a half times sweeter than sucrose (white sugar). Because of the worldwide increase in the consumption of sweeteners in soft drinks and foods containing high-fructose corn syrup (HFCS), fructose intake has quadrupled since the early 1900s.

Nutritional profile of fructose

Per 100g

• 398kcal / 1663kj
• 0g fats
• 100g carbs
• 0g protein

Fructose is absorbed directly into the bloodstream during digestion and has no impact on insulin production or blood glucose levels. Consequently, its GI value is much lower, on average around 19. It was once thought this made it a good substitute for table sugar, but there is now a growing body of research to question this.

Sweeteners such as HFCS have a higher GI value because they also contain glucose. It has been suggested that it is the glucose content of these sweeteners that may have contributed to the increase in cardiovascular disease and type 2 diabetes.

How does fructose affect your body?

Fructose is handled by the body in a different way to glucose as it is metabolised in the liver. As a result, blood sugar (glucose) levels do not rise as rapidly after fructose consumption compared to other simple sugars. However, when you eat too much fructose the liver cannot process it fast enough and instead, starts to make fats that are carried in the blood and and stored as body fat. Studies have shown that the consumption of large amounts of fructose may lead to increased appetite by impairing the body's ability to use insulin and to suppress circulating ghrelin (the appetite-stimulating hormone).

While most diabetics cannot tolerate sucrose, most can tolerate moderate amounts of fruit and fructose without loss of blood sugar control. Research is yet to show any detrimental health effects of moderate consumption of fructose as part of a balanced diet. However, it's worth keeping in mind that the excessive consumption of HFCS and other sweeteners may contribute to elevated blood sugar levels, blood fats and subsequent weight gain.

What is sucrose?

Sucrose is crystallised white sugar produced by the sugar cane plant and can be found in households and foods worldwide. Sucrose is a disaccharide made up of 50 per cent glucose and 50 per cent fructose and is broken down rapidly into its constituent parts.

Nutritional profile of sucrose

Per 100g

• 394kcal / 1680kj
• 0g fats
• 105g carbs
• 0g protein

Sucrose has a GI value of 65, this is because it is made up of both glucose (which impacts blood sugar) and fructose, (which has less of an impact). Diabetics should therefore be mindful of foods containing sucrose.

What is lactose?

Lactose is a sugar found naturally in milk and is a disaccharide made up of glucose and galactose units. It is broken down into the two parts by an enzyme called lactase. Once broken down, the simple sugars can be absorbed into the bloodstream.

Nutritional profile of lactose

Per 100g

• 398kcal / 1689kj
• 0g fats
• 95g carbs
• 0.3g protein

Whole milk has a GI value of 41 and is considered to be a low-GI food. Lactose is known to enhance the absorption and retention of minerals such as calcium, magnesium and manganese. Some people experience lactose intolerance – an inability to produce the lactase enzyme, this means lactose is not broken down and can lead to diarrhoea, bloating and other gastrointestinal symptoms.

So, is sugar bad for you?

There are two types of sugar: naturally occurring sugar like lactose in milk and added sugar, which includes table sugar (sucrose) as well as concentrated sources like fruit juice and syrup.

Current recommendations are that only 5 per cent of your daily calorie intake should consist of added or 'free' sugars. This equates to approximately seven teaspoons (30g) for an adult. To put this into perspective, one can of fizzy drink may contain seven teaspoons or more, so it's easy to reach the recommended daily amount quite quickly, especially when you consider the sugar added to processed foods.

Remember, virtually all the fibre, vitamins and minerals have been removed from white sugar (sucrose) so it's best to go sparingly. What's more, eating too many refined carbohydrates can raise blood sugar levels which negatively impacts our health, especially if you are insulin resistant or are diabetic. Eating excess sugar may also lead to weight gain, which increases your risk of heart disease and type 2 diabetes.

Find out more reasons why sugar is bad for you.

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This page was reviewed on 17 February 2025 by Kerry Torrens.

Jo is a registered nutritionist (RNutr) with the Association for Nutrition with a specialism in public health. Since graduating from the University of Westminster in 2010, Jo has worked in a variety of public and private contexts, delivering weight management programmes, community cookery projects, and corporate wellness packages. Alongside trying to grow more of her own fruit and veg at her allotment, Jo works as a Nutritionist and Health Coach for Second Nature. She has contributed articles to a number of nutrition websites, including Good Food..

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