Carbohydrates: Sugars, Starches and Fibre

Carbohydrates are found in foods from bread and rice to fruit, milk and beans. This lesson explains the main types, what happens when you digest them, and how carbohydrate and sugars appear on labels.

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What are carbohydrates?

Carbohydrates are one of the three macronutrients, alongside protein and fat. Chemically, they are made from sugar units. They range from single sugar units to long chains of hundreds or thousands of units.

The three main types

A simplified overview of the main types of carbohydrate.
Type	What it is	Examples of foods
Sugars	Small carbohydrates made of one or two sugar units, such as glucose, fructose, sucrose and lactose	Fruit, milk, honey, table sugar
Starches	Long chains of glucose units	Potatoes, bread, rice, pasta, oats
Fibre	Carbohydrates from plants that are not digested in the small intestine	Wholegrains, beans, lentils, vegetables, fruit

What happens when you digest carbohydrates

During digestion, starches and most sugars are broken down into single sugar units, mostly glucose, which are absorbed into the blood. Cells across the body can use glucose for energy, and the body can store some of it as glycogen in the liver and muscles.

Not every carbohydrate is handled the same way. Fibre passes through the small intestine without being digested; some is fermented by bacteria in the large intestine. How quickly a food raises blood glucose also depends on the food as a whole, including its fibre, fat and protein, how it was cooked and the portion eaten.

Carbohydrates and energy

As covered in Lesson 2, digestible (sometimes called “available”) carbohydrate provides about 4 kcal per gram. Fibre provides less, about 2 kcal per gram, because it is only partly fermented rather than fully digested.

Wholegrain and higher-fibre choices

Starchy foods can be refined (such as white bread or white rice) or wholegrain (such as wholemeal bread, brown rice and oats). Wholegrain and higher-fibre versions keep more of the grain, so they generally contain more fibre than refined versions. UK NHS guidance, for example, encourages choosing wholegrain or higher-fibre starchy foods where you can. Beans, lentils, vegetables, fruit and potatoes with their skins also provide carbohydrate along with fibre.

Free sugars, added sugars and naturally present sugars

Not all sugars are described the same way, and the definitions depend on the organisation or country:

WHO and UK: “free sugars”

The World Health Organization and UK guidance use the term free sugars. This includes sugars added to foods and drinks, plus sugars naturally present in honey, syrups, nectars, and unsweetened fruit juices, vegetable juices, smoothies and purées. Sugars found naturally in milk, and in whole fruit and vegetables, are not counted as free sugars.

United States: “added sugars”

US Nutrition Facts labels show Added Sugars beneath Total Sugars. This is a related but not identical idea to free sugars, so figures from the two systems are not directly interchangeable. See the FDA source below for its definition.

On UK labels, the sugars figure is total sugars: it includes free sugars and the sugars naturally present in milk, fruit and vegetables, so the label alone may not tell you how much is free sugar. The ingredient list can help: sugars may appear under many names, such as sucrose, glucose syrup, honey or fruit juice concentrate. Lesson 7 covers labels in more detail.

Reading carbohydrate on a label

UK and EU labels usually list carbohydrate with of which sugars underneath, and fibre (if shown) on a separate line. On US labels, Total Carbohydrate includes dietary fibre and sugars listed beneath it. So the same food can show different-looking carbohydrate figures in different countries.

Hypothetical example

A fictional cereal shows 60g carbohydrate per 100g, of which sugars 10g. In a 40g portion:

Carbohydrate: 60 ÷ 100 × 40 = 24g
Sugars: 10 ÷ 100 × 40 = 4g

This product is invented for learning; it is not a real product value.

Try it yourself (hypothetical food)

A fictional food label shows 30g carbohydrate per 100g. How much carbohydrate is in a 200g portion?

Use (carbohydrate per 100g ÷ 100) × portion in grams.

Look up carbohydrate in real foods

See carbohydrate, fibre and sugars for everyday foods using USDA FoodData Central data.

Open the Food Calorie Calculator

Lesson summary

Carbohydrates are made from sugar units and include sugars, starches and fibre.
Starches and most sugars are digested into simple sugars, mostly glucose; fibre is not digested in the small intestine.
Digestible carbohydrate provides about 4 kcal/g; fibre about 2 kcal/g.
Wholegrain and higher-fibre starchy foods generally contain more fibre than refined versions.
“Free sugars” (WHO/UK) and “added sugars” (US) are different definitions; check which one a source uses.
Label layouts differ by country, including whether fibre is counted within total carbohydrate.