Carbohydrates, what, why and why?

Along with fats and proteins, carbohydrates are one of the three macronutrients in our diet and their main function is to provide the body with energy. They appear in many different forms, such as sugars and dietary fiber, and in many different foods, such as whole grains, fruits, and vegetables. In this article, we explore the variety of carbohydrates that occur in our diet and their functions.

What are carbohydrates?

In their most basic form, carbohydrates are made from the building blocks of sugars, and can be classified according to the number of sugar units that are combined in their molecule. Glucose, fructose, and galactose are examples of single-unit sugars, also known as monosaccharides. Double-unit sugars are called disaccharides, among which sucrose (table sugar) and lactose (milk sugar) are the best known. Monosaccharides and disaccharides are generally called simple carbohydrates. Long-chain molecules, such as starches and dietary fibers, are known as complex carbohydrates. In reality, however, there are more distinct differences. Table 1 provides an overview of the main types of carbohydrates in our diet.

Table 1. Examples of carbohydrates based on the different classifications.

Carbohydrates are also known under the following names, which generally refer to specific groups of carbohydrates: ¹

• sugars
• simple and complex carbohydrates
• resistant starch
• dietary fibers
• prebiotics
• intrinsic and added sugars

The different names come from the fact that carbohydrates are classified according to their chemical structure, but also according to their role or source in our diet. Even leading public health authorities do not have common definitions aligned for different groups of carbohydrates. ^two

3. Types of carbohydrates

3.1. Monosaccharides, disaccharides and polyols

Simple carbohydrates, those with one or two sugar units, are also known simply as sugars. Examples are:

• Glucose and fructose: monosaccharides that can be found in fruits, vegetables, honey, but also in food products such as glucose-fructose syrups
• Table sugar or sucrose is a disaccharide of glucose and fructose, and occurs naturally in sugar beets, sugar cane, and fruits
• Lactose, a disaccharide consisting of glucose and galactose, is the main carbohydrate in milk and milk products.
• Maltose is a glucose disaccharide found in syrups derived from malt and starch.

Manufacturers, cooks, and consumers tend to add monosaccharide and disaccharide sugars to foods and they are called "added sugars." They can also appear as "free sugars" found naturally in honey and fruit juices.

Polyols, or so-called sugar alcohols, are also sweet and can be used in foods in a similar way to sugars, but they are lower in calories compared to regular table sugar (see below). They occur naturally, but most of the polyols we use are made by the transformation of sugars. Sorbitol is the most widely used polyol in foods and beverages, while xylitol is frequently used in chewing gums and mints. Isomalt is a polyol produced from sucrose, often used in confectionery. Polyols can have a laxative effect when eaten in too large amounts.

If you want to learn more about sugars in general, read our article Sugars: Tackling Common Questions , the article Addressing common questions about sweeteners , or investigate the opportunities and pitfalls of replacing sugar in baked goods and processed foods ( Sugars from a Food Technology Perspective ).

3.2.Oligosaccharides

The World Health Organization (WHO) defines oligosaccharides as carbohydrates with 3-9 sugar units, although other definitions allow slightly longer chain lengths. The best known are oligofructans (or in proper scientific terms: fructooligosaccharides), which consist of up to 9 fructose units and are found naturally in low-sweet vegetables, such as artichokes and onions. Raffinose and stachyose are two other examples of oligosaccharides found in some legumes, grains, vegetables, and honey. Most oligosaccharides are not broken down into monosaccharides by human digestive enzymes and are instead used by the gut microbiota (see our material on dietary fibers for more information).

3.3. polysaccharides

Ten or more, and sometimes as many as several thousand, sugar units are needed to form polysaccharides, which are generally distinguished into two types:

• Starch, which is the main energy reserve in root vegetables such as onions, carrots, potatoes, and whole grains. It has glucose chains of different lengths, more or less branched, and occurs in granules whose size and shape vary among the plants that contain them. The corresponding polysaccharide in animals is called glycogen. Some starches can only be digested by the gut microbiota rather than our own body's mechanisms: these are known as resistant starches.
• Non-starch polysaccharides, which are part of the dietary fiber group (although some oligosaccharides such as inulin are also considered dietary fiber). Examples are cellulose, hemicelluloses, pectins and gums. The main sources of these polysaccharides are vegetables and fruits, as well as whole grains. A distinctive feature of non-starch polysaccharides, and indeed of all dietary fibers, is that they cannot be digested by humans; hence its lower average energy content compared to most other carbohydrates. However, some types of fiber can be metabolized by intestinal bacteria, giving rise to beneficial compounds for our body, such as short-chain fatty acids. Learn more about dietary fibers and their importance to our health in our "whole grains" article and dietary fiber .

From here on, we will refer to "sugars" when talking about mono- and disaccharides, and "fibers" when talking about non-starch polysaccharides.

4. Functions of carbohydrates in our body

Carbohydrates are an essential part of our diet. Most importantly, they provide the energy for our body's most obvious functions, like moving or thinking, but also for "background" functions that most of the time we don't even notice. ¹ During digestion, carbohydrates consisting of more than one sugar are broken down into their monosaccharides by digestive enzymes, and then directly absorbed causing a glycemic response (see below). The body uses glucose directly as a source of energy in the muscles, brain, and other cells. Some of the carbohydrates cannot be broken down and are either fermented by our gut bacteria or transit through the intestine unchanged. Interestingly, carbohydrates also play an important role in the structure and function of our cells, tissues, and organs.

4.1. Carbohydrates as a source of energy and its storage

Carbohydrates primarily broken down into glucose are our body's preferred energy source, as cells in our brain, muscle, and all other tissues directly use monosaccharides for their energy needs. Depending on the type, one gram of carbohydrate provides different amounts of energy:

• Starches and sugars are the main energy-providing carbohydrates, supplying 4 kilocalories (17 kilojoules) per gram
• Polyols provide 2.4 kilocalories (10 kilojoules) (erythritol is not digested at all and therefore provides 0 calories)
• Dietary fiber 2 kilocalories (8 kilojoules)

Monosaccharides are absorbed directly from the small intestine into the bloodstream, from where they are transported to needed cells. Various hormones, including insulin and glucagon, are also part of the digestive system. They maintain our blood sugar levels by removing or adding glucose to the bloodstream as needed.

If not used directly, the body converts glucose to glycogen, a starch-like polysaccharide, which is stored in the liver and muscles as a readily available source of energy. When needed, for example between meals, at night, during spurts of physical activity, or during short periods of fasting, our body converts glycogen back to glucose to maintain a constant blood sugar level.

The brain and red blood cells are especially dependent on glucose for energy, and can use other forms of energy from fats in extreme circumstances, such as during very long periods of starvation. It is for this reason that our blood glucose must be constantly maintained at an optimal level. Approximately 130 g of glucose per day is needed to cover the energy needs of the adult brain alone.

4.2. The glycemic response and the glycemic index

When we eat a food that contains carbohydrates, the blood glucose level rises and then falls, a process known as the glycemic response. It reflects the rate of digestion and absorption of glucose, as well as the effects of insulin in normalizing the level of glucose in the blood. Several factors influence the speed and duration of the glycemic response:

• The food itself:
  • The type of sugars that form carbohydrates; e.g. fructose has a lower glycemic response than glucose, and sucrose has a lower glycemic response than maltose
  • The structure of the molecule; e.g. a starch with more branches is more easily broken down by enzymes and therefore easier to digest than others
  • The cooking and processing methods used
  • The amount of other nutrients in the food, such as fat, protein, and fiber
• The (metabolic) circumstances in each individual:
  • The extent of chewing (mechanical breakdown)
  • gastric emptying rate
  • Transit time through the small intestine (which is partially influenced by food)
  • the metabolism itself
  • The time of day the food is eaten

The impact of different foods (as well as food processing technique) on glycemic response is graded relative to a standard, usually white bread or glucose, within two hours of eating. This measurement is called the glycemic index (GI). A GI of 70 means that the food or drink causes 70% of the blood glucose response that would be seen with the same amount of carbohydrate from pure glucose or white bread; however, most of the time carbohydrates are eaten as a mixture and together with proteins and fats that influence the GI.

High GI foods cause a greater blood glucose response than low GI foods. At the same time, low GI foods are digested and absorbed more slowly than high GI foods. There is much discussion in the scientific community, but there is currently insufficient evidence to suggest that a diet based on low GI foods is associated with a reduced risk of developing metabolic diseases such as obesity and type 2 diabetes.

4.3. Bowel function and dietary fiber

Although our small intestines cannot digest dietary fiber, fiber helps ensure good bowel function by increasing the physical volume in the intestine and thus stimulates intestinal transit. Once indigestible carbohydrates pass into the large intestine, the intestinal microflora breaks down some types of fiber such as gums, pectins and oligosaccharides. This increases the total mass in the intestine and has a beneficial effect on the composition of our intestinal microflora. It also leads to the formation of bacterial waste products, such as short-chain fatty acids, which are released in the colon with beneficial effects on our health (see our articles on Dietary fiber for more information).

5. Summary

Carbohydrates are one of the three macronutrients in our diet and, as such, are essential for the proper functioning of the body. They come in different forms, ranging from starchy sugars to dietary fiber, and are present in many foods we eat. If you want to learn more about how they affect our health, read our article on Are carbohydrates good or bad for you?

references

1. Cummings JH & Stephen AM (2007). Carbohydrate terminology and classification. European Journal of Clinical Nutrition 61:S5-S18.
2. European Commission JRC Knowledge Gateway, Health promotion and disease prevention. Accessed 17 October 2019.