Carbohydrates – All You Need to Know

Undoubtedly, in the field of nutrition, proteins, fats, and carbohydrates are most often mentioned.

Each of these macronutrients has important functions in the human body.

In this article, we will look at carbohydrates – often associated with the enemy of weight loss. And, is that so? We will find out, as well as more interesting information about them and the processes in which they participate.

What will you learn?

You will learn what types of carbohydrates there are, how they are metabolized in the body and what their main functions are.

You will also understand why the glycemic index, as a method of determining the body’s physiological response to carbohydrates, has its limitations, as well as what negatives for the body can cause a diet rich in processed sugars.

What are carbohydrates

Carbohydrates are a large group of organic substances that have important biological functions in the body.

They belong to the three main macronutrients that we get through our food (proteins, carbohydrates, fats).

We get them mainly through food in the form of sugars, starch, and dietary fiber, and the end products that are reached after digestion are glucose, fructose, and galactose.

Molecular structure of carbohydrates

Carbohydrates are made up of molecules of carbon, hydrogen, and oxygen.

The name of carbohydrates is based on the idea that they are hydrates of carbon due to their general formula CnH2mOm, in which the ratio of hydrogen to oxygen is 2:1.

It is important to note that there are other compounds that correspond to the formula in terms of quality and quantity, but due to differences in properties, do not belong to the group of carbohydrates. This is, for example, acetic acid of formula C2H4O2.

There are other exceptions, which do not correspond to the general formula of carbohydrates, but in terms of properties belong to this class – deoxyribose with formula C5H10O4.

The molecular formula of glucose is C6H12O6.

Types of carbohydrates

Depending on their chemical structure, carbohydrates are divided into three main classes of saccharides (from the Latin Saccharum, sugar):

  • Sugar;
  • Oligosaccharides;
  • Polysaccharides.

These classes are divided into subclasses, which in turn are divided into different individual components.


  • Monosaccharides – Glucose, galactose, fructose, xylose
  • Disaccharides – Sucrose, lactose, maltose, trehalose
  • Polyols – Sorbitol, mannitol


  • Malto-oligosaccharides – Maltodextrin
  • Other oligosaccharides – Raffinose, stachyose, fructooligosaccharides


  • Starch – Amylose, amylopectin, modified starch
  • Polysaccharides other than starch – Glycogen, cellulose, hemicellulose, pectins, hydrocolloids

Monosaccharides are the simplest form of carbohydrates. They contain one (mono) sugar group.

Disaccharides are formed from two linked monosaccharides and, like them, are water-soluble.

Polyols are organic compounds that contain many hydroxyl groups.

Oligosaccharides are short chains of monosaccharides linked together in the form of disaccharides, trisaccharides, etc.

Polysaccharides, on the other hand, are long and complex chains of more than 10 interconnected monosaccharides that form a polymer.

The main carbohydrates we eat are:

  • Starch (bread and pasta);
  • Lactose (milk sugar – milk and some dairy products);
  • Sucrose (ordinary sugar);
  • Glucose, fructose (fruit)
  • Fiber (soluble and insoluble).

Dietary fiber

“Eat fiber!” You’ve probably heard this line, and it’s no coincidence that fiber plays an important role in nutrition.

So we will pay a little more attention to them.

Fiber, also called fiber or fiber, is found in unprocessed carbohydrate-rich foods – vegetables, fruits, whole grains, legumes.

There are two types of fiber:

  • Soluble fiber: oats, dried beans, peas, nuts, barley, flaxseed, bananas, apples, tomatoes, carrots;
  • Insoluble fiber: found mainly in the structures that make up the walls of plant cells – green beans, beets, green leafy vegetables, root vegetables, whole grains, seeds, nuts.

The benefits of fiber are:

  • Increase satiety;
  • Lower cholesterol;
  • Reduce the risk of colon cancer;
  • Food for beneficial bacteria.

The recommendations for minimum fiber intake is 25 g/day, and the optimal – 35 g for women and 48 g for men.

An interesting fact related to the topic of fiber and the health of the intestinal flora is the role of resistant starch in the diet.

Resistant starch is not broken down in the stomach or small intestine, but travels to the large intestine, where bacteria attach to it and use it for food or ferment it, which has some benefits for the body:

  • Improving insulin sensitivity – Resistant starch can improve insulin sensitivity and reduce glucose levels in response to meals;
  • Better satiety-resistant starch is converted into short-chain fatty acids by bacteria in the gut, which in turn trigger the release of hormones that reduce the desire for food (leptin, peptide YY, glucagon-like peptide);
  • Anti-inflammatory roles – the consumption of resistant starch allows the production of butyrate, which acts as a powerful anti-inflammatory agent for colon cells.

The recommendations for the amount of resistant starch are 6-12 g/day, up to 20 g/day. Amounts of more than 40 g could cause digestive problems.

Biological functions of carbohydrates

Although carbohydrates are relatively rare in quantity in the human body, they play important roles in the human body.

Energy source

Carbohydrates, especially glucose, are the preferred source of energy, but not in all situations.

It depends on the body and whether it is under load. For example, the muscles use mainly fatty acids at rest, and at high intensity – mainly glucose.

One gram of carbohydrates releases about 4 kcal.

Carbohydrates also have a reserve energy function through glycogen, and the main places where it is stored are the muscles and the liver.

Glycogen is a giant molecule that binds tens of thousands of glucose molecules. At any time, if necessary, glycogen can release the necessary amounts of glucose.

The reason for the binding of glucose molecules to glycogen is that large amounts of glucose cannot be maintained inside the cells because its high concentration will cause osmotic suction of large amounts of water, which in turn will inflate the cell and destroy it.

Carbohydrates play an important role in nutrition because they are the fastest acting source of nutrients for energy transfer, and at the same time, their reserves in the body are limited.

This does not mean that a person should follow a high-carbohydrate diet because glucose can also be synthesized from non-carbohydrate sources (gluconeogenesis).

There are five main metabolic pathways by which carbohydrates may be involved in the regeneration of adenosine triphosphate (ATP):

  1. Glycogenesis: synthesis of glycogen from glucose;
  2. Glycogenolysis: synthesis of glucose from glycogen;
  3. Glycolysis: from glucose to pyruvate;
  4. Krebs cycle and electron transport chain: from Acetyl-CoA to ATP, carbon dioxide, and water;
  5. Gluconeogenesis: synthesis of glucose from non-carbohydrate sources.

Building function of carbohydrates

Carbohydrates are involved in building most cells and cellular organelles.

Ribose and deoxyribose are structural elements of nucleic acids and are involved in the composition of chromosomes and ribosomes.

The main intercellular substance that builds bone and cartilage tissue are different types of chondroitin (chondroitin sulfates), which are one of the most important components of connective tissue (tendons and joints). This includes hyaluronic acid, which makes up the vitreous of the eye.

Protective functions

Some substances belonging to the group of carbohydrates are involved in important protective functions even at the cellular level.

For example, some oligosaccharides are embedded in cell membranes and serve as chemical identification marks – the cells in an organism recognize each other or reveal foreign cells that have penetrated from the outside – bacteria, infections, etc.

Glycosamine glycans are involved in various types of protective mucus secreted by the mucosa in organs such as the mouth, esophagus, intestines, stomach, and others. Mucus protects them from mechanical, chemical, and thermal damage.

These substances are also found in the composition of joint fluids, playing the role of lubricant for the joint surface.

Carbohydrate digestion and absorption processes

The digestion of carbohydrates, in turn, is an enzymatic hydrolysis of their glycosidic bonds, with the aim of breaking down complex forms such as oligo- and polysaccharides to the monosaccharides glucose, fructose, and galactose.

It is the monosaccharides that pass through the intestinal wall into the bloodstream, a process called nutrient absorption.

The breakdown of carbohydrates begins with their entry into our mouths, as the first step of the digestive tract.

If we have to be even more precise, the process begins even before that – through sight and smell and the signals that are sent to the brain. The digestive tract prepares for the breakdown of food and salivation begins. This is the so-called cephalic phase of digestion.

Decomposition in the mouth

In the mouth, thanks to the enzyme contained in saliva – amylase, begins the breakdown of carbohydrates.

Amylase randomly breaks glycosidic bonds inside the starch molecule. Thus, it is broken down into polysaccharides and a small amount of glucose and maltose.

The degree to which carbohydrates are broken down in the mouth is up to 20%.

This percentage depends on the length of stay of the food in the mouth. The less we chew and the faster we swallow food, the more it reduces the effect of amylase.

If you do an experiment and decide to keep a bite of bread in your mouth for a long time, you will feel the sweet taste of glucose and maltose.

Stomach and duodenum

In the stomach, the digestion of starch stops due to the low pH value – between 1 and 2 (for comparison, the pH of amylase in saliva is 7), and strongly acidic gastric juice, whose pepsin attacks and breaks down amylase itself.

The breakdown of starch ends in the duodenum, where the juice from the pancreas is poured, containing amylase with the same effect as in saliva.

The polysaccharides in starch are broken down into maltose, isomaltose, and glucose.

Small intestine

Hydrolysis of disaccharides begins in the small intestine.

Intestinal epithelial cells contain embedded enzyme molecules on their surface – maltase, isomaltase, lactase, and sucrase.

The enzymes maltase and isomaltase break down maltose and isomaltose to glucose, respectively.

Lactose and sucrose are hydrolyzed by the enzymes lactase and sucrose, respectively, with the breakdown of lactose leading to glucose and galactose and sucrose breaking down to glucose and fructose.

All of these end products from the breakdown of carbohydrates (hexoses glucose, fructose, and galactose, as well as several pentoses) can pass into the body through the intestinal mucosal cells and enter the bloodstream, which carries them to the liver and from there to other organs.

Absorption of monosaccharides

The passage of the end products from the digestion of carbohydrates in the blood takes place through various mechanisms.

Diffusion – through it pentoses pass through the membranes of intestinal cells, the process is slow;

Translocases help carry fructose and it appears quickly in the blood;

Transport that carries glucose and galactose is active – again through translocations that break down ATP – this is the fastest transfer, but it is also associated with energy expenditure.


The liver takes the number of monosaccharides it needs for energy transfer and glycogen stores and sends the rest in the form of glucose to be used by the body.

Although we get galactose and fructose from food, they are absent from the general circulation because both monosaccharides are converted to glucose by the liver, with the caveat that with high fructose intake, some of it can be converted to triglycerides.

Interestingly, fructose is the liver’s preferred source of energy so that it can replenish its glycogen stores.

Blood flow

Usually, about 20 grams of glucose circulates every hour and our body tries to keep these levels stable if they fall – to use new glucose to restore levels in blood and energy.

The rest of the glucose, if any, goes to the liver and muscles:

  • The liver can store 80-100 g of glycogen;
  • Muscle cells can store between 300 and 600 g of glycogen, depending on the amount of muscle mass an individual has.

Glycemic index, glycemic load, insulin index

Undoubtedly, the terms glycemic index and, a little more familiar, but more realistic, glycemic load are regularly accompanying the topic of carbohydrates.

There is a third term, more specifically, called the insulin index.

Let’s take a closer look.

Glycemic Index

The Glycemic Index (GI) determines the extent to which blood sugar will rise after eating a particular food, in other words, it measures how quickly a food raises blood sugar.

The glycemic index of glucose is taken as 100, and the values ​​of the glycemic index of other foods are determined depending on that of glucose.

Foods with a high glycemic index can raise blood sugar quickly and the resulting peak corresponds to the concentrations of insulin in the blood:

  • Sugar;
  • Sweets;
  • White bread;
  • Potatoes.

Low glycemic index foods recommended for maintaining stable blood sugar levels:

  • Beans;
  • Nuts;
  • Whole grain products;
  • Vegetables.

Limits of the glycemic index

The glycemic index is an interesting method for determining the physiological response to carbohydrates, but there are two main limitations:

  • The glycemic index is generally determined by eating food alone, on an empty stomach. The GI of a food changes when it is consumed with other foods – for example, although potatoes have a higher GI when consumed with vegetables and a source of protein, the total GI of a food can be significantly lower;
  • Some other factors, such as food processing, time of day consumption, and physical activity, also affect the glycemic index of food.

Glycemic load

Glycemic load (GL) is considered a more realistic method, as the glycemic load of a food is determined by its glycemic index and the number of carbohydrates consumed, ie. the portion.

The glycemic load is calculated on a linear basis:

(grams of carbohydrates) X (glycemic index of food) ÷ 100

The glycemic index and glycemic load are not always proportional – for example, watermelon has a high GI but contains a lot of water and few carbohydrates, which determines its low GL.

Insulin index

The glycemic index and glycemic load, although they have benefits in terms of glucose, its total amount, and determining how it will affect the body’s physiological response, are not good indicators of insulin response to a food.

For this purpose, the insulin index is used – it categorizes foods according to the insulin reaction they cause in the body after consumption.

This index directly measures the response of insulin in the blood to a food.

Carbohydrates in the diet

Carbohydrates are an integral part of life, and probably everyone knows that the brain and central nervous system prefer to use glucose as a source of energy.

Minimum recommendations for the amount of glucose

It is estimated that about 50 g of glucose is the minimum amount needed on a daily basis and it can be obtained through the process of gluconeogenesis.

Of course, this recommendation may vary, as individual factors, such as:

  • type and level of physical activity;
  • body size and composition;
  • the rest of the diet;
  • the specific goals of a particular person.

When the amount of carbohydrates in the diet is low, the body obtains energy through the production of ketones.

Ketones are a group of 3 molecules – acetoacetate, tri-beta-hydroxybutyrate, and acetone, and they can be considered as an alternative fuel.

Type of carbohydrates consumed

As it is already clear that food is not just fuel, but information and much more, the type of carbohydrates consumed affects the way the body will react.

The general advice for controlling blood sugar levels and insulin concentrations is that low glycemic index carbohydrates should be predominant – vegetables, legumes, whole grains.

Accordingly, those with a high glycemic index – processed carbohydrates, white rice, white flour (pasta, bread), cereals, sugar, and added sugar, should occupy a small part of the menu.

If the diet is dominated by such foods, it is possible to observe an increase in blood sugar, triglyceride levels, and “bad” cholesterol (LDL), and in some cases to develop insulin resistance.

Types of diets by manipulating carbohydrates

There are several main types of diets that use the method of manipulating the number of carbohydrates:

  • Low carb diets – a diet that limits carbohydrates in the food to a minimum, which aims to maintain lower insulin levels;
  • High-carbohydrate diets – a diet in which the main energy source coming from food are carbohydrates – the ratio between the two main energy sources is 20-30% fat to 70-80% carbohydrates;
  • Carbohydrate rotation – there are different options for the application of this method in the diet, but the basic principle is based on changing the number of carbohydrates consumed each day, and some of the varieties require a change in the amount of protein and fat;
  • Cheating day – this is a day when more carbohydrates are consumed, usually done in combination with a low-carb type of diet – a few low-carbohydrate days, followed by a cheating day.

In the world of nutrition, carbohydrates are often associated with the “devil” from whom you gain weight. That’s why low-carb diets are so popular when it comes to weight loss.

In fact, carbohydrates are not our biggest enemy and we can gain weight from them as well as from fat.

Which foods are “carb foods”

Before giving an example of some of the more commonly used “carb foods”, The Sized team would like to make an important clarification: it is incorrect to define foods in this way because these also contain other substances.

“Carb foods” are those with predominant carbohydrate content, but they usually contain other nutrients – proteins and/or fats.


If you hear the word “carbohydrates”, undoubtedly among the top places will be cereals:

  1. Buckwheat
  2. Oatmeal
  3. Barley
  4. Quinoa
  5. Spelt
  6. Rice
  7. Corn
  8. Wheat and others.


Fruits are a source of fruit sugar – fructose. The content of carbohydrates, sugars, and fiber in them varies according to the type of fruit.

  • Pineapple
  • Bananas
  • Grapes
  • Melon
  • Kiwi
  • Pears
  • Melon
  • Apples, etc.

This includes all dried fruits, which initially have high sugar content. It should be borne in mind that many of the dried fruits sold in the store network have additional added sugar.

Dairy products

Some dairy products also have a certain amount of carbohydrates, such as:

  • Yogurt
  • Milk
  • Cheese, etc.

Other sources of carbohydrates

The predominant carbohydrate foods include baked goods, pasta, confectionery, and snacks, and their list is endless. Here are waffles, pancakes, biscuits, bread, cake, croissant, pasta, spaghetti, noodles, popcorn, chips, ice cream, waffles, etc.

Although most nuts are high in fat, some also have a significant amount of carbohydrates.

Also, pay attention to some drinks.

The content of carbohydrates in most vegetables is low and is mostly fiber, but it is still worth noting them in the list.

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