Carbohydrates: what they are, how they are classified and what role they play in our diet

Carbohydrates are one of the three essential macronutrients for the human body, along with proteins and fats. They are the primary source of energy for our body and play a key role especially in the functioning of the brain, muscles and central nervous system. However, not all carbohydrates are the same, and understanding the differences is essential to making informed food choices. In this article we will explain in a simple but accurate way what carbohydrates are, how they are classified, what the main derivatives are and how they behave in our body.

What are carbohydrates?

Carbohydrates (or glucides) are organic compounds formed by carbon, hydrogen and oxygen. They are found mainly in plant-based foods and are used by the body to produce energy, mainly in the form of glucose. Each gram of carbohydrates provides approximately 4 kilocalories (kcal).

Our body can use carbohydrates immediately as a source of energy, or store them as glycogen in the liver and muscles. When carbohydrates are scarce, the body can also convert them into fat for long-term storage.

Carbohydrate classification

At the chemical level, carbohydrates are classified according to their molecular structure. We can divide them into four main groups:

• Monosaccharides (simple sugars): These are the smallest units. They cannot be broken down further. The most important are:
  • Glucose : present in the blood, it is the main source of energy.
  • Fructose : natural fruit sugar.
  • Galactose : present in milk.
• Disaccharides : formed by the union of two monosaccharides. Examples:
  • Sucrose (glucose + fructose): common table sugar.
  • Lactose (glucose + galactose): present in milk.
  • Maltose (glucose + glucose): Formed during the digestion of starch.
• Oligosaccharides : formed by 3 to 10 molecules of simple sugars. They are found for example in legumes and can ferment in the colon, contributing to the formation of intestinal gas. Some, such as fructooligosaccharides (FOS), have a prebiotic effect beneficial for the microbiota.
• Polysaccharides : long and complex chains of sugars. They can have an energetic or structural function. The main ones are:
  • Starch : present in cereals, potatoes, legumes. It is the energy reserve of plants.
  • Glycogen : is the glucose reserve of the human and animal organism.
  • Dietary fibers : such as cellulose, hemicellulose, pectin, inulin. They are not digestible but play a fundamental role in intestinal health, regulate the absorption of sugars and improve the sense of satiety.

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Simple and complex carbohydrates

In everyday practice, carbohydrates are often divided into:

• Simple : They include monosaccharides and disaccharides. They are easily and rapidly absorbed, for this reason they tend to quickly raise blood sugar levels. Some examples: white sugar, honey, syrups, fruit juices, packaged sweets.
• Complex : include oligosaccharides and polysaccharides. They have more complex structures and take longer to digest. The release of energy is more gradual and constant. Examples: whole wheat bread, whole wheat pasta, legumes, oats.

Caution: Refining foods can also turn a complex carbohydrate into a source of rapidly absorbed sugar. White bread, for example, has a similar glycemic impact to sugar.

Glycemic Index and Glycemic Load

To understand the effect of a food on blood sugar levels, two fundamental indicators are used:

• Glycemic Index (GI) : is a measure of how quickly a carbohydrate-containing food raises blood sugar levels compared to pure glucose (which has a GI of 100). A food with a high GI (e.g. white bread, puffed rice, baked potatoes) causes a rapid rise in blood sugar levels, followed by an equally rapid fall, which can stimulate hunger. In contrast, low GI foods (such as lentils, barley, vegetables) cause a slower and more gradual release of glucose into the blood.
• Glycemic Load (GL) : This takes into account not only the speed, but also the total amount of carbohydrates contained in a normal serving of food. It is calculated by multiplying the GI by the grams of carbohydrates present in the serving, and dividing by 100.

A food with a high GI can have a low glycemic load if consumed in small quantities. For example, watermelon has a high GI, but a low GL because it contains few carbohydrates per 100 g. Conversely, a food with a medium GI but consumed in large quantities can have a high glycemic load.

Understanding these differences helps you better manage your daily diet, especially for those who need to monitor their weight, blood sugar, or have conditions like diabetes. A diet rich in low glycemic index and load foods can help maintain stable energy levels, improve insulin sensitivity, and reduce the accumulation of visceral fat.

Carbohydrate Derivatives: What We Find in Food

In food products we can find different forms of carbohydrates, each with different characteristics and behaviors:

• Added simple sugars : such as sucrose, glucose, corn syrup, invert sugar. Often used in the food industry, they are associated with blood sugar spikes, increased insulin, and visceral fat accumulation.
• Starch : It is the most common form of carbohydrate in the Mediterranean diet. It is found in bread, pasta, rice, potatoes. Whole grain starches or those containing resistant starch (such as legumes) have a better metabolic impact.
• Resistant starch : It is not digested in the small intestine and reaches the colon, where it feeds the bacterial flora. It acts as fiber, improves insulin sensitivity and contributes to the health of the microbiota.
• Soluble and insoluble fibers : the first slow gastric emptying and modulate the absorption of sugars and fats. The second increases fecal volume and promotes intestinal transit.

Carbohydrates and low carb diet

In a low carb diet, the goal is to drastically reduce the consumption of carbohydrates, especially those with a high glycemic index or that are rapidly absorbed. The following are therefore eliminated or severely limited:

• Bread, pasta, sweets, rice, potatoes
• Refined sugars, sugary drinks

Instead, the following are preferred:

• Low sugar vegetables (zucchini, spinach, cucumber)
• Low glycemic fruits (berries)
• Prebiotic fiber sources such as inulin and FOS
• Foods containing resistant starch (green bananas, cooled potatoes)

The body, in the absence of available glucose, activates the production of ketone bodies from fats , entering the state of ketosis. In this condition, the brain and muscles learn to use fats as an alternative fuel, with potential benefits on body weight, appetite, mental focus and systemic inflammation.

Not all low-carb diets are the same: there are more moderate approaches (balanced low-carb) and more restrictive ones (keto or ketogenic diet), in which carbohydrate intake can drop below 20-30 grams per day. This approach is also used for therapeutic purposes, for example to manage drug-resistant epilepsy, metabolic syndrome or some chronic inflammatory conditions.

A fundamental aspect is the quality of the sources: a well-balanced low-carb diet must not be a "no-carb" or unbalanced diet rich only in saturated fats. Instead, it must focus on vegetables, healthy fats (extra virgin olive oil, dried fruit, oily fish), good quality proteins and fibers. Even small quantities of foods naturally rich in carbohydrates, if not refined, can find space, depending on the objectives and the individual metabolic response.

Carbohydrates are a fundamental component of the human diet, but they should not all be treated the same. Simple sugars, refined starches, fibers, and resistant starches have very different effects on metabolism. Understanding their structure and behavior in the body helps us build a more balanced, personalized, and conscious diet. Whether you follow a traditional regime, a low-carb diet, or a ketogenic diet, the secret is always in the quality of the carbohydrates chosen and in moderation of the quantities.