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

Carbohydrates are one of the three fundamental 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 for the functioning of the brain, muscles, and central nervous system. However, not all carbohydrates are created equal, and understanding their differences is essential for making informed food choices. In this article, we will explain, in a simple but accurate way, what carbohydrates are, how they are classified, what their main derivatives are, and how they behave in our body.

What are carbohydrates?

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

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

Classification of carbohydrates

Chemically, carbohydrates are classified based on 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 ones are:
  • Glucose: present in the blood, it is the main source of energy.
  • Fructose: natural sugar found in fruit.
  • 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 starch digestion.
• Oligosaccharides: formed by 3 to 10 simple sugar molecules. 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 beneficial prebiotic effect on the microbiota.
• Polysaccharides: long and complex chains of sugars. They can have an energy or structural function. The main ones are:
  • Starch: present in cereals, potatoes, legumes. It is the energy reserve of plants.
  • Glycogen: it is the glucose reserve of human and animal organisms.
  • Dietary fibers: such as cellulose, hemicellulose, pectin, inulin. They are not digestible but play a fundamental role in intestinal health, regulate sugar absorption, and improve satiety.

Simple and complex carbohydrates

In everyday practice, carbohydrates are often divided into:

• Simple: includes monosaccharides and disaccharides. They are easily and quickly absorbed, which is why they tend to raise blood sugar levels quickly. Some examples: white sugar, honey, syrups, fruit juices, packaged sweets.
• Complex: includes 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.

Note: food refining can turn even a complex carbohydrate into a rapidly absorbed sugar source. White bread, for example, has a glycemic impact similar 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 compared to pure glucose (which has a GI = 100). A food with a high GI (e.g., white bread, puffed rice, baked potatoes) causes a rapid increase in blood sugar, followed by an equally rapid drop, which can stimulate hunger. Conversely, foods with a low GI (such as lentils, barley, vegetables) result in a slower and more gradual release of glucose into the blood.
• Glycemic Load (GL): 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 100g. Conversely, a food with a medium GI but consumed in large quantities can have a high glycemic load.

Understanding these differences helps to better manage daily nutrition, especially for those who need to control body weight, blood sugar, or have conditions such as diabetes. A diet rich in foods with a low glycemic index and load can help maintain stable energy levels, improve insulin sensitivity, and reduce visceral fat accumulation.

Carbohydrate derivatives: what we find in foods

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: is the most common form of carbohydrate in the Mediterranean diet. It is found in bread, pasta, rice, potatoes. Whole starches or those containing resistant starch (such as those in 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 a fiber, improves insulin sensitivity, and contributes to the health of the microbiota.
• Soluble and insoluble fibers: the former slow down gastric emptying and modulate the absorption of sugars and fats. The latter increase fecal volume and promote 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 rapid absorption. Therefore, the following are eliminated or severely limited:

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

Instead, the following are preferred:

• Low-sugar vegetables (zucchini, spinach, cucumbers)
• Low-glycemic fruits (berries)
• Sources of prebiotic fibers such as inulin and FOS
• Foods containing resistant starch (green bananas, cooled potatoes)

In the absence of available glucose, the body activates the production of ketone bodies from fats, entering a state of ketosis. In this condition, the brain and muscles learn to use fats as an alternative fuel, with potential benefits for body weight, appetite, mental concentration, 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), where carbohydrate intake can even 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 certain chronic inflammatory conditions.

A fundamental aspect is the quality of the sources: a well-balanced low-carb diet should not be a "no carb" or an unbalanced diet rich only in saturated fats. Instead, it should focus on vegetables, healthy fats (extra virgin olive oil, nuts, oily fish), good quality proteins, and fiber. Even small amounts of naturally carbohydrate-rich foods, if unrefined, can find a place, depending on individual goals and metabolic response.

Carbohydrates are a fundamental component of human nutrition, but they should not all be treated the same way. 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 following a traditional diet, a low-carb diet, or a ketogenic diet, the secret always lies in the quality of the carbohydrates chosen and the moderation of quantities.