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 bodies and play a key role, especially in 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 dietary choices. In this article, we will explain simply but accurately what carbohydrates are, how they are classified, what their main derivatives are, and how they behave in our bodies.

What are carbohydrates

Carbohydrates (or glucids) are organic compounds made of carbon, hydrogen, and oxygen. They are mainly found in plant-based foods and are used by the body to produce energy, primarily in the form of glucose. Each gram of carbohydrates provides about 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. The most important are glucose (main energy source), fructose (natural fruit sugar), and galactose (found in milk).
• Disaccharides: formed by the union of two monosaccharides. Examples: sucrose (common table sugar), lactose (found in milk), maltose (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. Some, like fructooligosaccharides (FOS), have a beneficial prebiotic effect on the microbiota.
• Polysaccharides: long, complex chains of sugars. These include starch (plant energy reserve), glycogen (body's glucose reserve), and dietary fiber (cellulose, hemicellulose, pectin, inulin – indigestible but essential for intestinal health).

Simple and complex carbohydrates

In daily practice, carbohydrates are often divided into:

• Simple: include monosaccharides and disaccharides. They are easily and quickly absorbed, which is why they tend to raise blood sugar rapidly. 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.

Glycemic index and glycemic load

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

• Glycemic Index (GI): measures how quickly a carbohydrate-containing food raises blood sugar compared to pure glucose (GI = 100). A food with a high GI causes a rapid increase in blood sugar, followed by an equally rapid drop. Foods with a low GI (lentils, barley, vegetables) lead to a slower and more gradual release of glucose.
• Glycemic Load (GL): takes into account not only the speed but also the total amount of carbohydrates contained in a normal serving. It is calculated by multiplying the GI by the grams of carbohydrates 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. Understanding these differences helps to better manage daily nutrition, especially for those who need to control body weight, blood sugar, or have conditions like diabetes.

Carbohydrate derivatives: what we find in foods

In food products, we can encounter 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: 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 have a better metabolic impact.
• Resistant starch: is not digested in the small intestine and reaches the colon, where it feeds beneficial gut bacteria. It acts as fiber, improves insulin sensitivity, and contributes to microbiota health.
• Soluble and insoluble fibers: the former slow down gastric emptying and modulate the absorption of sugars and fats. The latter increase stool bulk and promote intestinal transit.

Carbohydrates and low-carb diet

In a low-carb diet, the goal is to drastically reduce carbohydrate consumption, especially those with a high glycemic index or rapid absorption. Therefore, foods like bread, pasta, sweets, rice, potatoes, refined sugars, and sugary drinks are eliminated or severely limited.

Instead, preference is given to: low-sugar vegetables (zucchini, spinach, cucumbers), fruits with low glycemic impact (berries), sources of prebiotic fibers like inulin and FOS, and foods containing resistant starch (green bananas, cooled potatoes).

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 drop below 20-30 grams per day. A fundamental aspect is the quality of sources: a well-balanced low-carb diet should focus on vegetables, healthy fats (extra virgin olive oil, nuts, oily fish), good quality protein, and fiber.

Conclusion

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.