Our bodies are chemical (or rather biochemical) machines, which are undergoing constant changes. The nutrients we introduce through our food, once absorbed in the intestines, undergo a series of chemical changes that will lead them to be “destroyed” (catabolism) to form new molecules (anabolism). Together, catabolism and anabolism make up the well-used term “metabolism”. One part of the process of catabolism, called oxidative catabolism, has a particular job: obtaining energy from nutrients. Our body needs a continuous supply of energy to perform all its functions. Even when we are asleep, we consume energy, and lots of it! Energy expenditure “at rest” is about 65% of total expenditure, while for physical activity, it represents on average about 25%.
Given the constant need for energy, the body must be continually deriving it from nutrients. However, not all nutrients are a source of energy, but only “caloric” ones (proteins, lipids and carbohydrates). They are called caloric because the unit of measurement of energy in nutrition is the calorie, or rather the kilocalorie (kcal). Actually, there are two units of measurement: the kcal and the kilojoule (kj). The first describes energy in terms of heat, and the second in terms of work exerted, and it is important to know that they are not equivalent in numbers: 1 kcal is roughly the same as 4 kj. Since the international unit of measurement is the kj, on the nutritional labels of food products, the value of energy contained is reported either only in kj or, more frequently, both in kj and in kcal. Make sure, therefore, when reading the nutritional labels, to check the energy value (expressed per 100 g of product or per portion) – it is not enough to look at the number, we must also consider the unit of measurement.
Caloric nutrients can be either energetic or structural. The first ones, essentially the carbohydrates, mostly serve to be broken down in our bodies to produce energy, on average 4 kcal / g of carbohydrates. Even from the breaking-down of proteins, our body can obtain 4 kcal / g, but in reality these nutrients are poorly used for energy purposes and their job is instead to build structure (they are defined as structural caloric nutrients). Lipids are both energetic and structural; they are used both to produce energy (9 kcal / g) and for structural purposes. In fact, they are not only made up of the subcutaneous adipose tissue, which is very important in the right quantity because it reduces heat loss and protects the internal organs in the event of trauma, but also the membranes of any type of cells.
Our body is also able to extract energy from another molecule, namely ethyl alcohol, which however is not considered a nutrient, as it is not necessary for the normal development of physiological functions. It is therefore possible to be teetotal without causing any harm. Moderate consumption of alcoholic beverages (2-3 glasses of wine a day) is allowed in adults, keeping in mind however that this also is a source of energy: the body gets about 7 kcal / g from the oxidative catabolism of a gram of ethyl alcohol.
So when we create our diet, and choose the foods to consume, we must remember that they contain nutrients that provide energy, and if these add up to more than the energy we consume, they will be “stored”, mainly in adipose tissue, and will contribute to an increase in body weight. We must therefore learn to distinguish between two different characteristics of each food: the energy density (the possible caloric yield), and the nutritional density that also takes into account the concentration of acaloric nutrients. For example, a plate of salad has a high nutritional density and a low density of energy, while for sweets, it is the exact opposite. It is obvious which is the better choice…
Department of Agricultural and Food Sciences
University of Bologna