Energy: the body’s fuel

The energy derived from the oxidation of the three energy nutrients (proteins,carbohydrates and fats) is expressed in calories and this is how the energy output of these three macronutrients is usually conveyed.The calorie translates energy into units of heat and is defined as the heat required to increase the temperature of one gram of water by one degree centigrade.When we speak about the calories in food we are really speaking about kilocalories,or 1,000 calories.The kilocalorie (kcal),therefore,is the heat required to increase the temperature of one kilogram of water by one degree centigrade.One kilocalorie (kcal) is equal to 1,000 calories.

The energy output of micronutrients can also be expressed in kilojoules,or units of energy – instead of units of heat,as is the case with kcal.1 kcal = 4.184 kilojoules.

When nutrients oxidise,their energy output is expressed in kcal per g of nutrient.This is one of the fundamental pillars of dietetics,as it allows for the conversion of kcals into grams of nutrients and,subsequently,grams of food,making it possible to draw up guidelines and recommendations on consumption and to plan diets and meals.Of course,it also allows for the reverse – enabling the study of how people and populations feed themselves based on the grams of food they consume.

Each of the three energy nutrients provides a different energy output:

• Carbohydrates provide 3.75 kcal/g,although this is usually rounded up to 4 kcal/g.
• Proteins provide 4 kcal/g.
• Fats provide 9 kcal/g.
• The energy value of ethyl alcohol – 7 kcal/g – is also usually taken into account.

Human beings use the energy obtained from food for various ends:

• Basal metabolism to maintain the body’s vital functions,as,even while sleeping,the body needs energy to maintain organs such as the brain (which accounts for a large proportion of basal energy requirements) and the heart,in addition to the respiratory system,muscle tone and circulatory system.
• Thermogenesis,or the production of heat to maintain body temperature,occurs during the process of oxidising foods.Protein has a higher thermogenetic effect than the other two nutrients.
• Physical activity; the energy used will be greater or lesser depending on whether the activity undertaken is light,moderate or intense.

A balanced diet results from the right combination of the different foods types,and should ensure that energy and nutrient requirements are met.Both excess consumption and deficiencies should be avoided,as the objective is to keep individuals in optimal health conditions at all stages and in all physiological conditions of their life.The recommended distribution of energy that should be obtained from the three energy nutrients is:

• 50-60% from Carbohydrates.
• 25-35% from Fats.
• 10-15% from Proteins.