Foods give the essential energy for the functioning and growth of our organism. Fats, proteins, and carbohydrates are different types of energy providers, and their calibrated transformation (or metabolism) gives the energy for physiological functions to occur. The energetic equilibrium of an organism is reached when the food provides exactly the amount of energy that the organism uses. Food/nutrition is the primary engine of human activities and has many complex interrelationships with other systems that concern biology, medicine, culture, and sociology. The eating/nutrition system has proven itself through scientific research and epidemiology rather complex as the human organism, according to the principles of thermodynamics, is an open system in dynamic equilibrium that exchanges energy and matter with the external environment to maintain anatomical and functional integrity. Maternal metabolism is designed to provide adequate nutrition for fetal growth in the form of glucose, ketones, lipids, and protein. Insulin sensitivity is one of the metabolic adaptations that occurs in pregnancy in order to support the growing fetus’s needs. In early pregnancy, insulin sensitivity increases, stimulating the uptake of glucose into adipose stores in preparation for the energy demands of later pregnancy. Amino acids have an important role as an important fetal nutrient, and maternal metabolic changes that spare amino acids for fetal and placental use support growth. The nutritional management of pregnant women is complex, as weight changes must be carefully evaluated, and a higher protein intake does not entail any benefit for the health and well-being of women and the fetus.