Fats are essential nutrients that have a significant role in the human diet and are essential to provide energy. Fatty acids are present in several types of lipids, such as triglycerides and phospholipids. Fatty acids differ among them, depending on the number of double bonds and on the length of the hydrocarbon chains. If there are no double bonds, the fatty acids are considered saturated and show a linear structure. Compounds with double bonds are unsaturated and have bent structure. The saturated fatty acids are usually solid at room temperature and the unsaturated fatty acids are liquid at that very same temperature. These compounds are of recognized value as raw materials for drug delivery systems, such as lipid nanoparticles. The behaviour of the macroscopic aspects of fat polymorphisms is directly influenced by the melting point, the crystallization and their polymorphic transformations. In this work, we revise the most critical factors contributing for the long-term stability of lipid nanoparticles, as well as the influence of the polymorphism on the loading capacity for drug molecules.