Nutrients are substances needed by the body to sustain basic functions, in order to survive, grow, and reproduce, and are optimally obtained by eating a balanced diet. Thus, glucose and other carbohydrates, amino acids, and lipids are essential cellular nutrients, with certain mechanisms to sense their availability in mammalian cells. The capability to sense and respond to variations in environmental nutrient availability is a key requisite for survival. Thus, cells must be able to store nutrients when they are abundant and access them when nutrients are scarce. Moreover, nutrient levels in circulation need to stay within certain safe ranges. Therefore, cells must be able to sense nutrient levels in order to react appropriately. Various pathways that sense intracellular and extracellular levels of carbohydrates, amino acids, lipids, and different metabolites are integrated and coordinated at the organismic level via hormonal signals. Throughout food abundance, nutrient-sensing pathways employ anabolism and storage, whereas food scarcity activates homeostatic mechanisms. There are four nutrient sensing pathways: