Final answer:
When insulin levels are low, the body uses fats for energy; when they are high, it uses carbohydrates. Insulin regulates the storage and utilization of energy by promoting glucose uptake and storage as glycogen, or by stimulating fat breakdown when its levels are low.
Step-by-step explanation:
When insulin levels are low, it pushes the body to use fat as a primary fuel. However, when insulin levels are high, it pushes the body to use carbohydrates for fuel. Insulin, a hormone secreted by the pancreas, plays a crucial role in regulating blood glucose levels by facilitating the uptake of glucose into the cells. Following a meal, insulin stimulates the storage of blood glucose as glycogen in the liver and muscle. This causes a decrease in blood glucose levels. On the other hand, low insulin levels signal the body to break down stored fats into fatty acids that can be used as an alternative fuel source, a process known as lipolysis. Insulin also promotes lipogenesis, the synthesis of fats when in excess. During fasting or low carbohydrate intake, insulin levels drop, prompting energy production from fats.