Final answer:
Carbohydrates such as glucose are used immediately for energy or stored as glycogen in the absorptive state. When fasting, the body maintains energy by converting this glycogen back into glucose. In starvation, the body uses fats and proteins for energy, underscoring the significance of carbohydrates in metabolic processes.
Step-by-step explanation:
Sources of Carbohydrates in Metabolic States
Within a 24-hour period, the human body transitions through various metabolic states, including the absorptive (fed), postabsorptive (fasting), and, rarely without prolonged fasting, the starvation states. Carbohydrates play significant roles in these metabolic changes. During the absorptive state, carbohydrates are ingested, digested, and absorbed. Glucose, a primary carbohydrate, enters the bloodstream and is used immediately for energy or stored as glycogen in the liver and muscles.
As the body enters the postabsorptive state, insulin levels fall, and glucagon is released to convert glycogen back into glucose to maintain energy supply. When the body faces prolonged fasting and enters starvation, glycogen stores are depleted, and the body begins utilizing fatty acids and proteins for energy. The liver produces ketone bodies, which the brain can use as an energy source. If starvation persists, muscle proteins break down, highlighting the vital importance of carbohydrates in the diet.