Starvation, a prolonged lack of food, triggers complex metabolic adjustments to ensure survival. This occurs when energy intake falls below the level required for basic bodily functions. The body prioritizes and switches between different internal fuel sources, adapting its metabolism to conserve energy and sustain essential processes. This allows humans to endure periods without food by strategically utilizing stored nutrients.
Immediate Energy Reserves
In the initial hours of insufficient food intake, the body first uses glucose, its most readily available energy source. Glucose is primarily stored as glycogen, a complex carbohydrate, in the liver and muscles. Liver glycogen maintains stable blood glucose levels for the brain and red blood cells. Muscle glycogen fuels the muscles for local activity.
These limited glycogen reserves typically deplete within 24 to 48 hours of continuous food deprivation. Once liver glycogen stores are exhausted, the body must find alternative ways to produce glucose for the brain and other glucose-dependent tissues. This depletion marks a significant metabolic shift, moving the body beyond its immediate carbohydrate reserves.
The Shift to Fat Metabolism
As glycogen stores diminish, the body transitions to using fat as its primary energy source. Fat is stored in adipose tissue as triglycerides, which are highly efficient energy reserves. These triglycerides break down into fatty acids and glycerol for energy production. Fatty acids become the main fuel for most tissues, sparing any remaining glucose for the brain.
During this stage, ketogenesis occurs in the liver. When glucose is scarce, the liver converts fatty acids into ketone bodies (e.g., acetoacetate, beta-hydroxybutyrate). These water-soluble molecules cross the blood-brain barrier, providing an alternative fuel source for the brain and other organs that typically rely on glucose. This adaptation reduces the brain’s reliance on glucose, thereby conserving protein stores. Ketone bodies typically become a significant brain fuel after two to three days of fasting, with levels rising during prolonged food deprivation.
Protein Breakdown: A Critical Stage
While fat serves as the primary energy source during starvation, the body eventually breaks down its own protein when fat reserves become severely low. This process primarily affects muscle tissue, which is a substantial protein reservoir. Amino acids released from muscle protein are transported to the liver for gluconeogenesis, converting them into new glucose molecules.
This glucose is essential for functions that cannot readily use fat or ketones, such as certain brain cells and red blood cells. While this mechanism helps maintain glucose levels, it comes at the expense of muscle mass. The breakdown of protein leads to muscle wasting, impacting physical strength and overall bodily function. This stage signifies deeper energy depletion, as the body sacrifices functional tissue to sustain life-sustaining processes.
Body’s Adaptations to Prolonged Starvation
The body implements broader physiological adaptations to cope with prolonged starvation. A decrease in the metabolic rate reduces overall energy expenditure to conserve resources. This slowdown affects various bodily functions, contributing to lethargy and reduced activity.
Hormonal changes also orchestrate these metabolic shifts. Insulin levels, which promote energy storage, decrease, while hormones like glucagon, epinephrine, norepinephrine, and cortisol, which mobilize stored energy, increase. Thyroid hormone levels also tend to decrease, further reducing the metabolic rate. These adjustments prioritize energy distribution, ensuring organs with stringent glucose requirements, such as the brain, receive a sustained supply, even if other tissues adapt to alternative fuels.