A lack of sufficient caloric intake triggers a complex physiological response designed to preserve life. When the body does not receive energy from food, it initiates a survival sequence, mobilizing stored resources and altering metabolic processes to conserve fuel. This state of energy deprivation, commonly called hunger, forces the body to shift its primary fuel source from external food to internal reserves. These internal adjustments impact virtually every bodily system, from immediate changes in brain chemistry and mood to long-term systemic deterioration.
The Body’s Immediate Energy Shift
The body’s initial response to short-term hunger involves a rapid shift in fuel metabolism to maintain a stable supply of glucose for the brain. Within the first few hours of fasting, the liver breaks down stored carbohydrates (glycogenolysis), releasing glucose into the bloodstream. This hepatic glycogen reserve is relatively small, often depleting within 24 hours. As glycogen stores diminish, the body transitions to using fat as its main energy source (lipolysis). Triglycerides stored in adipose tissue are broken down into glycerol and free fatty acids, which become the primary fuel for most tissues, sparing glucose for the brain and red blood cells.
The liver also begins ketogenesis, converting fatty acids into ketone bodies. These ketones can cross the blood-brain barrier and serve as an alternative fuel for the brain, helping to preserve muscle tissue. The brain can derive up to 30% of its energy from ketones within three days of fasting. Simultaneously, the potent appetite-stimulating hormone ghrelin is secreted by the stomach lining. Ghrelin levels rise significantly during fasting, signaling negative energy balance to the hypothalamus, which initiates the drive to seek food.
Impact on Cognitive Function and Mood
The brain demands a constant and substantial energy supply, primarily glucose, making it highly sensitive to changes in fuel availability. When blood glucose levels drop (hypoglycemia), cognitive functions become impaired. Individuals may experience difficulty concentrating, reduced mental clarity, and impaired decision-making abilities.
This compromised mental state is often accompanied by significant psychological and emotional changes, commonly described as “hanger.” Increased irritability, anxiety, and mood swings are frequently reported. This is linked to the stress response triggered by low glucose, which prompts the body to release stress hormones, particularly cortisol and catecholamines.
These hormones promote the breakdown of stored resources to raise blood sugar. The release of cortisol, even at levels comparable to psychological stress, increases self-reported hunger and affects the brain’s control centers for food intake. This hormonal surge impacts neural activity, further linking the metabolic stress of hunger to emotional volatility. The connection between low glucose and negative mood demonstrates the brain’s dependence on stable fuel for optimal emotional regulation.
Immune System Suppression and Physical Deterioration
Prolonged hunger forces the body beyond temporary coping mechanisms, leading to systemic physical deterioration and immune suppression. The body begins to break down protein from non-adipose tissues (catabolism) to generate amino acids for gluconeogenesis—the creation of new glucose. This desperate attempt to maintain blood glucose results in significant loss of muscle mass, known as muscle wasting or atrophy.
The catabolism of skeletal muscle is often exacerbated by systemic inflammation, which interferes with muscle protein synthesis. This prolonged lack of necessary macronutrients severely compromises the immune system, as the production of immune cells slows down due to the scarcity of required building blocks. This suppression leaves the body highly vulnerable, drastically reducing the ability to defend against infections.
The strain of chronic hunger also extends to vital organs, contributing to multiorgan failure. The heart, being a muscle, begins to atrophy, reducing its pumping efficiency. The liver and kidneys are placed under excessive stress as they process the byproducts of continuous tissue breakdown.