When a person engages in resistance training but consistently fails to consume adequate calories, they create low energy availability (LEA). This chronic energy deficit forces the body into survival mode, where the high energy demands of lifting weights clash with an insufficient fuel supply. The body’s primary goal shifts from adapting to exercise, such as building muscle, to simple survival and energy conservation. This prevents the necessary recovery and repair processes required after intense physical stress, undermining the goals of weight lifting and compromising overall health.
The Body Enters a Catabolic State
The immediate physiological response to an energy deficit is a shift toward catabolism, the process of breaking down complex molecules for energy. When intake is too low to cover basal metabolic needs and the high expenditure of weight training, the body searches for internal fuel sources. Since glycogen stores are quickly depleted, the body turns to muscle tissue itself.
This results in an increased rate of skeletal muscle protein breakdown, a state that directly counters the goal of muscle growth (hypertrophy). The body sacrifices muscle protein to produce glucose for immediate energy and to supply amino acids for other vital bodily functions. Studies show that low energy availability can decrease muscle protein synthesis rates by a significant margin, even when protein intake is high. Therefore, instead of gaining or even maintaining muscle mass, the individual experiences sarcopenia, or muscle wasting.
This process of breaking down muscle for fuel prevents the necessary rebuilding and strengthening that resistance training is supposed to stimulate. Even with consistent training, the body cannot repair the microscopic tears in muscle fibers without sufficient energy and building blocks. A meta-analysis suggests that an energy deficit of approximately 500 kcal per day is enough to prevent gains in lean mass, illustrating how quickly this catabolic state can impair physical progress.
Performance Degradation and Training Plateaus
The acute consequence of under-fueling is a rapid decline in training quality and a halt in strength progression. Chronic low energy availability leads to persistent fatigue that is not alleviated by rest. This fatigue is compounded by a lack of mental focus, often described as “brain fog,” which impairs the brain-muscle connection necessary for effective lifting.
The body’s inability to replenish muscle glycogen stores between sessions directly reduces power output and strength capacity. An individual may find their lifting numbers stagnate or even decrease, regardless of their effort in the gym. This creates a training plateau, where the physical stress of the workout can no longer drive positive adaptation.
The reduced ability to recover between sets or between training days forces a lower intensity in workouts, further blunting the muscle-building stimulus. This cyclical problem means the person is training hard but their body is functionally detraining, making the entire resistance training effort counterproductive to strength and size goals. Studies show that low energy availability can impair explosive power, such as measured by a countermovement jump, within a matter of days or weeks.
Systemic Health and Metabolic Slowdown
Chronic low energy availability triggers widespread systemic consequences, as the body prioritizes life-sustaining functions. The most notable long-term effect is metabolic adaptation, where the body actively lowers its basal metabolic rate (BMR) to conserve energy. This survival mechanism means the body requires fewer calories for basic functions, making it progressively harder to lose weight or maintain a lower body weight without further reducing food intake.
This state of energy deficit severely disrupts the endocrine system, a collection of glands that produce hormones that regulate metabolism, growth, and mood. Stress hormones, particularly cortisol, become chronically elevated, which further promotes muscle breakdown and fat storage around the midsection. Conversely, anabolic hormones like testosterone and triiodothyronine (T3), a key thyroid hormone that regulates metabolism, are reduced. The suppression of T3 effectively slows down numerous bodily processes, and the reduction in testosterone impairs muscle recovery and overall energy levels.
Furthermore, the immune system is significantly suppressed when energy is chronically restricted. The body diverts resources away from non-immediate survival functions, making the individual more susceptible to frequent illnesses and delaying the healing time for injuries. Psychological health also suffers, with common symptoms including increased irritability, anxiety, and disruptions to sleep quality. This comprehensive breakdown of essential systems illustrates how chronic underfueling turns a fitness pursuit into a serious health risk, a syndrome known as Relative Energy Deficiency in Sport (RED-S).