A runner’s persistent fatigue often signals more than simple exhaustion. When it endures after workouts, slows down easy runs, and fails to improve even with rest, the body may be operating with two simultaneous shortages. The issue isn’t always training intensity. Sometimes, insufficient nutrition fails to meet the demands of the effort, and the two problems can produce nearly identical effects, complicating diagnosis.
Cortney Berling, a registered dietitian and certified running coach, describes these as two sides of the same imbalance: energy output surpassing recovery input. The key difference lies in duration and location. Overtraining leaves legs feeling heavy for days after a strenuous session, while underfueling causes a widespread energy drain that can last weeks without correction.
Berling explains the distinction through a basic mismatch. Overtraining forces the body to perform beyond muscle capacity in a short period, while underfueling deprives the system of calories needed to sustain even moderate activity. Both conditions deplete glycogen stores, but one represents a sprint beyond limits while the other resembles a marathon running on empty.
Yet symptoms often overlap. Mental fog, irritability, and sudden appetite loss aren’t solely psychological effects of pushing too hard—they also signal the body lacks sufficient fuel for basic function. A 2021 narrative review in Sports Medicine found evidence of low energy or low carbohydrate availability in 86 percent of overtraining studies, leading the authors to suggest that many cases attributed to overtraining may actually reflect underfueling.
For new runners, confusion frequently begins with mileage. Someone transitioning from casual running to marathon training, adding volume and intensity too rapidly, may hit a performance wall their body cannot adapt to. The result is fatigue that only resolves when training load decreases.
Even with ideal training progression, fatigue can persist if nutrition doesn’t align with demands. Berling’s clients often assume their diets provide adequate fuel, only to discover meals lack the proper balance. A high-protein diet without sufficient carbohydrates, for instance, leaves muscles without quick energy. Timing is equally critical: fueling before, during, and after runs is essential, even if daily calorie intake appears sufficient.
For runs under 60 minutes, 30 grams of simple carbohydrates, such as a banana or two graham crackers, consumed 10 minutes before starting provides necessary energy. Longer efforts require 0.5 to 1 gram of carbohydrates per pound of body weight, taken one to two hours before the run. During runs exceeding 60 minutes, 30 to 60 grams of carbohydrates per hour helps maintain performance levels.
The post-run period demands equal attention. Berling recommends a 3:1 carbohydrate-to-protein ratio, approximately 60 grams of carbohydrates and 20 grams of protein, within 30 minutes of finishing. Her preferred recovery smoothie combines fruit, milk, Greek yogurt, whey protein, and granola for an easily digestible boost.
When fatigue doesn’t improve despite dietary and training adjustments, Berling advises consulting a sports dietitian or physician. Some persistent exhaustion cases stem from underlying conditions like thyroid dysfunction, anemia, or gastrointestinal issues that disrupt nutrient absorption. Blood tests can detect deficiencies in iron, vitamin B12, or ferritin, common among endurance athletes, while a dietitian can verify whether caloric intake aligns with actual energy needs rather than perceived requirements.
