The effects of energy deficiency (ED) on health-related outcomes are well described in the literature. Few studies have addressed the effect of controlled ED on performance, and particularly whether exercise (EX), which is known to preserve lean mass, can offset potential declines in performance during ED. PURPOSE: 1) To determine the effects of short-term ED on performance and metabolism and 2) to compare if these effects vary when ED is achieved through caloric restriction (CR) alone or through CR and EX. METHODS: Six young, healthy, and physically fit men (25.2±2.4 y; 79.7±7.7 kg; 9.6±1.5% body fat) participated in this 4-way crossover study. ED conditions were characterized by an energy availability (EA) of 15 kcal/kg fat free mass (FFM), once accomplished via CR to 30 kcal/kg FFM and prescribed EX (15 kcal/kg FFM; ED+EX) and once via CR and no EX (ED-EX). Control conditions were characterized by an EA of 40 kcal/kg FFM, accomplished both with (C+EX) and without EX (C-EX). Conditions lasted 4 days. Body composition, substrate metabolism, aerobic and anaerobic performance, strength, and indices of well-being were measured before and after each condition. RESULTS: Body weight decreased in ED+EX (-1.8±0.4 kg, p=0.004) and ED-EX (-2.4±0.3 kg, p<0.001); whereas FFM decreased only in ED-EX (p=0.06). No significant changes were found in anaerobic performance, leg curl, and leg extension. Bench press performance decreased only in ED+EX (p=0.04). VO2max increased by 6.2±1.7% in ED+EX (p=0.007), but not in ED-EX. Submaximal HR and RPE increased only in ED-EX (p≤0.05). Fasting glucose decreased in ED+EX (-11.9±2.3% p=0.008), but not in ED-EX. In both ED conditions, there was an increase in fasting free fatty acids (ED+EX: 96±25%, p=0.02; ED-EX: 175±98%, p=0.07) and submaximal fat oxidation (ED+EX: 129±51%, p=0.004; ED-EX: 126±21%, p = 0.001). In ED-EX, but not in ED+EX, there was a decrease in positive mood (p=0.04), self-confidence (p=0.02), sense of fitness (p=0.08), physical energy (p=0.06), and recovery (p=0.07). CONCLUSIONS: Despite similar changes in body weight and substrate metabolism during both ED conditions, reductions in FFM, submaximal performance indices, and indices of well-being were only observed in ED-EX. EX seems to be an effective means to maintain performance, FFM, and well-being during short-term ED.