To assess the impact of rationed versus full estimated energy provisions on markers of physiological strain in response to a simulated 250 km multistage ultramarathon (MSUM), on two separate occasions, the ultraendurance runner performed a laboratory simulated MSUM, with rationed (RP: 3303±75 kcal⋅day -1 ) and full (FP: 7156±359 kcal⋅day -1 ) provisions. Total daily energy expenditure was determined using dual-method indirect calorimetry. Resting metabolic rate, iDXA, and body water were measured at baseline, day 3, and post-MSUM. Blood, urine, and feces were collected, and mood state was measured, d 1 to 5 (before and after running) to determine various physiological strain indices. Heart rate, RPE, thermal comfort, gastrointestinal symptoms, and non-protein oxidation rates were measured every 30 min during running. Data were analyzed using single-subject design analysis and interpreted using Cohen's effect size. Energy expenditure was lower on RP (6943±145 kcal⋅day -1 ) than FP (7486±143 kcal⋅day -1 ) (Cohen's δ=-3.1). More pronounced exertional strain (RPE δ=1.2, thermal conform δ=0.6, rectal temperature δ=1.0, and plasma cortisol concentration δ=1.7) was observed on RP as the MSUM progressed. Total carbohydrate and fat oxidation during running decreased (0.76 vs. 1.82 g⋅min -1 ; δ=-3.9) and increased (0.91 vs. 0.54 g⋅min -1 ; δ=3.7), respectively, more profoundly on RP as the MSUM progressed. Gastrointestinal symptoms were modestly lower in RP (δ=-0.26). Exercise-induced leukocytosis, cytokinaemia, and neutrophil responses were higher on RP. Iron status markers were trivial. Higher mood disturbance and fatigue were reported on RP. The ultraendurance runner presented greater physiological and psychophysiological disturbances, in response to a laboratory simulated MSUM, on rationed energy provisions, despite the lighter pack-weight.