Adequate nutrition reportedly can help reduce the degree of muscle loss and improve muscle quality in hospitalized patients recovering from trauma. This study investigated the association between nutritional intake and changes in muscle quality and quantity in these patients. The handgrip strength (HGS) and body composition of 52 patients admitted to the trauma ward were measured at 1-week intervals. According to their dietary intake, they were categorized into the hypocaloric nutrition group (HNG; < 70% of recommended caloric intake) and the isocaloric nutrition group (ING; ≥ 70% of recommended caloric intake). Within one week, body mass index (24.3 ± 4.4 kg/m² vs. 23.4 ± 4.5 kg/m²), body fat percentage (24.1% ± 9.8% vs. 17.2% ± 9.2%), and skeletal muscle mass (28.6 ± 4.9 kg vs. 27.5 ± 4.3 kg) significantly decreased in the ING compared with those in the HNG. Although the skeletal muscle mass decreased, the ING’s left HGS significantly increased (26.6 ± 9.6 kg vs. 28.5 ± 10.1 kg). The ING also consumed a significantly greater amount of protein (beyond the recommended amount) than the HNG (72.6 ± 43.2 → 100.8 ± 27.0% vs. 58.6 ± 25.9 → 49.5 ± 20.1%; p = 0.039). In bioelectrical impedance vector analysis, the vectors of the ING shifted more within the normal range of the 75% tolerance ellipse than those of the HNG (23% vs. 10%). These results suggest that, although the muscle mass quantitatively decreased during trauma recovery, adequate nutritional support helps preserve muscle quality.