Differences Between Habitual Low and Normal Carbohydrate Intake in Body Composition and Maximal Isometric Strength in Resistance-Trained Men

Introduction: Most studies evaluating low-carbohydrate diets in resistance-trained individuals have focused on short-term ketogenic interventions. Whether habitual carbohydrate intake maintained under free-living conditions is associated with body composition and muscle strength remains unclear. Therefore, this study compared body composition and maximal isometric strength between recreationally resistance-trained men with a habitual low or normal carbohydrate intake. Methods: A cross-sectional analytical study was conducted in 53 recreationally resistance-trained men aged 18–35 years. Habitual dietary intake was assessed using three non-consecutive 24 h dietary recalls. Participants were classified as having low (<3 g/kg/day; n = 28) or normal (3–5 g/kg/day; n = 25) carbohydrate intake; participants also reported maintaining their respective dietary pattern for at least three months. Body composition was estimated using the Ross and Kerr anthropometric model, whereas fat mass was calculated using the Martin equation. Maximal isometric handgrip and knee extension strength were also assessed. Between-group comparisons were performed using Welch’s t-test or the Mann–Whitney U test, and multiple linear regression models were adjusted for age, height, and relative energy intake. Sensitivity analyses were additionally adjusted for relative protein intake. Results: Participants with a habitual low carbohydrate intake presented significantly lower adipose mass (β = −2.42 kg; 95% CI: −4.49 to −0.35; p = 0.02), adipose percentage (β = −4.64%; 95% CI: −7.47 to −1.80; p < 0.01), fat mass (β = −2.33 kg; 95% CI: −4.08 to −0.57; p = 0.01), and body fat percentage (β = −4.06%; 95% CI: −6.52 to −1.59; p < 0.01) than participants with a normal carbohydrate intake. No significant differences were observed in muscle mass, handgrip strength, or absolute knee extension strength after adjustment. In sensitivity analyses additionally adjusted for relative protein intake, associations with adiposity were attenuated and no longer statistically significant, whereas low carbohydrate intake was associated with lower absolute and relative handgrip and knee extension strength (all p < 0.05). No significant associations with skeletal muscle mass were observed. Conclusions: In recreationally resistance-trained men, habitual carbohydrate intake below 3 g/kg/day was associated with lower adiposity in the primary analyses; however, these associations were attenuated after additional adjustment for relative protein intake. Conversely, associations with lower maximal isometric strength emerged after adjustment for protein intake, while no associations with skeletal muscle mass were observed. These findings suggest that the associations of habitual carbohydrate intake with adiposity and muscle strength may be influenced by overall dietary composition and should be interpreted cautiously given the cross-sectional design.