Protein2026-07

Effect of Intra-Workout Protein-Carbohydrate Co-Ingestion Versus Isocaloric Carbohydrate During Resistance Training on Muscle Fibre Hypertrophy and Oxidative Capacities in Young Men: A Randomized Controlled Trial.

Svensson Michael, Stål Per, Högström Magnus, Nyström Hagfors Linda et al. — Nutrients

Summary

This study investigated whether consuming a protein-carbohydrate mix during resistance training offers greater muscle growth benefits than consuming only carbohydrates, especially when individuals are not training on an empty stomach. Researchers found that while resistance training significantly improved muscle size and strength, adding protein during the workout did not provide extra benefits compared to an equal-calorie carbohydrate drink. This suggests that for non-fasted individuals, the timing of protein intake during a workout might not be as crucial as the training itself.

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Key points

Resistance training significantly boosts muscle size, strength, and metabolic fitness.
For young men who aren't training fasted, consuming protein with carbohydrates during a workout didn't provide extra muscle growth benefits compared to just carbohydrates.
This suggests that the overall training and sufficient calorie intake might be more important than intra-workout protein timing in non-fasted states.
Due to the study's small size, these findings should be interpreted with caution.

What the study looked at

This research aimed to understand if adding protein to a carbohydrate drink consumed during resistance training could lead to greater muscle growth and improved fitness compared to drinking only carbohydrates, especially when individuals were not training on an empty stomach. Previous studies often looked at fasted training, but this one focused on real-world scenarios where people usually aren't fasted. Seventeen physically active young men participated in an 8-week supervised resistance training program. They were randomly divided into two groups: one consumed a protein-carbohydrate drink during their workouts, while the other consumed an equal-calorie carbohydrate-only drink. Researchers measured changes in muscle fiber size, body composition, strength, and other fitness markers before and after the intervention. Both groups experienced significant improvements in muscle size, strength, and overall fitness due to the resistance training. However, the study found no additional benefits for muscle growth or other outcomes in the group that consumed protein during their workouts compared to the group that only had carbohydrates. The researchers noted that the small number of participants means these findings should be considered exploratory.

Dietary takeaway

For individuals who are not training on an empty stomach, this study suggests that the primary focus for muscle growth should be consistent resistance training and ensuring adequate overall protein intake throughout the day. While protein is crucial for muscle repair and growth, consuming extra protein *during* a workout might not offer additional benefits if you're already well-fed. Remember, this is just one study with a small number of participants, so these findings are not definitive and more research is needed to confirm them.

Abstract

Protein intake in relation to resistance training (RT) is often studied under fasted conditions, although most real-world training is performed in a non-fasted state. This study examined whether intra-workout carbohydrate plus protein (CHO:P) co-ingestion enhances muscle hypertrophy compared with isoenergetic carbohydrate (CHO) alone when RT is performed in a non-fasted state. Additionally, baseline testosterone, insulin-like growth factor 1 (IGF-1), capillary density, and maximal oxygen uptake (VOmax) were evaluated in relation to gains in muscle fibre cross-sectional area (fCSA). : Seventeen physically active young men completed the study (CHO:P, n = 8; CHO, n = 9) as part of an 8-week supervised RT program. Assessments included body composition, physical performance, blood variables, and muscle biopsies obtained before and after the intervention. Fibre-type-specific and distributional (percentile-based) analyses of fCSA were performed. Given the limited sample size, analyses should be considered exploratory. : No significant between-group differences were observed for any outcome (all > 0.05). In contrast, RT induced significant increases in lean mass, muscle strength, VOmax, fat oxidation capacity, and fCSA (all < 0.05). For example, type IIA fCSA median increased from 5232 μm (P10-P90: 2917-7286) to 5925 μm (3794-8422 μm). Distributional analysis revealed that type IIA fibre fCSA increased across the full percentile range, indicating a broad hypertrophic response, whereas type I fibres showed more modest and less uniform changes. A marked fibre-type transition was observed, with type IIX fibres decreasing from ~1.2% (0.5-3.2%) to ~0.0% (0.0-1.3%) and IIA/IIX hybrid fibres from ~8.9% (7.3-12.2%) to ~4.4% (1.1-5.6%). Baseline IGF-1 showed an exploratory association with increases in fCSA ( = 0.04), whereas testosterone, capillary density, and VOmax did not (all > 0.05). : Resistance training performed in a non-fasted state resulted in robust improvements in muscle hypertrophy, metabolic capacity, and physical performance. No additional benefit of intra-workout protein co-ingestion was detected compared with isoenergetic carbohydrate intake under the present experimental conditions. However, given the small sample size and variability in responses, modest effects cannot be excluded. The observed association between baseline IGF-1 and hypertrophy should be interpreted cautiously and requires confirmation in larger studies.

View on PubMed →DOI: 10.3390/nu18142307