You just crushed your workout. Now you're frantically chugging a protein shake, racing against the clock to catch that mythical 'anabolic window.' For years, lifters have debated the optimal time to consume protein to maximize gains. Is it better to fuel up before you lift or replenish immediately after?
The internet is full of conflicting advice, and even past scientific reviews have been murky. But a new 2025 systematic review and meta-analysis published in Nutrients by Casuso and Goossens set out to provide a clearer answer by looking at the evidence in a more rigorous way.
Let's break down what they found and what it means for your training.
A Better Way to Ask the Question
Previous meta-analyses on this topic often had a critical flaw: they frequently compared a group taking protein (either before or after) to a group taking a placebo. This tells us that protein is better than nothing, but it doesn't tell us if timing matters when total daily protein is the same.
This new analysis was different. The researchers only included randomized controlled trials that met a strict criterion: they had to directly compare a group taking protein before a workout to a group taking protein after the workout, with total daily protein and calories matched between them. This isolates the variable of timing, giving us a much cleaner look at its specific effect.
After screening over 1700 studies, they found only five that fit their strict criteria (representing six reports). These studies involved a mix of young trained men and women, older untrained adults, and older individuals with type 2 diabetes.
The Main Verdict: Timing Doesn't Seem to Matter for Muscle Growth or Upper Body Strength
After pooling the data, the researchers came to a clear conclusion for most outcomes: protein timing doesn't significantly impact muscle growth or strength gains.
Here’s a quick rundown of the key findings:
- Lean Body Mass: There was no significant difference in lean mass gains between the pre- and post-workout protein groups (Standardized Mean Difference [SMD]: -0.08). Your overall muscle growth wasn't affected by when you had your shake.
- Fat Mass: Similarly, timing had no effect on changes in fat mass.
- Muscle Thickness: Direct measurements of bicep, triceps, and vastus lateralis (quad) thickness showed no significant differences between groups.
- Upper Body Strength: For chest press 1RM, the primary measure of upper body strength across five studies, timing made no difference (SMD: 0.07).
For the majority of your goals—getting bigger and stronger overall—this study suggests that as long as you're hitting your daily protein target, the precise timing around your workout is far less important than we've been led to believe.
The Surprising Twist: A Potential Edge for Leg Day?
While the overall message was clear, the analysis uncovered one interesting exception. When looking specifically at leg press 1RM, the data showed a notable advantage for the pre-workout protein group.
Consuming protein before training led to a greater increase in leg press strength compared to consuming it after (SMD: 0.70).
Before you immediately switch to pre-workout shakes on leg day, it's crucial to understand the context. The authors urge this finding be interpreted with caution for a few key reasons:
- Very Small Data Pool: This finding is based on data from only two studies involving a total of 53 subjects.
- Low Certainty: The overall certainty of the evidence for this specific outcome was rated as 'very low.'
The researchers proposed a potential hypothesis for this finding. They noted that leg muscles have a higher mitochondrial content than arm muscles and are the main site for breaking down Branched-Chain Amino Acids (BCAAs) for energy. Therefore, providing protein (and thus BCAAs) before a leg workout might offer a readily available fuel source, potentially improving performance by keeping the muscles further from failure during a set.
This is a fascinating idea, but it's just a hypothesis that needs to be tested in future, larger studies.
Practical Takeaways for Lifters and Coaches
So, what does this all mean for you in the gym?
- Focus on Total Daily Protein First: The most important variable for muscle growth is consuming enough protein throughout the day (typically 1.6-1.8 g/kg for athletes). Timing is a secondary, and likely minor, detail.
- The 'Anabolic Window' is More Like a 'Barn Door': Don't stress about slamming a shake within 30 minutes of your last rep. This study looked at a window from 15 minutes before to 2 hours after training. As long as you get a high-quality protein meal in within a few hours of your workout, you're likely maximizing your recovery and growth.
- Consider Experimenting on Leg Day: If you're an advanced athlete looking for every possible edge, you could try having your protein shake before your next leg session. It's a low-risk strategy. Just don't expect it to be a magic bullet.
- Be Critical of the Evidence: This meta-analysis is a step forward, but the authors are transparent about its limitations. The small number of available studies and the limited methodological quality of some of them mean the science is still evolving. The certainty of the evidence for all outcomes was rated 'low' to 'very low.'
In conclusion, this rigorous 2025 analysis confirms what many coaches have suspected: for most people, consistency with total protein intake trumps precise nutrient timing. Nail the fundamentals first. The exact timing of your protein shake is likely not the thing holding you back from making progress.
Reference:
Casuso, R. A., & Goossens, L. (2025). Does Protein Ingestion Timing Affect Exercise-Induced Adaptations? A Systematic Review with Meta-Analysis. Nutrients, 17(13), 2070. https://pubmed.ncbi.nlm.nih.gov/40647175/