7-Day Creatine ‘Wash-In’ Boosts Muscle Mass in Women — But Not Long-Term, Study Reveals

New research suggests short-term creatine supplementation increases lean body mass in females, but effects may be temporary and linked to water retention.

A recent study published in Nutrients¹ has found that a seven-day creatine monohydrate (CrM) “wash-in” period—where participants took 5 grams daily without exercise—led to increased lean body mass (LBM), particularly in women. However, when combined with long-term resistance training (RT), creatine did not provide additional muscle gains beyond what RT alone achieved.

Key Findings: Short-Term Gains, No Long-Term Advantage

The study involved healthy, inactive adults who were divided into two groups: one receiving CrM supplements² and a control group. After the initial seven-day wash-in phase, the supplement group showed significant increases in total LBM, especially in the trunk region, suggesting possible water retention effects. However, after 12 weeks of resistance training, both groups gained similar amounts of muscle (around 2 kg), indicating that creatine did not enhance long-term muscle growth beyond exercise alone.

Notably, the benefits were sex-specific:

• Women saw greater LBM gains during the wash-in phase and maintained slightly higher trunk LBM over the full study period.
• Men did not experience significant LBM changes from creatine at any point.

Why Does This Matter?

Increasing lean body mass has well-documented health benefits, including improved bone strength, better blood sugar control, and reduced mortality risk. While resistance training remains the most effective way to build muscle, the role of creatine—a popular performance supplement—has been debated.

Previous research³ suggests that creatine enhances training capacity, but its direct impact on muscle protein synthesis is unclear. This study highlights that:

• A short-term creatine load (without exercise) can temporarily increase LBM, possibly due to fluid retention.
• Long-term creatine use alongside RT may require higher maintenance doses (beyond 5 grams/day) to sustain benefits.

Study Limitations and Future Research

The study had several limitations, including:

• No tracking of menstrual cycles, which could influence water retention in women.
• Lack of hydration measurements, making it difficult to distinguish between muscle growth and fluid shifts.
• Self-reported dietary data, which may underestimate actual intake.

Future studies should explore different dosing strategies, monitor hydration levels, and account for hormonal fluctuations to better understand creatine’s effects on muscle mass.

Conclusion

While a seven-day creatine wash-in may boost lean body mass—especially in women—these gains appear short-lived and may not translate to long-term muscle growth when combined with resistance training. For those considering creatine supplementation, the findings suggest that its primary benefits may lie in enhancing workout performance rather than directly increasing muscle mass over time.

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