Menstrual Cycle vs. Oral Contraceptives: Effects on Muscle Protein Metabolism After Resistance Exercise

Condition(s) studied

Investigational drug(s) / intervention(s)

Protein Beverage: Participants will consume 16 half-hourly (8 hours) isoenergetic, isonitrogenous beverages containing 1.2 g/kg/d protein.

Stable Isotope Tracer [D5]phenylalanine: Protein beverages will be enriched with the stable isotope \[D5\]phenylalanine. The use of \[D5\]phenylalanine will allow for determination of muscle protein synthesis.

Stable Isotope Tracer [13C]phenylalanine: Protein beverages will be enriched with the stable isotope tracer \[13C\]phenylalanine. The use of \[13C\]phenylalanine will allow for the determination of the fate of amino acids in the body (incorporation into body protein or oxidation) which can be used to determine protein requirements.

Study summary

The muscles of the body are constantly breaking down old proteins and building new ones. These two processes, protein breakdown and protein synthesis, together are known as protein turnover. Protein turnover is essential for maintaining healthy muscle.

Despite its importance, females have historically been underrepresented in protein metabolism research. A long-standing assumption has been that fluctuations in female sex hormones such as estrogen and progesterone, whether across the natural menstrual cycle or in individuals using oral contraceptives (OCs), make metabolism and training responses too variable to study. Because of this, many researchers have excluded female participants for logistical reasons.

Resistance exercise, such as weightlifting, is the most effective way to increase muscle size and strength. Each resistance-training session triggers muscle protein synthesis (MPS), the process by which new muscle proteins are built. Consuming dietary protein or individual amino acids further increases the rate at which new proteins are formed. Over time, higher rates of protein synthesis support muscle growth and the maintenance of other lean tissues in the body.

The purpose of this study is to examine how menstrual cycle phases and OC use influence the synthesis of proteins in both muscle tissue and the rest of the body. Improving scientific understanding in this area will support more effective, evidence-based training and nutrition recommendations for females.

Eligibility

Primary outcome measure(s)

• Muscle Protein Synthesis (MPS) — 8 hours
  Muscle protein synthesis (MPS) will be assessed during an 8-hour post-exercise and feeding period using a stable isotope tracer, L-\[ring-D5\]phenylalanine (\[D5\]Phe). The incorporation of the \[D5\]Phe tracer into myofibrillar muscle proteins, collected from muscle biopsies (pre-exercise and post-8h feeding), will be analyzed using liquid chromatography-tandem mass spectrometry (LC-MS/MS) to determine the fractional synthesis rate (FSR) of muscle proteins. This will help us understand how menstrual cycle phases and oral contraceptive use influence MPS, providing insight into optimizing protein intake and training adaptations in female athletes.

Trial sites (1)

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