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

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.

Conditions

• Female Sex Hormones
• Menstrual Cycle
• Oral Contraceptives
• Protein Metabolism

Interventions

DIETARY_SUPPLEMENT

Protein Beverage

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

DIETARY_SUPPLEMENT

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.

DIETARY_SUPPLEMENT

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.

Sponsors & Collaborators

• University of Toronto

  lead OTHER

Study Design

Allocation

RANDOMIZED

Purpose

BASIC_SCIENCE

Masking

NONE

Model

CROSSOVER

Eligibility

Min Age

18 Years

Max Age

40 Years

Sex

FEMALE

Healthy Volunteers

Yes

Timeline & Regulatory

Start

2025-07-20

Primary Completion

2026-09-01

Completion

2026-09-01

Countries

• Canada

Study Locations