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NCT Number: NCT07258576

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

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.

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Key information

Conditions

Female Sex Hormones Menstrual Cycle Oral Contraceptives Protein Metabolism

Age range

18 year–40 year

Sex eligibility

Female

Study type

Interventional

Phase

Not applicable

Primary location

Goldring Center for High Performance Sport

Toronto, Ontario, M5S 2C9, Canada

Location status: Recruiting

Location contact

Ines Kortebi, PhD Candidate

CONTACT

[email protected]

6479392755

Who can participate

Healthy volunteers accepted: Yes

Only the study team can determine whether someone qualifies for participation.

Inclusion criteria

  • Age: 18-40 years
  • BMI between 18.5-29.9 kg/m2 (non-obese)
  • Recreationally active (resistance train minimum twice a week)
  • Using monophasic or triphasic oral contraceptives for >1 year (for oral contraceptive users)
  • Have regular menstrual cycles (21-35 days) for the past 3 months and discontinued any hormonal contraceptive use for at least 6 months (non-oral contraceptive users)
  • Must meet a progesterone sufficiency test (non-oral contraceptive users)

Exclusion criteria

  • Mid-luteal progesterone levels <16umol
  • Chronic disease diagnosis (cardiovascular, thyroid, diabetes)
  • Current or recent remission of cancer
  • Regular use of non-steroidal anti-inflammatory drugs (NSAIDs; except low-dose aspirin), anticoagulants
  • Use of prescription drugs that would impact muscle protein synthesis, e.g. Statins, Lithium, ADHD medication.
  • Insertion of intrauterine device (IUD) - exception: copper
  • Use of emergency contraception in the last 3 months (e.g. Plan B)
  • Severe food allergies (e.g. soy, nuts)
  • Smoking, use of performance enhancing drugs (growth hormones, testosterone)

Treatment and study plan

Protein Beverage

Dietary Supplement

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

Stable Isotope Tracer [D5]phenylalanine

Dietary Supplement

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

Dietary Supplement

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.

Primary outcomes

  1. Muscle Protein Synthesis (MPS)

    Time frame: 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.

Secondary outcomes

  1. Whole Body Protein Synthesis (WBPS)

    Time frame: 4 hours

    Whole body protein synthesis (WBPS) will be measured over a 4-hour period using the Indicator Amino Acid Oxidation (IAAO) methodology. Participants will consume a stable isotope tracer, L-[1-13C]phenylalanine ([13C]Phe), and breath samples will be collected every 30 minutes during the final 4 hours of the metabolic trial. The enrichment of 13CO₂ in breath samples will be analyzed to determine phenylalanine oxidation rates, providing an estimate of whole-body protein turnover and dietary protein utilization. This will allow us to assess whether hormonal fluctuations influence protein metabolism and nutritional requirements in female athletes.

Study contacts

Contact information is provided by the study sponsor or research team.

Daniel R Moore, PhD

CONTACT

[email protected]

4169464088

Ines Kortebi, PhD Candidate

CONTACT

[email protected]

Sponsors and collaborators

Lead sponsor

University of Toronto

Other

Registry information

Official study title

The Effect of Menstrual Cycle Phase and Oral Contraceptive Use on Muscle Protein Metabolism Post-Resistance Training

Acronym: MCOC

Important dates

Study start
2025
Primary completion
2026
Study completion
2026
First posted
Dec 2, 2025
Registry last updated
Dec 2, 2025

OpenTrials presents study information sourced from ClinicalTrials.gov. The official registry record should be consulted for the latest information.

View the official ClinicalTrials.gov record (opens in a new tab)

This listing is for discovery and informational purposes only. It is not medical advice, does not guarantee that a study is recruiting, and does not determine eligibility. Contact the study team and a qualified healthcare professional when considering participation.

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