Oral Melatonin is a commercially available product available alone and as a constituent in a number of supplements. Previous research suggests that short-term supplementation with oral melatonin may amplify the recovery response to damaging resistance exercise via modulation of subsequent immune and inflammatory responses. However the effects of oral melatonin on neutrophil and monocyte invasion/migration, a critical step in the resolution of skeletal muscle tissue homeostasis, has not been examined. An oral melatonin supplement (5mg) will be provided three times daily beginning 24-hours before and ending 48-hours after an acute bout of damaging resistance exercise (total 15mg/day for 3 days).

Goals:

1. To investigate the effect of melatonin on systemic and cellular responses following an acute bout of damaging resistance exercise.
2. To investigate the effect of melatonin on measures of functional performance before and during recovery from an acute bout of damaging resistance exercise.

Resistance exercise of sufficient intensity can result in localized damage to skeletal muscle tissue. The resolution of tissue homeostasis following damaging exercise is mediated largely by neutrophils and monocytes of the innate immune system (1). Neutrophils and monocytes originate in the systemic circulation and infiltrate the damaged tissue following activation and migration in response to damage associated molecular cues. These cells are reported to execute a number of processes fundamental to recovery, including the phagocytosis of opsonized cellular debris (2), and secretion of mitogenic factors that stimulate both the proliferation and differentiation of myogenic precursor cells (3). Disruption of this cellular response has been shown to lead to impaired muscle regeneration, and a subsequent deficit in muscle fiber size (4). Accordingly, a successful innate immune response is paramount for optimal tissue regeneration.

Several supplements have been introduced to the market with purported claims of enhanced recovery from exercise. One potential mechanism for enhanced recovery is through augmentation of the immune response, which may allow for a more pronounced stimulus to enhance repair (1). Previous research suggests that short-term supplementation with melatonin, an endogenously produced indoleamine, may amplify the recovery response. Melatonin receptors are expressed on most immune cell types, including monocytes, neutrophils, and Th1 lymphocytes (5). Melatonin has previously been shown to activate monocytes (6), and treatment with melatonin has been shown to upregulate the recruitment of human monocytes through interaction with the melatonin receptor (MLTr) in vitro. Melatonin has also been shown to alter cytokine production and increase the production of cells of both the innate and adaptive immune system (7). Binding of melatonin to MLTr's expressed on immune cells may modulate recruitment of these cells to damaged tissue, which may in turn regulate the ensuing recovery response. Theoretically, this could reduce recovery time following damaging exercise, improving quality of training and potentially allow for enhanced performance over time (8,9,10). Nevertheless, the effect of short-term melatonin supplementation on innate immune cell and inflammatory responses during recovery from resistance exercise has not been examined.

Goals:

1. To assess systemic and cellular responses to an acute bout of resistance exercise between participants ingesting melatonin versus placebo, including creatine kinase (CK), complete blood counts (CBC), serum interleukin-8 (IL-8), serum monocyte chemoattractant protein-1 (MCP-1), serum melatonin, serum C-reactive protein (CRP), cell receptor expression (monocytes: activated CD11b, CCR2, MLTr1A; neutrophils: activated CD11b. CXCR2, MLTr1A), and neutrophil/monocyte invasion/migration dynamics (cell index, CI).
2. To assess functional measures of recovery following an acute bout of resistance exercise between participants ingesting melatonin versus placebo, including subjective sleep quality/duration, fatigue, soreness and stress (Hooper Questionnaire), perceived Recovery (Perceived Recovery Status Scale, PRSS), active range of motion (AROM), pain pressure threshold (PPT), countermovement jump (CMJ), squat maximal voluntary isometric contraction (MVIC), and objective sleep and physical activity (Accelerometry).

Method:

Randomized, double-blind, placebo controlled parallel-groups trial comparing the effect of supplementation with Melatonin versus Placebo on systemic and cellular responses and functional recovery from damaging resistance exercise.

Melatonin or placebo supplementation (randomly assigned) will occur over a period of 3 days beginning 24-hours prior to completion of an acute bout of damaging resistance exercise through a 48-hours post-exercise recovery period.

Objective sleep and physical activity (accelerometry) will be assessed 24-hours before (-24H), 24-hours Post (24H) and 48-Hours (48H) post-exercise. Diet (carbohydrates, fats, proteins, total calories and micronutrients) will be assessed at Pre (0), 24H and 48H post-exercise. CBC, CK, IL-8, MCP-1, CRP, Hooper, PRSS, AROM, PPT, CMJ, MVIC and melatonin will be assessed at Pre (0) immediately- post (IP) 4-hours post (4H), 24H and 48H post-exercise. Neutrophil cell index will be assessed at Pre (0), 4H and 24H post-exercise. Monocyte cell index (CI) will be assessed at Pre (0), 24H and 48H post-exercise. Neutrophil cell receptor expression will be assessed at Pre (0), 4H and 24H. Monocyte receptor expression will be assessed at Pre (0), 24H and 48H.

Inclusion Criteria:

• Male or female between the ages of 18-40 years old.
• Healthy and ready for physical activity as determined by the Physical Activity Readiness Questionnaire (PAR-Q+) and Medical History Questionnaire (MHQ).
• Active resistance training for a minimum of 6 months as defined by 3 resistance training sessions (muscle strengthening activities such as free weights, weight machines, or calisthenics, etc.) per week with at least one lower body session as determined by the training history questionnaire.
• Premenopausal, with an identifiable onset of menses (early follicular phase) as determined by the menstrual status questionnaire (female participants only)..
• Not currently pregnant and no intention to become pregnant for the duration of participation (female participants only).
• Currently free from and willing to abstain from dietary supplements that are viewed by study investigators to confound the outcomes of the study (e.g., creatine, beta-alanine) for the duration of the study. Participants currently using supplements will be permitted to enroll in the study following a 4-week washout period of these supplements.
• Willing to adhere to all pre-testing visit instructions including abstaining from exercise for the duration of the study and abstaining from alcohol for 24 hours prior to visit 2 as well as for 24 hours before visit 3 until completion of the study.
• Currently be consuming ≤ 300mg caffeine per day on average and willing to keep caffeine intake consistent throughout the duration of the study. Participants must also be willing to abstain from caffeine intake for 12 hours prior to visits 3, 4 and 5 and not consume caffeine the morning of these visits.
• Free from previous or current lower body injuries that are viewed by the investigators to potentially limit ability of the participant to perform the exercise intervention or functional assessments.
• Not regularly taking any type of prescription or over-the-counter medication which might affect the assessments, or having any chronic illnesses, which require medical care.
• Considered by the study investigators to have a high likelihood of successful venipuncture following an initial examination of the participants antecubital fossa by a certified phlebotomist and verbal discussion of the participants blood draw history (including history of unsuccessful venipunctures, known issues with the locating/palpating of veins in the antecubital fossa by a phlebotomist, and whether these difficulties are more apparent on one arm compared to the other).

Exclusion Criteria:

• Individual does not agree to participate in this study.
• Individual does not meet inclusion criteria to participate in the study protocols as determined via the PAR-Q+, medical history questionnaire, training history questionnaire, and/or the caffeine consumption questionnaire.
• Currently taking melatonin
• Regularly taking any type of prescription or over-the-counter medication which might affect the assessments, or having any chronic illnesses, which require medical care.
• Not currently meeting requirements for resistance trained status.
• Current known pregnancy or intent to become pregnant during the study period.
• Not regularly having periods or amenorrheic, as determined by Menstrual Status Questionnaire (MSQ).
• Currently taking any performance-enhancing drug (determined from health and activity questionnaire).
• Currently taking a nutritional supplement viewed by the research team to confound the outcomes of the study and not willing to abstain from taking the supplement during the course of the study or not willing to undergo a 4-week wash-out period prior to participating if required.
• Evaluated as having a low likelihood of successful venipuncture by a certified phlebotomist.