Abstract
Background
The inverse association between physical activity and cancer risk may be mediated by higher melatonin levels. However,few studies have examined the effect of increased physical activity on melatonin levels.
Methods
The parent study was a randomized controlled trial (RCT) which randomized 51 men and 49 women to a 12-month moderate-to-vigorous aerobic exercise intervention (‘exercisers’) and 51 men and 51 women to a stretching control (‘controls’). Participants were aged 40-75 years, and previously sedentary. Levels of the principal urinary metabolite of melatonin 6-sulphatoxymelatonin (aMT6s), corrected for creatinine levels, were measured in spot morning urine samples by immunoassay at baseline and 12-months. Changes in levels between exercisers and controls were compared using generalized estimating equations for linear regression.
Results
We observed no statistically significant difference in the change in aMT6s levels from baseline to 12-months in exercisers compared with controls (change in aMT6s levels: exercisers, +6.5%; controls, +13%; P=0.66). There was no evidence of effect modification by age, sex or body mass index.
Conclusions
A 12-month moderate-intensity exercise intervention did not affect levels of aMT6s.
Impact
Further research needs to focus on other potential mechanisms through which physical activity may reduce the risk of cancer.
Keywords: aMT6s, cancer, exercise, sedentary, sex hormones
Introduction
Regular moderate-to-vigorous intensity physical activity (PA) lowers cancer risk.(1) Possible mechanisms include decreasing sex hormones, metabolic hormones, and inflammation levels.(1) Largely unexplored mechanisms include effects of melatonin (5-methoxytryptamine), a hormone secreted primarily by the pineal gland during the dark phase of the light-dark cycle.(2) PA may increase melatonin levels, resulting in decreased estrogen production, improved fat metabolism, and reduced cancer risk.(3) Few studies have examined the effects of PA on melatonin.(4) In the only randomized controlled trial (RCT) to date, a 16-week exercise intervention in healthy women (18-30 years) had no effect.(5) To our knowledge, no studies have examined the effect of a long-term PA intervention. The primary aim was to determine the effect of a 12-month exercise intervention vs. control on the principal urinary metabolite of melatonin,6-sulphatoxymelatonin (aMT6s) levels in sedentary men and women. We also investigated associations betweenaMT6s and changes in markers of cell proliferation (Ki67) in colon biopsies obtained at baseline and 12-months,(6) and cross-sectional associations between aMT6s and levels of previously measured biomarkers, anthropometric measurements, and self-reported measures of sleep quality and duration.
Materials and Methods
This study is ancillary to the A Program Promoting Exercise and an Active Lifestyle study (http://clinicaltrials.gov/show/NCT00668161).(6) Briefly, 102 men and 100 women, 40-75 years, sedentary (<90 minutes/week of moderate-to-vigorous exercise in past 3 months), no history of cancer or serious illness, and <2 alcoholic beverages/day, were randomized into an exercise (exercisers) or control (controls) group. Randomization was stratified by sex, use of non-steroidal anti-inflammatory drugs, smoking status, and, among women, menopausal status and current use of postmenopausal hormone therapy. The intervention was a 12-month program, with a goal of 60 minutes/day, 6 days/week of moderate-to-vigorous aerobic exercise at 60-85% of maximal heart rate (determined by baseline VO2max). Fasting blood and spot urine samples, colon biopsies, anthropometrics and questionnaire data were collected at baseline and 12-months. The study was approved by the Institutional Review Board of the Fred Hutchinson Cancer Research Center, and all participants signed informed consent.
aMT6s and creatinine levels were assayed from the same spot urine samples at the Biomarker Core Laboratory (Yerkes National Primate Research Center, Emory University), using commercially available competitive ELISA kits (aMT6s: ALPCO Diagnostics; sensitivity 0.14 ng/ml, inter and intra-batch coefficients variation <15%; creatinine: Sigma Diagnostics Creatinine reagents).
Statistical analysis
aMT6s was not normally distributed, and was log-transformed. We assessed baseline associations with aMT6s using analysis of variance to compare means between groups and Spearman's partial correlation coefficient ρ, adjusted for age and body mass index (BMI), to assess baseline correlations. We compared percent change in mean aMT6s levels from baseline to 12-months in exercisers and controls by generalized estimating equations for linear regression. Secondary analyses examined associations between adherence to the intervention with change in aMT6s adjusted for age, sex and BMI. We assessed statistical significance at P<0.05. Statistical tests were two-sided and analyses were conducted in SAS 9.1 (SAS Institute, Cary, NC).
Results
After excluding participants with missing or extreme aMT6s levels, the analyses included 95 women (33% premenopausal; 46 exercisers and 49 controls) and 100 men (49 exercisers and 51 controls). Baseline aMT6s levels were significantly higher among women compared with men (14.4 vs. 8.0 ng/mg creatinine; P=0.001). In men, aMT6s levels were positively correlated with insulin-like growth factor (IGF-1; ρ=0.23, P=0.03) and the IGF-1/IGFBP (binding protein)-3 ratio (ρ=0.22, P=0.03); and negatively correlated with mean Ki67 staining in colon biopsies (ρ= -0.28, P=0.01). For women, aMT6s levels were positively correlated with androstenedione (ρ=0.27, P=0.01) and dehydroepiandrosterone sulfate (ρ=0.24, P=0.02). aMT6s was not associated with other metabolic hormones (Table 1) or measures of sleep quality and duration (data not shown). The exercise intervention had no effect on aMT6s levels(change in aMT6s: exercisers, +6.5%; controls, +13%; P=0.66). There was no evidence of effect modification by age, sex or BMI (Table 2).Similarly, there was no effect on aMT6s levels within subgroups defined by adherence to the intervention (data not shown).
Table 1. Correlations with baseline urinary 6-sulphatoxymelatonin (ng/mg creatinine).
| Men (n=100) | Women (n=95) | |||||||
|---|---|---|---|---|---|---|---|---|
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|
|
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| Pearson correlation | Partial Pearson correlation* | Pearson correlation | Partial Pearson correlation* | |||||
| ρ | P-value | ρ | P-value | ρ | P-value | ρ | P-value | |
| Age (years) | -0.01 | 0.94 | -0.04 | 0.73 | ||||
| Body mass index (kg/m2) | 0.01 | 0.94 | 0.03 | 0.80 | 0.14 | 0.18 | 0.12 | 0.24 |
| Waist circumference (cm) | 0.06 | 0.53 | 0.09 | 0.39 | 0.07 | 0.52 | 0.05 | 0.62 |
| Hip circumference (cm) | 0.02 | 0.81 | 0.03 | 0.81 | 0.14 | 0.19 | 0.12 | 0.25 |
| Waist-to-Hip ratio | 0.09 | 0.37 | 0.14 | 0.18 | -0.07 | 0.53 | -0.07 | 0.52 |
| Total Fat Mass (kg)# | 0.06 | 0.57 | 0.09 | 0.42 | 0.10 | 0.34 | 0.09 | 0.39 |
| Dexa % Body fat# | 0.10 | 0.33 | 0.14 | 0.19 | 0.18 | 0.08 | 0.18 | 0.08 |
| Total Abdominal Fat (cm2) | 0.05 | 0.64 | 0.10 | 0.34 | 0.15 | 0.13 | 0.14 | 0.17 |
| Intra-abdominal Fat (cm2) | -0.07 | 0.46 | -0.03 | 0.81 | 0.07 | 0.50 | 0.06 | 0.60 |
| Subcutaneous Fat (cm2) | 0.13 | 0.21 | 0.20 | 0.07 | 0.15 | 0.14 | 0.14 | 0.19 |
| VO2 Maximum (ml/kg/min) | 0.09 | 0.36 | 0.02 | 0.89 | -0.09 | 0.44 | -0.10 | 0.33 |
| Moderate to vigorous Met mins/wk | 0.14 | 0.18 | 0.08 | 0.45 | -0.10 | 0.33 | -0.10 | 0.35 |
| Steps per Day | 0.03 | 0.75 | 0.08 | 0.47 | -0.04 | 0.69 | -0.05 | 0.67 |
| Energy Intake (kcal)‡ | 0.01 | 0.85 | 0.04 | 0.70 | -0.01 | 0.91 | -0.01 | 0.92 |
| 3-A-G (ng/ml) † | -0.09 | 0.36 | -0.10 | 0.35 | - | - | - | - |
| DHT (pg/ml) † | -0.04 | 0.71 | -0.04 | 0.71 | - | - | - | - |
| Estradiol (pg/ml) † | -0.06 | 0.53 | -0.06 | 0.53 | - | - | - | - |
| Free Estradiol (pg/ml) † | -0.04 | 0.71 | -0.04 | 0.68 | - | - | - | - |
| Bioavailable Estradiol (pg/ml) † | -0.04 | 0.71 | -0.04 | 0.68 | - | - | - | - |
| Testosterone (ng/dl) † | -0.01 | 0.94 | 0.00 | 0.96 | 0.13 | 0.20 | 0.11 | 0.31 |
| Free Testosterone (ng/dl) † | 0.06 | 0.58 | 0.06 | 0.55 | 0.11 | 0.27 | 0.06 | 0.57 |
| Bioavailable Testosterone (ng/dl) † | 0.06 | 0.57 | 0.06 | 0.55 | - | - | - | - |
| Prolactin (ng/ml) † | - | - | - | - | -0.12 | 0.23 | -0.13 | 0.20 |
| Androstenedione (pg/ml) † | - | - | - | - | 0.24 | 0.02 | 0.27 | 0.01 |
| DHEAS (ug/dl) † | - | - | - | - | 0.26 | 0.01 | 0.24 | 0.02 |
| Insulin† | 0.01 | 0.95 | -0.01 | 0.95 | 0.17 | 0.10 | 0.13 | 0.24 |
| Glucose (mg/dl) † | 0.02 | 0.81 | 0.02 | 0.85 | 0.03 | 0.79 | 0.04 | 0.69 |
| HOMA score† | 0.01 | 0.95 | -0.01 | 0.93 | 0.15 | 0.14 | 0.11 | 0.30 |
| SHBG (ng/ml) † | -0.10 | 0.32 | -0.11 | 0.27 | -0.06 | 0.54 | -0.02 | 0.88 |
| IGF-1 (ng/ml) † | 0.21 | 0.03 | 0.23 | 0.03 | 0.05 | 0.65 | 0.07 | 0.49 |
| IGFBP-3 (ng/ml) † | 0.03 | 0.74 | 0.06 | 0.56 | -0.01 | 0.91 | 0.00 | 0.98 |
| Ratio IGF-1 : IGFBP-3† | 0.22 | 0.03 | 0.22 | 0.03 | 0.05 | 0.66 | 0.06 | 0.55 |
| CRP (mg/L)†± | -0.11 | 0.29 | -0.13 | 0.21 | 0.13 | 0.23 | 0.08 | 0.53 |
CRP, C-reactive protein; DHT, dihydrotestosterone; IGF, insulin-like growth factor; IGFBP, insulin-like growth factor binding protein; SHBG, sex hormone-binding globulin.
Partial correlation adjusted age.
Men, n=97; Women, n=94.
Men, n=95; Women, n=94.
Partial correlation adjusted for age and body mass index.
Men, n=96; Women, n=93.
Table 2. Baseline and 12-month follow-up urinary 6-sulphatoxymelatonin geometric means (ng/mg creatinine), overall and within subgroups.
| Baseline | 12-month follow-up | P† | |||||
|---|---|---|---|---|---|---|---|
|
|
|
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| Exercisers Mean (95%CI) | Controls Mean (95%CI) | Exercisers Mean (95%CI) | Change (%) | Controls Mean (95%CI) | Change (%) | ||
| Overall | 6.0 (4.8-7.6) (n=95) | 6.7 (5.3-8.3) (n=100) | 6.4 (5.2-7.9) (N=95) | 0.4 (6.5) | 7.5 (6.2-9.2) (n=99) | 0.9 (13.1) | 0.66 |
| Age (years) | |||||||
| <55 | 7.2 (5.4-9.7) (n=46) | 6.4 (4.7-8.7) (n=54) | 6.7 (5.0-9.0) (n=46) | -0.5 (-7.1) | 8.0 (6.1-10.4) (n=53) | 1.6 (24.8) | 0.18 |
| ≥55 | 5.1 (3.6-7.1) (n=49) | 7.0 (5.0-9.7) (n=46) | 6.1 (4.6-8.3) (n=49) | 1.1 (20.6) | 7.0 (5.2-9.5) (n=46) | 0.1 (0.7) | 0.29 |
| Sex | |||||||
| Men | 4.9 (3.7-6.6) (n=49) | 5.4 (4.0-7.3) (n=51) | 5.2 (3.9-7.0) (n=49) | 0.3 (6.7) | 5.7 (4.3-7.6) (n=50) | 0.3 (5.8) | 1.00 |
| Women | 7.5 (5.3-10.6) (n=46) | 8.3 (6.0-11.5) (n=49) | 7.9 (5.9-10.6) (n=46) | 0.4 (5.7) | 10.0 (7.8-12.8) (n=49) | 1.7 (20.4) | 0.55 |
| Body mass index (kg/m2) | |||||||
| <25 | 7.2 (4.5-11.4) (n=23) | 4.6 (2.6-8.2) (n=19) | 7.2 (4.3-12.0) (n=23) | -0.0 (-0.3) | 7.7 (5.0-11.9) (n=19) | 3.1 (67.1) | 0.17 |
| 25-29 | 4.7 (3.3-6.6) (n=32) | 6.1 (4.1-9.0) (n=37) | 5.2 (3.9-7.0) (n=32) | 0.6 (12.0) | 6.9 (4.7-10.1) (n=37) | 0.8 (13.4) | 0.96 |
| ≥30 | 6.7 (4.6-9.8) (n=40) | 8.5 (6.4-11.3) (n=44) | 7.1 (5.1-9.8) (n=40) | 0.4 (5.7) | 8.1 (6.3-10.4) (n=43) | -0.4 (-4.6) | 0.59 |
P-value comparing the changes from baseline to follow-up between exercisers and controls.
Discussion
Physical activity is associated with lower risk of cancer;(1) however the mechanisms remain largely unknown. The effect of PA may be mediated by higher melatonin levels but data are limited. Results from observational studies are conflicting,(4) and a short-term exercise intervention had no effect on aMT6s.(5) Results from our study add to these findings and, when taken together, provide evidence suggesting that PA does not substantially influence melatonin levels in either the short- or long-term. While our results suggest possible sex-related difference in associations between aMT6s levels and metabolic hormones, and Ki67 staining, they should be interpreted cautiously as correlations were modest. Study strengths include the RCT design, 12-month duration, excellent adherence and low drop-out rates, and adequate power (80% power to detect a 60% relative change in aMT6s between groups).
Other potential mechanisms through which PA may reduce the risk of cancer need to be examined.
Acknowledgments
Financial support: This study was funded by NIH grants R01 CA77572 and U54 CA116847 (C. Duggan L. Xiao and A.McTiernan). A.P. Thrift was supported by an Early Career Fellowship from the National Health and Medical Research Council of Australia.
Footnotes
Disclosures: The authors have no disclosures.
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