Melatonin is believed to protect against breast cancer and is produced by the pineal gland. The rate-limiting step is influenced by the light/dark cycle, with synthesis increased in the dark and reduced in the light. Overnight production of melatonin correlates with urine production of 6-sulfatoxymelatonin.
The authors studied women in relationship to their melatonin production, light exposure and exercise. Urine was collected for the determination of 6-sulfatoxymelatonin(the metabolite of melatonin) and light exposure was measured. Records of exercise times and durations were also kept.
The most significant result was that exercise increased the amount of melatonin production and this was especially true if the exercise was late in the day. The production of 6-sulfatoxymelatonin was increased as long as the person exercised but it did not matter if the exercise was moderate or intense. The amount of 6-sulfatoxymelatonin was related to the duration of exercise. A relationship was not found between melatonin and the amount of light exposure, perhaps due to the difficulty of measuring the amount of light properly.
CONCLUSION: The authors conclude that exercise seems to have a protective effect for breast cancer due to the increased melatonin produced during exercise.