Melatonin supplements, often taken to help insomniacs and jet-lagged travellers sleep, may make it dangerous for those with cardiovascular problems to stand, reported a researcher in yesterday's issue of the Journal of Physiology.
The study by Chester A. Ray, associate professor of medicine at Penn State College of Medicine, suggests that melatonin supplements may make symptoms worse for the half a million people - many of them women and the elderly - with orthostatic intolerance, a condition in which the cardiovascular system cannot maintain adequate blood pressure and blood flow to the brain when a person stands, causing dizziness and fainting.
"Melatonin supplements can hinder nervous system responses that enable the body to compensate for the drop in blood pressure when a person moves from a seated to standing position," said Ray.
Melatonin is a hormone made by the pineal gland in the brain and secreted at night on a circadian, or 24 hour, cycle, to make people feel sleepy. Available in synthetic form, dietary supplements of melatonin have been linked to improved sleep, prevention of jet lag, and more. Studies have also shown that melatonin supplementation may reduce blood pressure and decrease blood plasma levels of norepinephrine, a neurotransmitter responsible for raising blood pressure.
Melatonin is also taken by astronauts in space, two-thirds of whom experience orthostatic intolerance immediately after touchdown on Earth.
"When astronauts return to Earth, there is a chance that, for safety reasons, they will need to make a quick exit from the spacecraft," noted Ray. "This study suggests that taking melatonin supplements for sleep could exacerbate orthostatic intolerance, making them unable to evacuate the shuttle and compromising their safety."
For this study, 12 healthy volunteers were given either a three-milligram tablet of melatonin - a dose often used and available at nutrition stores - or a non-medicated placebo pill. Fifty minutes later, each volunteer was asked to lie down and place both legs in a lower body negative pressure (LBNP) chamber, a device that reduces pressure to the legs, mimicking the effect of a person standing up.
Ray used microneurography to record the sympathetic nerve activity to the blood vessels of the muscles. Sympathetic nerve activity starts a chain reaction that causes the blood vessels to constrict, increasing blood pressure and ensuring adequate blood flow reaches the entire body. After taking measurements at rest, the pressure in the LBNP was decreased and all measures were taken again.
To see if melatonin had an impact during other forms of physical stress, volunteers performed a handgrip exercise test and a cold pressor test where a hand is placed into ice water for a short period of time. These tests were also done on two separate occasions, once after taking the melatonin supplement and once taking a placebo.
Results showed that nerve activity under orthostatic stress was reduced in those volunteers who had taken melatonin compared to those who had taken placebo. Ray noted that nerve activity was not affected in volunteers who had taken melatonin who underwent the handgrip or cold pressor tests.
"This finding suggests that melatonin has specific physiological effects, namely, decreasing nerve activity, when someone stands, but not necessarily during other types of physical activities," concluded Ray.
Additional studies are to investigate whether melatonin decreases orthostatic tolerance in women, elderly and astronauts.
The study titled 'Melatonin attenuates the sympathetic nerve responses to orthostatic stress in humans', was published in the 15 September issue of the Journal of Physiology.