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A study done at UCLA Medical Center in California concludes that one reason acupuncture is effective is because the needles increase the release of nitric oxide. The study tested 24 volunteers with electroacupuncture – one group having electroacupuncture on meridian points and the control group having electroacupuncture on non-meridian areas. They collected samples of nitric oxide throughout the study and found the enhancement.
Source
Department of Obstetrics and Gynecology, David Geffen School of Medicine at University of California Los Angeles, Harbor-UCLA Medical Center, Torrance, California. “Responses of Nitric Oxide-cGMP Release in Acupuncture Point to Electroacupuncture in Human Skin In Vivo Using Dermal Microdialysis.” In Microcirculation, 2009 May 26:1-10.