Background:The current study was designed to evaluate potential Yoga specific changes of brain structure in comparison with an active sport group and a passive control group. Materials and Methods:We conducted a longitudinal structural MRI study to investigate changes in gray matter (GM) density following a Yoga intervention. In order to check the repeatability of the results two data acquisition periods (cohorts) with the same study design were done. Yoga naive subjects (n = 19/n = 20) received a Yoga intervention once a week for 75 minutes over a period of 10 weeks. Control groups included active sport subjects (n = 10/n = 22) and passive subjects (n = 17/n = 14). All participants could choose their group assignment by their own preference. A longitudinal and group comparison was done by voxelbased-morphometric analyses before and after the intervention. In addition, blood pressure was taken before and after the study period as a controlling instrument. Finally, the results of the two cohorts were reported into a comprising data set for a general conclusion on Yoga effects. Results:Initially, hippocampal GM density in the Yoga group was significantly lower than in both control groups already before the intervention. It was equally and independently present in both cohorts. The longitudinal MRI measurements showed a significant increase in the right hippocampal GM density as a specific effect of the Hatha Yoga intervention. Coincidentally, the blood pressure measurement detected a significant reduction in both systolic and diastolic blood pressure in the Yoga group but not in the sport or passive group. Conclusions:Various neurodegenerative and neuropsychiatric diseases as well as long lasting stress lead to structural shrinkage of the hippocampal GM. In contrast, stress reduction was shown to be associated with an increase in the hippocampal GM density. The Yoga specific effect in this study can be considered as positive impact on both stress management and its underlying neural structure. These effects have been already proven in prevention and therapy of neurodegenerative and neuropsychiatric diseases. The Yoga-induced stress reduction can be furthermore supported by the findings of blood pressure reduction. Blood pressure is well known as an indirect indicator for stress reduction. Since the GM density was detected lower in the two Yoga cohorts twice compared to both control groups, one may postulate a pre-existing but unconscious higher stress vulnerability when voluntarily deciding for the Yoga intervention.

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