Diosgenin content is a novel criterion to assess memory enhancement effect of yam extracts

Several studies have suggested that some kind of Dioscorea species (yam) or yam-contained herbal medicines have cognitive enhancement effect. However, it has been unknown what is a crucial factor for cognitive enhancement in each Dioscorea species. In this study, we aimed to investigate whether one of the main and brain-penetrating components in yams, diosgenin, can be a novel criterion to assess memory enhancement effect of yam extracts. Although our previous studies showed that administration of diosgenin or diosgenin-rich yam extract enhanced cognitive function in normal mice and healthy humans, we have never evaluated whether the effect depends on diosgenin content or not. Therefore, we compared memory enhancement effects of low diosgenin-contained general yam water extract with diosgenin-rich yam extract on cognitive function in normal mice. We found that unlike diosgenin-rich yam, administration of general yam water extract did not enhance object recognition memory in normal mice. LC-MS/MS analyses revealed that after administration of general yam, diosgenin concentration in the brain did not reach to the effective dose because of the low diosgenin content in the original yam extract. On the other hand, when diosgenin was artificially added into general yam, the extract showed memory enhancement in normal mice and promoted neurite outgrowth in neurons. Our study suggests that diosgenin is actually an active compound in yams for memory enhancement, and diosgenin content can be a criterion for predicting cognitive enhancement effect of yam extracts.