Dioscorea nipponica, also known as Japanese yam, has a history of use in traditional East Asian medicine for rheumatic conditions, including rheumatoid arthritis (RA). However, beyond traditional use, there is some scientific research suggesting its potential efficacy for RA. The main bioactive components are steroidal saponins, such as dioscin, which have demonstrated anti-inflammatory effects in various experimental models.

Preclinical studies, particularly in animal models of arthritis, have shown that extracts of Dioscorea nipponica can reduce inflammation, suppress immune responses, and alleviate joint damage. For example, studies in collagen-induced arthritis rats suggest that the extract inhibits pro-inflammatory cytokines (such as TNF-α and IL-1β) and modulates immune function. Some in vitro research also points toward its ability to impede the activation of key inflammatory pathways (like NF-κB).

Despite these promising findings, clinical evidence in humans is very limited. There are only a few small-scale clinical trials or case reports, and these are not of high methodological quality. As such, the evidence supporting the use of Dioscorea nipponica for RA in humans remains preliminary, justifying a modest evidence rating.

In summary, while there is scientific rationale and some animal research supporting its use, robust clinical validation is lacking. Most claims are based on preclinical data and traditional use.