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Traditional Chinese medicine

Chinese yamDioscorea oppositifolia (also written Dioscorea opposita)

A mild starchy root eaten as food and simmered in spleen formulas; a review found only two human trials of it among 27 studies.

Botanical name
Dioscorea oppositifolia (also written Dioscorea opposita)
Part used
rhizome (the starchy underground tuber, peeled and dried)
Also called
pinyin Shan yao (山药); Chinese yam, Dioscoreae rhizoma, Huai shan yao
Used in
Traditional Chinese medicine
Taken for
Loose stools and diarrhea, Appetite, Indigestion
Sold as
dried slices, powder, fresh tuber, decoction, pills in formulas

The plant

Chinese yam is the underground tuber of Dioscorea oppositifolia (the name Dioscorea opposita Thunb. is also in wide use), a climbing vine whose starchy tuber is eaten across East Asia.

The pharmacy drug is the dried rhizome, sold as slices or powder. It is also a vegetable, cooked in soups and porridge. In formulas it is used with other mild herbs, and it is one of the ten ingredients of Shen Ling Bai Zhu San, the classic Song-dynasty formula for what the texts call spleen weakness with loose stools. A systematic review describes it as a medicinal food used across East Asia for gastrointestinal health, and then counts the evidence: 27 studies, of which only two were in people. The same review notes that the plant part, extraction method and dose differed widely between studies, which makes them hard to compare.

How the traditions use it

Traditional Chinese medicine (about TCM)

Shan yao is classed in the Chinese materia medica as sweet and neutral, entering the spleen, lung and kidney channels. The tradition lists it as a tonic that supports what the texts call the spleen and stomach and also the lung and kidney, and uses it where the texts describe weak digestion with poor appetite, loose stools or chronic diarrhea. It is valued as a mild, non-drying staple rather than a strong herb, and it sits next to coix, lotus seed and poria in the spleen-supporting formulas. Shen Ling Bai Zhu San, the most-cited of these, comes from the Song-dynasty text Taiping Huimin Heji Ju Fang and lists yam, lotus seed, coix, lablab bean, poria, atractylodes, ginseng, amomum, platycodon and licorice. These are the tradition’s own classifications and formulas, taken from the standard English materia medica and a review of the formula.

What is in it

Yam tuber is mostly starch, with polysaccharides and saponins that research groups study; a review inside the paper notes that gut enzymes can convert the saponin dioscin to diosgenin. Reviews report in animal and laboratory work that yam polysaccharides and extracts reduce intestinal inflammation, preserve the mucosal barrier and shift the gut microbiota, and a systematic review draws the same picture from 25 animal studies. These are preclinical findings. The same review notes that the plant part, the extraction method and the dose varied widely between studies, which is one reason their results are hard to combine.

What the human trials show

Single-ingredient human evidence for yam in the gut is thin. A systematic review of Dioscoreae Rhizoma and gastrointestinal function searched eight databases and found 27 studies, only two in people and 25 in animals, excluding multi-herb formulas. The two human trials reported improved gut-microbiota balance, better glycemic control and better post-surgical recovery, including wound healing and fewer infections; the authors list the lack of high-quality human studies, small samples and heterogeneity as limits. In animal models yam preparations reduced inflammation, supported the mucosal barrier and changed the microbiota.

Most clinical data are on Shen Ling Bai Zhu San, a ten-herb formula, so they cannot be attributed to yam. A meta-analysis of 14 studies with 1,335 participants with chronic gastritis found the formula beat conventional therapy on the effective rate (risk ratio 1.29, 95% CI 1.21 to 1.37), though the authors rated the quality of evidence as not high and call the safety data unclear. A network meta-analysis of five Chinese formulas for ulcerative colitis, 64 randomized trials with 5,456 patients, concluded that formulas added to mesalazine did better than mesalazine alone. A separate review of the formula in colitis notes that most of its trials did not describe their randomization. A pooled analysis of 26 animal studies of the formula in diarrhea found lower diarrhea scores (standardized mean difference -1.40) but rated the evidence very low.

What has not been tested: whole yam as a food in a controlled gut trial, any dose-ranging, and the formula against placebo in irritable bowel syndrome with diarrhea, for which we found no randomized trial.

StudyDoseWhat it found
Systematic review of Dioscoreae Rhizoma and gut function: 27 studies, 2 in humans and 25 in animals PMID 41010469Varied by plant part and extractHuman trials reported better microbiota balance, glycemic control and post-surgical recovery; authors cite lack of high-quality human studies
Meta-analysis of Shen Ling Bai Zhu San in chronic gastritis: 14 studies, 1,335 participants (a ten-herb formula) PMID 36156925Formula; doses not standardizedEffective rate better than conventional therapy, RR 1.29 (95% CI 1.21 to 1.37); evidence quality not high
Network meta-analysis of five formulas added to mesalazine in ulcerative colitis: 64 randomized trials, 5,456 patients (includes Shen Ling Bai Zhu San) PMID 39109018Formulas; add-on to mesalazineTCM plus mesalazine more effective than mesalazine alone; analysis of TNF reduction listed the formula (OR 0.19, 95% CI 0.08 to 0.39)
Meta-analysis of 26 animal studies of Shen Ling Bai Zhu San in diarrhea PMID 36188567Animal dosesLower diarrhea scores (SMD -1.40, 95% CI -2.03 to -0.87); authors rate evidence quality very low

How much the studies and the texts used

We found no gram dose for single-herb yam in a human gut trial that we could tie to a source. In the tradition the dried slices are simmered in decoctions or ground to a powder and cooked into porridge, and the fresh tuber is eaten as a vegetable. In the formula trials the doses were those of the formula; the review of the formula in colitis states that its dose follows tradition and has not been standardized or checked in toxicology studies. A reader who sees a yam extract or powder for sale is looking at a product that has not been tested at a defined gut dose in a controlled human trial.

Doses here describe what studies and pharmacopoeias used. They are not a recommendation for any person; a clinician or pharmacist who knows your medicines and conditions is the right person to ask.

Side effects and cautions

As a cooked vegetable, Chinese yam has a long record of use in East Asia, and the human trials in the review did not flag harms in the abstract. Pregnancy, breastfeeding and child data for extracts are absent from the sources we read. The Dioscorea genus is large and the species differ: a different species, Dioscorea bulbifera (air potato), is the subject of a review that pairs its pharmacology with reports of liver toxicity, so a label that says only “yam” or “Dioscorea” does not identify which plant is inside. For the formula products, a review of Chinese proprietary medicines in Singapore tabulated reports of excess toxic heavy metals and undeclared drugs in such products between 1990 and 1997; that finding concerns the products listed there and not every formula.

Interactions

We found no documented drug interaction for yam as a food or for yam extract in the sources read. Because the reviewed human trials reported effects on glycemic control, the evidence includes a blood-sugar effect, which is relevant to anyone on glucose-lowering medicine. No drug-transporter or enzyme effect in people is reported in the sources we used.

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Often paired or compared

TCM Coix Coix lacryma-jobi var. ma-yuen TCM Lotus seed Nelumbo nucifera TCM Poria Wolfiporia cocos (syn. Poria cocos) TCM Atractylodes Atractylodes macrocephala (bai zhu); A. lancea (cang zhu) TCM Codonopsis Codonopsis pilosula

Questions people ask

Are all yams the same?

No. Dioscorea is a large genus. The gut formulas use the tuber of Dioscorea oppositifolia, while a different species, Dioscorea bulbifera, is the subject of a review that reports liver toxicity alongside its pharmacology. A label that says only yam does not name the species.

Are there human trials of Chinese yam for digestion?

A systematic review found only two human trials of yam alone, among 27 studies in total, and reported better microbiota balance, glycemic control and post-surgical recovery. The authors note small samples and few high-quality studies.

What is Shen Ling Bai Zhu San?

A Song-dynasty ten-herb formula that includes Chinese yam, lotus seed, coix, poria, atractylodes, ginseng, amomum, lablab bean, platycodon and licorice. Reviews pool clinical trials of the whole formula in chronic gastritis and ulcerative colitis, and the results cannot be assigned to yam alone.

How does TCM describe yam?

TCM classes it as sweet and neutral, entering the spleen, lung and kidney channels, and uses it as a mild support for what the texts call the spleen. This is the tradition’s own vocabulary.

Sources