Water of Metal: Shan Yao 山藥

味酸皆屬金，五味[子]為之主，枳實為木，豉為火，芍藥為土，薯蕷為水。

All sour belongs to metal, for it is governed by Wuweizi, and Zhishi is wood, [Dan Dou] Chi is fire, Shaoyao is earth, and Shuyu [Shan Yao] is water.

Shuyu (薯蕷) - better known perhaps by its contemporary name, Shanyao (山藥) - is Dioscorea, the Wild Chinese Yam. The Shennong Bencao Jing describes its properties this way:

署豫　味甘小溫。主治傷中，補虛羸，除寒熱邪氣，補中益氣力，長肌肉。久服耳目聰明，輕身不饑延年。一名山芋。生山谷

Shuyu tastes sweet and slightly warm. It governs damage to the center, supplementing deficiency emaciation, eliminating cold and hot pernicious qi, supplementing the middle, increasing qi and strength, and growing the tendons and flesh. Prolonged taking sharpens and brightens the ears and eyes, lightens the body, eliminates hunger and extends life. Another name for it is Shan Yu (“Mountain Tuber”). It grows in Mountains and Valleys.

The Shen Nong's description highlights key functions of  supplementing (補) and benefiting or increasing (益). This is in keeping with the Five Phase Dynamics of Metal and Water. Metal generates Water, and so by bringing the activity of Metal into the realm of Water, we would expect Shanyao to have a tonifying effect.

Shanyao, c/o Kamwo 

Those of you familiar with Zhang Zhongjing’s formulas are already aware that this herb is not a common ingredient in the Shang Han Lun or Jin Gui Yao Lue. However, it does appear in one very famous and widely used formula, Shen Qi Wan (腎氣丸) or Jin Gui Shen Qi Wan (金匱腎氣丸) as it is sometimes known after the Jin Gui Yao Lue, in which it first appeared.

In addition to Shen Qi Wan, Shuyu or Shanyao also appears in Shen Qi Wan’s lesser known cousin formula, Gua Lou Qu Mai Wan (栝蔞瞿麥丸), and in a tonic formula that bears its name, Shu Yu Wan (薯蕷丸).

The fact that all of these formulas are honey pills, or wan, indicate that they are to be taken for a long time, and have the primary function of tonifying deficiency, which is very much in keeping with the functions described in the Shen Nong Ben Cao Jing.

Shen Qi Wan and Shu Yu Wan both are listed in the Jin Gui Yao Lue’s Chapter on Vacuity Taxation. Shen Qi Wan is indicated for a pattern of “vacuity taxation with lumbar pain, urgent cramping in the lesser abdomen and unsmooth urination” (虛勞腰痛，少腹拘急，小便不利者，八味腎氣丸主之), while Shu Yu Wan is indicated for “vacuity taxation with every insufficiency [and] the hundred diseases of wind qi.” (虛勞諸不足，風氣百疾，薯蕷丸主之). Gua Lou Qu Mai Wan is listed in the Chapter on urinary disorders, where it described as treating a pattern of unsmooth urination with thirst, attributed to pathological accumulation of water (小便不利者，有水氣，其人若渴 ).

The diseases listed here all involve some kind of deficiency, and two of them specifically mention impaired fluid metabolism, which has direct relevance to Shanyao’s function as the Water of Metal.

In the case of Shanyao, its Sour Flavor brings the action of Metal, which is to gather and astringe, to the phase of Water. What this means is in practice is that it has a gathering, astringing, or securing effect on the Kidneys. This makes sense, given that in both Shen Qi Wan and Shu Yu Wan, it is paired with Dihuang, the Water of Water.

As the horary herb of Water, Dihuang is able to replenish its own phase (in the same way that we saw Wuweizi, as the Metal of Metal, replenishes the Lungs). Shanyao supports this activity by its Sour flavor and gathering action. Its ability to treat "damage in the center" (治傷中) can also indirectly benefit Water; the center belongs to Earth, and Earth controls Water, and so by strengthening Earth, Water can be regulated to prevent leakage; the same way that an earthen dam can contain a body of water to prevent it from flooding or draining away.

In short, Shanyao is able to gather and astringe fluids and both post and pre-natal essence to tonify deficiency. This relationship can hopefully illuminate more broadly the way in which Metal can generate Water. The herbal pairing of Shanyao with Dihuang occurs not only Shen Qi Wan, but in the many post-classical formulas dervied from it, such as Liu Wei Dihuang Wan (六味地黄丸) and its many variations, as well as You Gui Wan (右歸丸) and Zuo Gui Wan (左歸丸).

Shen Qi Wan can be understood as the combination of three herbs that support the Yin (Dihuang, Shanyao, Shanzhuyu) with three herbs that drain pathological fluid (Fuling, Zexie, Mudanpi) and two herbs that support the Yang (Guizhi and Fuzi).

The later formulas essentially take one or more of these functions as primary.

For example, Liu Wei Dihuang Wan removes the Yang tonics and its variations such as Zhi Bai Dihuang Wan may add additional herbs to clear heat or supplement Yin. And You Gui Wan takes Shen Qi Wan but removes the draining herbs, and adds additional Yang tonics such as Tu Su Zi and Lu Jiao Jiao. All of these formulas, however, contain both Shanyao and Dihuang, because they are all supplementing the Kidney, and Shanyao’s ability to tonify Water when combined with Dihuang is unsurpassed.

In the case of Gualou Qu Mai, while there is deficiency, it is primarily of the Kidney’s ability to transform and steam fluids. Water collects below, leading to unsmooth urination but does not rise to moisten the upper burner, leading thirst. In this case, Fuzi (Water of Wood) is used to stimulate the Kidney to transform fluids in the lower burner, while Shanyao (Metal of Water) helps to gather fluids in the upper burner, when paired with the fluid generating and thirst quenching Gualougen / Tianhuafen.

This ability of Shanyao to gather, contain, and astringe is an important property that is used in many post-classical formulas as well, such as Wan Dai Tang (完帶湯), which is often used to treat vaginal discharge, or Shen Ling Bai Zhu San (參苓白術散), whose formula pattern often presents with watery loose stools. Shanyao is a key ingredient in both of these formulas.

This herb has somewhat less to discuss than the others, due to its low use in classical formulas, but it is still quite interesting and hopefully will expand your use of the formulas where it is used.