Hip,
To understand a TCM prescription, one also needs to understand the "Five Flavours/Tastes" to understand each herb's property + Understand the "King-Minister-Guiding Herb principle". I shall not do a lengthy discussion of that here but focus on my personal Formula Interpretation here to reduce complexity
"Bu Zhong Yi Qi Tang":
Very well known + often used prescription in TCM. A possible read might be here:
http://www.acupuncture.com/herbs/buzhongyi.htm
One thing to note in TCM is that if you look at individual herbal contraindications (TCM type, not pharmacological), alot of people will not have body conditions suitable to eat it. Almost every herb will have contraindications. For instance, Astragalus isnt suitable for people with alot internal heat fairly common especially in people with "yin deficiency". Hence, TCM combines various herbs to reduce side effects so a person with specific contraindications can better tolerate it. For instance, Hip took Rhemannia raw. For TCM, both Cooked and Raw format are both said to be very greasy/"dampening" on the stomach (creates digestive/stomach problems) if you eat it alone. Just go touch some Rhemmania externally and you will notice.
For TCM, it is mostly incorrect to eat a herb standalone without reducing it's side-effects. Effects are also much reduced too without combination from a TCM outlook and can actually make a pre-existing condition worse instead, especially with contraindications. This is very crucial.
This is one very user-friendly English TCM book of individual herb properties i follow much lately. Read contradindications of Rhemmania (cooked) under page 780 here:
https://books.google.com.sg/books?id=AmCVN_uxXD8C&printsec=frontcover&dq=a materia medica for chinese medicine plants 780&hl=en&sa=X&ved=0CBoQ6AEwAGoVChMI5_T26q6byAIVw0iOCh16ugTP#v=onepage&q=a materia medica for chinese medicine plants 780&f=false
http://old.tcmwiki.com/wiki/shu-di-huang
Personally, my 2 cents interpretation of the high Formula total effective rate is that it works particularly well for chronic conditions without even finding the Illness Origin as in my personal opinion, chronic conditions often involves:
1. Spleen and stomach digestive problems
2. Deficient energy Causing Internal body Heat (low quality energy) yet also causing feeling cold symptoms (due to lack of high-quality energy) at the same time especially in lower half of body (one possible lousy analogy here is poor car oil/fuel quality causing more crappy polluting gases like carbon monoxide and vice versa)
3. Blood insufficiency (for ease of explanation simplicity, think of it as inclusive of low Red Blood Cell and low White Blood Cells for instance, loss of excessive menstruation blood for women)
TCM role of herbs in "Bu Zhong Yi Qi Tang" (Ginseng and Astragalus Combination):
1. Bai Zhu (atractylodes) and Chen Pi (tangerine peel) tonifies and helps with the spleen and stomach
2. Huang Qi (Astragalus) and Dang Shen (Codonopsis) together tonify Deficient Energy and "Raise energy (yang) upwards to reach the upper body half. Ginseng, being relatively more expensive, is commonly substituted for here in TCM by Codonopsis which has a similar functions but at 1:3 ratio for much cheaper herbal cost.
3. Dang Guai helps with blood insufficiency and nutrient problems
4. Gan Cao (licorice root) is almost in every TCM formula because it's "sweetness" property helps to harmonize the harsh effects of herbs
5. Sheng Ma (black rhizome) and Chai Hu clears "strong fever-type heat" here and has similar functions to Astragalus especially in raising energy upwards.
6. Chai Hu (blupureum) is commonly used for stagnation uses, but here, it's used like Astragalus's function here to raise energy. Personally, i do not like to use Chai Hu as it's been felt by some TCM practitioners to consume "yin energy" and hence they nullfy side effect they add alittle Rhemannia to tonify "yin energy"
Liu Wei Di Huang Wan (Rhemannia 6):
Can be said to be the most Ultra-Famous Household name Formula in Asia. Most people in Asia have taken them in one time or another at least in their lives. Alot alot of people (even healthy people) take this on a daily basis (especially pill-form) including me (i take a very very modified and self-personalized decocted version of this for health maintenance). Ask around in every Chinese family in Asia and most mums will have heard of this/currently consuming. In Asia, the extent is such that every man and their donkey will have heard or used it before = LOL!
With it's household fame, there are many spawned-off modified versions including different herbs for Rhemania 6 along with modified versions like Rhemania 8 too etc. Their functions are largely the same with minor differences as they center upon Rhemannia.
TCM View of Herb Functions in Liu Wei Di Huang Wan (Rhemannia 6):
1. Shu Di Huang (Rhemannia): Tonifies "Jing/Stored Potential Energy Reserves' and Blood only. This herb's effects takes afew weeks/months to be felt as it does not directly tonify "Qi" which we currently use at the moment, so it's effects are much slower to be visibly felt.
2. Shan Zhu Yu (Cornus) with Mu Dan Pi (Moutan): tonifies liver and clears heat
3. Shan Yao (Dioscorea): Qi Tonic
4. Ze Xie (Alismatis) and Fu Ling (Poria): Both are used to drain dampness/greasy side effects by Rhemannia. Ze Xie (Alismatis) is said to have a stronger function here than Fu Ling (Poria)
http://www.itmonline.org/arts/rehm6.htm
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My Personal TCM Viewpoint/Comparison of "Bu Zhong Yi Qi Tang" (Ginseng and Astragalus Combination) Versus "Liu Wei Di Huang Wan" (Rhemannia 6):
For the record, i have taken all the all the herbal formulas above in their original format combination in both pill form and decoction form and have also taken all the individual herbs above in various modifications or part of other formulas for many years, especially Rhemannia 6.
Once again, the below is my personal rudimentary-knowledge of the 2 formulas so take it with a grain of salt
GAC: Ginseng and Astragalus Combination
1. In TCM, GAC is often used for moderate to severe chronic conditions to immediate life-danger body conditions where the energy deficiency and other conditions is very serious (for instance very low appetite, spontaneous sweating, fever etc). For immediate life-threatening conditions, Ginseng (more expensive) is in place of Codonopsis (Dang Shen) as the former works very fast and is able to tonify "Jing/Stored Energy Reserves' almost instantly.
Rhemania 6, on the other hand, can be used by people anywhere from no health ailments (general body maintenance) to chronic conditions too. Rehmannia 6 can only be used in cases with no serious immediate conditions seen, like major appetite loss/profuse sweating/fever.
2. For TCM, Rhemannia 6's effects focuses on tonifying "Jing/stored energy reserves" while GAC focuses on immediate dangerous conditions by tonifying "Qi/energy" instantly. Effects for the Rhemannia 6 can take weeks/many months to be felt, while for GAC, it can be felt within afew days (improvements visible via less/no profuse/spontaneous sweating/improved appetite/reduced fever etc). Hence, Rhemannia 6 and variants are particularly taken in Asia during Winter to increase energy reserves
3. In TCM, both formulas improves cognitive functions (brain-fog feeling) too but GAC works faster than Rhemannia 6 due to the intrinsic usage of Ginseng as compared to Rhemannia.
4. For when GAC is appropriately used and an individual is more stable, GAC is stopped and highly modified formulas of Rhemannia 6 might be used or other entirely different tonification formulas is used. IMHO, GAC is not meant for use in the long run unless said individual has chronic symptoms.
An overly simplified TCM Viewpoint might be that GAC is used in very chronic/life-threatening cases while Rhemannia 6 is usable in chronic conditions with no outwardly dangerous symptoms. GAC is a much much more complicated formula than Rhemannia 6 and it should ideally be taken only under close TCM medical supervision.
Thanks, Sushi/the other mod (dont know your name yet) for the difficult task of helping in moderation of everyone's postings