"What you eat is what you supplement," is the biggest misconception people have about Traditional Chinese Medicine (TCM), which misinterprets the TCM principle of "nourishing an organ with a corresponding organ." The five organs in TCM are actually a combination of the functions of a series of organs and tissues. The five organs in TCM are not anatomically localized; instead, they are categorized according to the depth and level of their functional evolution. If the human body is seen as a great tree or a pagoda, the roots of the tree and the base of the pagoda are what TCM calls the "Kidney," and the gradually ascending trunk and leaves are the Zang-fu organs such as the Heart, Liver, Spleen, and Lungs.
The TCM Kidney is the "foundation of innate life." If the "Kidney" is damaged, it can be fatal. Therefore, TCM has the saying, "a long-term illness eventually affects the Kidney," which means that if any disease is not brought under control, it will eventually impair the "Kidney," leading to "Kidney deficiency." "Kidney deficiency" is merely a description of the stage of a disease's progression. For instance, in the late stages of a patient with heart failure, Fuzi (Aconite root) might be used to restore yang and rescue the patient from collapse, and Fuzi is a herb that warms and supplements the Kidney yang. The TCM "Kidney" is the foundation of the human body. When the leaves and trunk are severely infested by insects, the roots are also hard to escape; if the base of the pagoda becomes loose, the pagoda itself will collapse.
For treating Kidney deficiency, TCM herbal medicines rarely use flowers or leaves of plants; they are generally roots, stems, or seeds. Examples of the former are Gouji (Dogwood root) and Shanyao (Chinese yam), and examples of the latter are Shanyurou (Cornus fruit) and Gouqi (Goji berry). The roots and stems of a plant are analogous to the roots of the human body, and the seeds are similar to stem cells. Supplementing the "Kidney" with these is the true meaning of what TCM calls "nourishing an organ with a corresponding organ."
In addition to plant roots, animal bones, and even human placenta are used as excellent medicines to supplement the Kidney. For example, pig spinal cord is an ingredient in the "Dabuyin Wan" (Great Yin-Supplementing Pill); human placenta is the TCM herb "Zheheche" (Purple River Carriage), used to treat congenital developmental deficiencies and low immunity caused by illness and weakness. As for eating pig brains to nourish the brain, it is based on the same principle. Because the pig brain is essentially spinal cord, it also supplements the Kidney. Using animal brains to supplement the body is simply leveraging the effect of nourishing the Kidney to make the roots strong, and enhanced brainpower is merely one of the benefits that flourish when the roots are robust.
As for practices like "eating pig liver to nourish the liver," this is an abuse or even a misunderstanding of "nourishing an organ with a corresponding organ." TCM posits that the Blood is governed by the Liver. If one must say that eating liver nourishes the liver, what is being supplemented is the Blood, which is controlled and managed by the TCM "Liver."