The “Huang Di Classic of Medicine” stated that “with the sustenance of Zheng Qi in the body, no evils can intervene (正氣存内，邪不可干)”. What is Zheng Qi (正氣) in traditional Chinese medicine (TCM)? According to Dao theory, the Universe originated from one Dao, which split into Yin and Yang. The interaction between these two opposing forces in the body generates Zheng Qi, also known as vital energy [1] . Understanding the concept of Zheng Qi is essential for practitioners of Traditional Chinese Medicine (TCM), as it forms the basis of many treatment modalities.

According to Traditional Chinese Medicine (TCM) theory, two types of energy contribute to the generation of Zheng Qi in the body, which is responsible for maintaining health and vitality. Acquired Qi (Pectoral Qi or Zong Qi) is formed by combining vital essences extracted from atmospheric air and dietary food, while Primordial Qi is inherited from parents and primarily derived from the Universe ( Figure 1 ) [2] [3] . The combination of these two types of energy in varying proportions leads to the generation of Zheng Qi. While both Acquired Qi and Primordial Qi are required for Zheng Qi generation, a sufficient supply of Acquired Qi can prevent the overuse of Primordial Qi, thus slowing down its rate of exhaustion. This concept forms the basis of promoting health and longevity in TCM [4] - [8] .

Zheng Qi has two main functions: Wei Qi (Yang in nature) and Ying Qi (Yin in nature) [5] [9] . Wei Qi protects the body against external pathogens, similar to innate immunity, while Ying Qi nourishes the visceral organs with vital essences, thereby promoting their function, including that of adaptive immunity. The generation of Zheng Qi requires the functioning of visceral organs, specifically the Lung, Spleen, and Stomach, which extract Air Qi and Grain Qi (Acquired Qi) from atmospheric air and food. Additionally, the Kidney helps mobilize the Primordial Qi [9] . However, the amount of Primordial Qi is fixed from conception and cannot be easily replenished during a lifetime [3] [5] [10] [11] . Maintaining the Yin/Yang balance of visceral organs to ensure their optimal physiological functions is crucial for the efficient generation of Acquired Qi and the mobilization of Primordial Qi. Chinese tonifying herbs, categorized into Yang-invigoration, Qi-invigoration, Yin-nourishing, and Blood-enriching, can restore this balance and Qi/Blood function in visceral organs. This enables the efficient generation of Acquired Qi and mobilization of Primordial Qi, preventing the overuse of Primordial Qi [3] [11] - [16] .

Yang/Qi deficiency is linked to chronic fatigue syndrome in Western medicine [8] , while Yin deficiency is associated with autoimmune diseases [17] . Pharmacological studies of Chinese tonifying herbs have shown that restoring Yin/Yang balance and Qi/Blood function affects various cellular functions, leading to the resumption of normal physiological functions. These include enhancing mitochondrial ATP generation with Yang and Qi herbs and potentiating the adaptive immune response with Yin herbs. Blood herbs can also induce nitric oxide (NO) production in cultured vascular endothelial cells and erythropoietin (EPO) production in cultured hepatocytes [18] . Recent studies have shown that the status of Zheng Qi generation can be assessed by measuring the natural killer (NK) cell activity and antigen-induced T/B cell proliferation in an ex vivo mouse model [19] .

The use of herbal formulations in Traditional Chinese Medicine (TCM) is a holistic approach focused on the overall health and well-being of an individual [7] [11] [18] [20] . The principle of using multiple herbs for disease prevention and treatment is based on the belief that a combination of herbs, guided by TCM theory, can produce a more comprehensive therapeutic effect than a single herb alone [20] [21] [22] [23] . In cases of sub-optimal health conditions, diagnosing the deficiency pattern in specific visceral organs guides the formulation of herbs based on relevant functional categories and meridian tropism (targeted visceral organs) [24] [25] . The functional characterization of Chinese herbs relies on thousands of years of human usage, guided by Traditional Chinese Medicine (TCM) theories, including Yin and Yang, the Five Elements, and the Meridian theory [23] [26] [27] . Administering herbs or herbal formulations can restore a normal Yin/Yang balance and Qi/Blood function in visceral organs. This leads to improved physiological function and health outcomes, presumably through the enhancement of Zheng Qi generation [3] [27] . Mycelium cultures of Cordyceps sinensis (Cs4) ( Table 1 ) and Lingzhi spore ( Table 2 ), popular herbal health products, were found to enhance Zheng Qi generation. This was assessed by measuring natural killer cell activity and antigen-induced T/B cell proliferation in an ex vivo mouse model.

Chinese medicine often uses tonifying herbs and multi-component herbal formulations to treat deficient body conditions. To ensure the quality control of Chinese herbal products, functional assays related to Yin/Yang and Qi/Blood functions should be included. This is in addition to chemical marker analysis and pharmacological assays for specific medical claims [6] [21] [26] . Chinese tonifying herbs are classified into four categories: Yang-invigorating, Qi-invigorating, Yin-nourishing, and Blood-enriching. These herbs can be used alone or combined to address deficiencies in body function in sub-optimal health conditions [3] . Yang-invigorating herbs stimulate mitochondrial ATP generation. Qi-invigorating herbs improve cellular glutathione redox status, leading to an increase in mitochondrial ATP generation capacity [28] . Yin-nourishing and blood-enriching herbs possess specific pharmacological activities, measurable by antigen-induced T/B cell proliferation and EPO/NO production in hepatocytes and endothelial cells, respectively [18] [19] . Additionally, the capacity of herbal health products to boost Zheng Qi production can be evaluated by measuring NK cell activity and antigen-induced T/B cell proliferation in an ex vivo mouse model [19] . These functional assays serve as biomarkers for the quality control of Chinese herbal products.

TCM often utilizes Chinese tonifying herbs and herbal formulations to rebalance Yin/Yang and Qi/Blood functions in visceral organs, thereby enhancing the efficient production of Zheng Qi (i.e., physiological functions) and promoting good health outcomes. To ensure quality control of Chinese herbal health products, functional assays measuring Yin/Yang and Qi/Blood functions, as well as Zheng Qi production, should be included. These assays can act as biomarkers for assessing the quality and efficacy of herbal health products within the framework of TCM theory.