Progress in the study of Yang-deficiency constitution in terms of Traditional Chinese Medicine: a narrative review

Abstract

The Yang-deficiency constitution has been a hot topic of interest in recent years. Important research progress has been made in quantitative diagnostic criteria and auxiliary identification technologies, modern biological inter-pretations of the constitution characteristics, the association between the Yang-deficiency constitution and diseases, as well as the mechanisms of regulating the constitution. Yet, there are still some deficiencies and limitations. Herein, we systematically reviewed the research progress on the Yang-deficiency constitution by searching articles published in China National Knowledge Infrastructure Database, China Science and Technology Journal Database, Wanfang Database, PubMed, and Embase. The future development direction of the Yang-deficiency constitution should focus on developing objective constitution identification techniques, establishing animal models of Yang-deficiency constitution, conducting constitutional and disease-related research with a high level of evidence, combined with research on the full chain of the nine constitutions so as to obtain more scientific and objective conclusions.

1. INTRODUCTION

The Yang-deficiency constitution is one of nine Traditional Chinese Medicine (TCM) constitutions based on the comprehensive manifestation of individual inherited, physiological, physical, and psychological conditions. It determines the response, susceptibility, and progress of certain diseases. People with Yang-deficiency constitution exhibit cold intolerance and mental atrophy, which usually appears before the occurrence of illness, directly impacting their quality of life.¹ Over the years, several clinical studies have suggested a close connection between the Yang-deficiency constitution and the incidence and development of diseases such as chronic fatigue syndrome, hypertension, and infertility.² The 2017 national epidemiological survey data on the TCM constitution showed that 16.41% of the population have the Yang-deficiency constitution, ranking first among all imbalanced constitutions.³ In this paper, we reviewed the progress in studying the Yang-deficiency constitution, analyzed the current limitations of the studies on Yang-deficiency constitution, and proposed directions for future research.

2. STUDIES ON YANG-DEFICIENCY CONSTITUTION

2.1. Establishment of quantitative diagnostic criteria and auxiliary identification technologies

The Constitution in Chinese Medicine Questionnaire⁴ (including the Body Constitution Questionnaire to Assess Yang-deficiency) was developed in 2005 as a measurement instrument to identify different TCM body constitution types. Based on the reliability and validity tests of the questionnaire, the "Classification and Determination of Constitution in TCM" standard (including the standard for Yang-deficiency Constitution) was developed a year later after multidisciplinary cross-validation by TCM clinical experts, epidemiologists and constitution experts. This standard was recognized as an industry standard by the China Association of Chinese Medicine in 2009 and has since been promoted and implemented nationwide. Using the Constitution in Chinese Medicine Questionnaire and the standard of Classification and Determination of Constitution in TCM as objective classification instruments, a nationwide epidemiological survey, which included of a total of 129 963 cases, was conducted to clarify the distribution characteristics of Yang-deficiency constitution among the general population. From 2007 to 2017, the incidence of the Yang-deficiency constitution in the general population increased from 9.04% to 16.75%, ranking first among the imbalanced constitutions. Moreover, the incidence was higher in women and East Chinese residents.^3,5

In addition to the standardized identification instruments, researchers have developed a series of modern auxiliary identification technologies. The evaluation methods based on the four TCM diagnostics (inspection, listening and smelling, inquiring, and palpation), such as facial feature assessment and objective tongue image differentiation technique, are often used to assess the Yang-deficiency constitution at a macroscopic view. Facial feature assessment is done using near-infrared technology. Ina previous study, 5150 facial images and videos were collected for analysis, and the preliminary facial characteristics of Yang-deficiency constitution were obtained: pale face, lusterless eyes, pale lips, minimal absolute values of facial shape (wide at the top and narrow at the bottom, with a short and shallow philtrum).⁶ Moreover, studies on tongue imaging and analysis systems revealed that the length and volume of the tongue, as well as the characteristics of the tongue being with or without tooth marks, can be used to identify the tongue of people with Yang-deficiency and neutral constitution.⁷

Evaluation methods based on physiological characteristics, such as Yang-deficiency constitution's sensation of chill and cold intolerance, have also been applied. Infrared thermography assessment can reflect the human body's heat distribution. It has been found that the temperature of the whole body is lower for people with Yang-deficiency constitution, especially in the abdomen, compared with other body constitutions. Moreover, the governor vessel in people with Yang-deficiency constitution was warmer, while the temperature of the middle energizer, left and right chest, stomach, abdomen, and extremities were lower. Also, discontinuous or even disconnected heat maps of the governor's vessel were observed for those with higher Yang-deficiency scores, with the disconnections often occurring at the Dazhui (GV14) and Mingmen (GV4) points.⁸ Therefore, a Yang-deficiency constitution assessment can be performed in the winter by scanning only the lower extremities.⁹

Evaluation methods based on the principle of bioelectrical impedance, such as the application of TCM meridian health detector and computer-aided scanning detection system (EASY-SCAN), reflect the characteristics of energy and metabolism of Yang-deficiency constitution. Studies using the TCM meridian health assessment instrument found that people with Yang-deficiency constitution generally had low physical energy and metabolism compared to those with neutral constitution, and most had autonomic nerve disorders. In addition, the energy values in the kidney, spleen, heart, small intestine, bladder, and triple energizer meridians were generally low for those with Yang-deficiency constitution.¹⁰ Moreover, EASY-SCAN revealed that people with Yang-deficiency constitution had lower basal metabolism and visceral function than those without Yang-deficiency constitution, while the accumulation of oxidative free radicals was higher, suggesting that people with Yang-deficiency constitution aged more easily.¹¹

Based on the principle of natural endowment of the constitution, through the study of genome-wide expression profiles, a gene classifier was constructed to reveal the genetic characteristics of the Yang-deficiency constitution from a microscopic point of view. The results of constitution classification were consistent with the genomic classification, and the genomic classifiers could preliminarily distinguish between Yang-deficiency and neutral constitution. Among different genes, RGS6, mGluR5, GAPDHL19, and IKZF1 could assist in the determination of the Yang-deficiency constitution.¹²

2.2. Interpreting the physiological characteristics of Yang-deficiency constitution using modern biological interpretations

Multidisciplinary systematic studies have been conducted in biophysics, genomics, proteomics, metabolomics, and intestinal microecology. These studies suggested that people with Yang-deficiency constitution tend to manifest energy metabolism disorders and immune hypo-function before the first disease attack. These studies provided modern biological interpretations for the physiological characteristics of the Yang-deficiency constitution.

2.2.1. Sleep physiological parameters

Sleep physiological parameters were measured and showed a tendency towards drowsiness and energy metabolism deficiency in people with Yang-deficiency constitution. The mattress sleep tracking system was used to monitor the physiological parameters of people with Yang-deficiency constitution during nighttime sleep. It was found that the total sleep time was biased towards the upper limit of the normal range, the percentage of deep sleep was decreased, the sleep quality was low, the lowest nocturnal oxygen saturation was lower than the normal range, and the mean heart rate was slow in the normal range.¹³ Due to the lack of Yang-Qi, Yang-deficiency constitution was characterized by drowsiness, tiredness after more sleep, and low energy metabolism level.

2.2.2. Biochemical tests

Biochemical tests further suggested that before disease attack, people with Yang-deficiency constitution have reduced functions of the hypothalamic-pituitary-adrenal axis and the hypothalamic-pituitary-thyroid axis, as well as cyclic nucleotide system and immune system disorders. Serum corticosterone levels, cAMP/cGMP ratio, and interleukin 1β (IL-1β) and thyrotropin (TSH) levels were significantly increased, while serum cortisol, adrenocorticotropic hormone (ACTH), cGMP, and unconjugated thyroxine (FT4) levels were significantly decreased in those with Yang-deficiency constitution, suggesting an association between Yang-deficiency constitution and the reduced functions of the hypothalamic-pituitary-adrenal axis and hypothalamic-pituitary-thyroid axis, cyclic nucleotide system disorders, and immune system disorders.¹⁴

2.2.3. Genomic and transcriptomic assays

Multi-omics studies including genomic [single nucleotide polymorphism (SNP)], epigenomic (miRNA), and transcriptomic assays, revealed the specific genetic characteristics of the Yang-deficiency constitution, characterized by immune system disorders (reduced immune surveillance and upregulated expression of inflammatory cytokine-related genes). Endocrine and genetic messaging were also decreased and regulated by epigenetic miRNAs. Compared with people with the neutral constitution, those with Yang-deficiency had specific SNP distributions (PPARD rs2267669 and rs2076167, APM1 rs7627128 and rs1063539) and specific haplotype distributions (Hap25 of APM1), which are the genetic basis for the characteristic classification.¹⁵ Compared with the neutral type, 267 genetic loci with different expressions were detected in people with Yang-deficiency constitution, of which 150 were up-regulated and 117 were down-regulated. The genes that were up-regulated mainly involved inflammation-related genes, the cyclic adenosine monophosphate response element-binding gene, the cyclic adenosine monophosphate response element modulator gene, etc.; genes that were downregulated were primarily related to genetic messaging, the methylenetetrahydrofolate reductase gene, etc. Among them, down-regulation of thyroid hormone receptor β expression provided a molecular biological explanation for cold intolerance in people with Yang-deficiency constitution.¹⁶ Further studies revealed that the differential expression of the miRNA target genes in the saliva of people with Yang-deficiency constitution was enriched in the thyroid hormone signaling pathway, which confirmed that cold intolerance was regulated by epigenetic miRNAs.¹⁷ Also, compared with people with Yin-deficiency, Yang-deficiency constitution genes with different expressions were enriched in 35 significantly different signaling pathways, mainly involving immune function, energy metabolism, lipid metabolism, glucose metabolism, nervous system, and the tumor-cancer signaling pathways. Among them, there were three pathways with completely opposite expressions, namely FFAR2, GPR109B, and NAMPT, which suggested differences in gene expressions related to energy metabolism, especially glucose and lipid metabolism, between the constitution types of Yang-deficiency and Yin-deficiency. These studies further support the compensatory gene expressions for coldness intolerance in Yang-deficiency and heat intolerance in Yin-deficiency.¹⁸

2.2.4. Proteomics studies

Differentially expressed serum proteins in the Yang-deficiency type were enriched in molecules associated with endocrine and energy metabolism disorders as well as decreased immune function. Compared with the neutral type, 85 differentially expressed proteins, including 64 up-regulated proteins and 21 down-regulated proteins, were screened.¹⁹ Signaling pathway enrichment analysis indicated that the Yang-deficiency constitution was prone to endocrine and energy metabolism disorders and decreased immune function. Among them, PI3K-Akt, which has a central role in insulin signaling, adipocyte function, and glucose metabolism, was the most significantly up-regulated pathway.¹⁹

2.2.5. Metabolic end product studies

Metabolic end product studies^20,21 of the Yang-deficiency constitution showed the presence of energy metabolism and glucolipid metabolism disorders in people with Yang-deficiency constitution. Multivariate statistical analysis on the endogenous metabolic differences in the serum, urine, and feces of people with Yang-deficiency constitution was performed using NMR-based metabolomics with the neutral constitution as control. Lactate, very low-density lipoprotein/low-density lipoprotein, N-acetyl glycoprotein, fatty acids, and unsaturated fatty acids were decreased in the serum, while glutamine, glucose, phosphatidylcholine, and high-density lipoprotein were increased in people with Yang-deficiency constitution. Moreover, creatinine was decreased in urine, while lactate, dimethylamine, citric acid, and hippuric acid were increased; in feces, butyrate, acetate, methionine, succinate, and glucose were decreased, while propionate, alanine, and lactate were increased. These potential biomarkers indicated the presence of energy metabolism disorders, lipid metabolism disorders, and glucose metabolism disorders, as well as intestinal dysbiosis and altered kidney function in people with Yang-deficiency constitution.^20,21

2.2.6. Intestinal microbiota

The unique structural characteristics of the intestinal microbiota of people with Yang-deficiency constitution affect the host metabolism.^22,23 DNA sequencing was performed on the V3-V4 region of the 16S rRNA of all bacteria in the fecal samples from individuals with Yang-deficiency constitution using Illumina high-throughput sequencing. The abundance of intestinal microbiota in individuals with Yang-deficiency constitution showed a higher heterogeneity than for individuals with neutral constitution. In terms of microbial abundance, the content of Faecali bacterium was significantly reduced in people with Yang-deficiency constitution compared with people with the neutral constitution. Furthermore, the abundance of this bacteria was negatively associated with the degree of depression, which provided an intestinal microecological explanation for the psychological characteristics of introversion and dispiritedness in people with Yang-deficiency constitution.²² Further analysis was performed using multivariate statistical methods, including factor analysis to explore the structural diversity of the intestinal microbiota and the fecal metabolomics data. It was found that Leuconostoc and Rothia, which were significantly more abundant in Yang-deficiency constitution, were negatively correlated with energy, protein, and amino acid metabolism. Defluviitaleaceae_incertae_sedis and Butyricimonas were negatively correlated with glucose metabolism; Ruminococcus, which was significantly reduced in Yang-deficiency constitution, was positively correlated with glucose metabolism, energy metabolism, as well as protein and amino acid metabolism.²³

2.3. Clarifying the association between Yang-deficiency constitution and diseases and elucidating the mechanism of action of constitution conditioning interventions

2.3.1. Association between Yang-deficiency constitution and diseases

The researchers searched the literature for all clinical studies on diseases related to the Yang-deficiency constitution conducted in China and abroad using the standard of Classification and Determination of Constitution in TCM since its publication until December 31, 2018. A total of 214 clinical studies were included, with a total sample size of 75 569 cases involving 112 diseases. The results showed that Yang-deficiency constitution was the most common among individuals with infertility, osteoporosis, irritable bowel syndrome, osteoarthritis of the knee, menstrual cramps, polycystic ovary syndrome, menopausal syndrome, rheumatoid arthritis, urinary calculi, colorectal polyps, recurrent miscarriage, hypertension, and cervical spondylosis, with a proportion of 19.3% to 39.6% reported in the literature, which was significantly higher than the national norm of 9.04%. Twenty-six studies have shown that the Yang-deficiency constitution was a risk factor for diseases such as osteoporosis and chronic fatigue syndrome (95% confidence interval of OR: 2.227 to 9.057).² Therefore, TCM constitution identification and regulation could help promote the prevention and treatment of these diseases.

2.3.2. Regulating Yang-deficiency constitution for the prevention and treatment of related diseases

According to the principle of prescription in accordance with the constitution, the relative stability of the Yang-deficiency constitution suggests that its regulation must be a long-term process. The main herbs for the regulation of the Yang-deficiency constitution should be warm and moisturizing in nature. Roucongrong (Herba Cistanches Deserticolae), Bajitian (Radix Morindae Officinalis), and Suoyang (Herba Cynomorii Songarici) that are warm and moisturizing to the kidney Yang, as well as Lurong (Cornu Cervi Pantotrichum), Lujiaojiao (Colla Cornus Cervi) and Lujiaoshuang (Cornu Cervi Degelatinatum) that warm and dredge the governor vessel, may be used. Meanwhile, when selecting appropriate medicine, it is also necessary to take into account the conditioning method of reinforcing Yang from Yin [Shudihuang (Radix Rehmanniae Praeparata)) or equally reinforcing Yin and Yang (Tusizi (Semen Cuscutae)] as well as warming the interior and dispersing the cold [Fuzi (Radix Aconiti Lateralis Preparata), Ganjiang (Rhizoma Zingiberis)]. Guifudihuang Wan (桂附地黄丸) or Yanghe decoction (阳和汤) is considered the main prescription for Yang-deficiency constitution conditioning.²⁴ Guifudihuang Wan decreases Yang-deficiency constitution scores and improves subjective symptoms. Moreover, constitution conditioning interventions can assist in improving the symptoms of sub-optimal health, depression, hypertension, osteoporosis, chronic cough, chronic fatigue syndrome, obesity, insomnia, and other diseases in individuals with Yang-deficiency constitution as the main type of constitution, as well as improving the health and quality of life of patients.^{25,⇓,⇓,⇓,⇓,⇓,⇓-32}

2.3.3. The mechanisms of regulating Yang-deficiency constitution

Clinical studies have shown that Guifudihuang Wan increases Th1 (%), Th1/Th2, decreases Th2 (%), increases IL-2, INF-γ, TNF-α, IL-4, IL-6, IL-10, increases plasma 5-HT, serum Cor, and plasma ACTH levels, and decreases norepinephrine levels. It has been suggested that the mechanism of action of Guifudihuang Wan in regulating the Yang-deficiency constitution is related to the regulation of neuro-endocrine functions, energy metabolism, and cellular immunity.³³ Metabolomic studies have also shown that Guifudihuang Wan increases the levels of alanine, glutamine, α-glucose, isoleucine, betaine, and propylene glycol levels in individuals with Yang-deficiency constitution, mitigates energy metabolism disorders, and increases the energy production of the body.³⁴ Long-term administration of small doses of Jingui Shenqi pills (金匮肾气丸) can increase IgM levels that decrease with aging, regulate the changes of immunoglobulins that occur with aging, and improve the immune function of elderly individuals with Yang-deficiency constitution.³⁵

Moreover, acupuncture or moxibustion has been most commonly used for the non-pharmacological management of the Yang-deficiency constitution. The mechanisms are related to regulating the hypothalamic-pituitary-adrenal axis function and the improvement of immune indicators in individuals with Yang-deficiency constitution.^36,⇓-38 In addition, Qigong and medicinal food can also be used to improve the Yang-deficiency constitution by adjusting energy metabolism.^39,40

3. LIMITATIONS AND COUNTERMEASURES OF YANG-DEFICIENCY CONSTITUTION

3.1. Limitations of Yang-deficiency constitution

Although researchers have carried out a series of studies on the constitution identification techniques, the microscopic characteristics of Yang-deficiency constitution, the association between Yang-deficiency constitution and diseases, as well as the mechanisms of action of constitution-regulating treatments, and a relatively systematic research approach have been established, there are still some limitations.

3.1.1 Shortcomings of constitution identification techniques

At present, the Constitution in the Chinese Medicine Questionnaire and the standard of Classification and Determination of Constitution in TCM are still used as the main instruments for identifying the Yang-deficiency constitution. In practice, there are multiple entries that require feedback and certain questions that are difficult to understand; therefore, further revisions and simplifications are required. The standard of Classification and Determination of Constitution in TCM remains an industry standard, which restricts its promotion and application to a certain extent. The accuracy of constitution identification can be affected by differences in subjective judgment during the use of questionnaires. Thus, an increasing number of researchers have begun to apply modern objective diagnostic technologies. However, objective and reproducible technical parameters have not yet been obtained due to the lack of big data support.

3.1.2. Limitations of microscopic research on the physiological characteristics of Yang-deficiency constitution

Currently, researchers have applied systems biology methodologies to conduct microscopic studies on the characteristics of the Yang-deficiency constitution. However, most of these studies were conducted in patients in the prevention stage and lacked further validation in clinical trials, animal experiments, and cellular experiments. In particular, the lack of mature animal models of the Yang-deficiency constitution has become a bottleneck for microscopic research on TCM constitution.

3.1.3. Clinical research issues related to the correlation between Yang-deficiency constitution and potentially associated diseases

The current research on the association between the Yang-deficiency constitution and diseases is still based on literature research or cross-sectional surveys, and there is a lack of high-quality clinical studies with large sample sizes, and systematic evaluation and Meta-analysis of clinical studies.² In clinical studies on the regulation of the Yang-deficiency constitution for the prevention and treatment of related diseases, the intervention methods are relatively single,^41,42 the conditioning schemes are not uniform, and there is a lack of evidence-based outcome evaluation. Meanwhile, the effect of constitution-conditioning intervention outcomes is mostly evaluated according to the standard of Classification and Determination of Constitution in TCM, the recognized quality of life scale, or the clinical symptoms of constitution-related diseases. However, the Classification and Determination of Constitution standard in TCM is a diagnostic standard and should not be used directly as an evaluative standard.⁴³ Therefore, the lack of evaluation standards for the intervention outcome of the Yang-deficiency constitution is also an important issue limiting the development of clinical research.

3.2. Research countermeasures and prospects of Yang-deficiency constitution

In view of the above-mentioned problems and limitations, the future scientific research development direction of the Yang-deficiency constitution should focus on the following aspects.

3.2.1. Focus on developing objective constitution identification techniques with big data support

Accurate identification of the constitution types is a prerequisite for clinical research. Given the shortcomings of Yang-deficiency constitution identification techniques, literature research, expert consultation, and epidemiological surveys should be performed to further revise and simplify the Constitution in Chinese Medicine Questionnaire,⁴⁴ upgrade the standard of Classification and Determination of Constitution in TCM to a national standard, and officially declare it as an international standard to expand the scope of application. Furthermore, modern diagnostic technologies may be appropriately applied to establish databases on the biological characteristics of the constitution types for research and development of objective constitution identification technologies. These two aspects may also be combined to facilitate accurate identification.

3.2.2. Using modern language to interpret the microscopic characteristics of constitutions in-depth and establishing animal models of the Yang-deficiency constitution

The chronological research model of "Yang-deficiency constitution in the prevention stage-Yang-deficiency constitution in the diseased stage-Yang-deficiency constitution in the diseased stage after intervention" may be selected to further verify the molecular mechanism of the association between constitution and potentially related diseases through clinical trials. Furthermore, in order to break through the limitation of the microscopic research on the physiological characteristics of Yang-deficiency constitution, efforts also should be made to break the research bottleneck — the construction of animal models of the Yang-deficiency constitution. The current animal models used in TCM research mostly focus on TCM symptom patterns and diseases. How to simulate humans and make animals appear as Yang-deficiency constitution in their natural state, which has the phenotypic characteristics of Yang-deficiency constitution and has the genetic characteristics that can be passed down for generations, is a difficult point in research. Therefore, we proposed a concept that could be used to construct an animal model of spontaneous Yang-deficiency constitution by the method of "screening- reinforcing traits-mating and passing of the same constitution", which combines multiple factors of nature and nurture.^45,46 This can be used as a platform to conduct further microscopic studies on Yang-deficiency constitution.

3.2.3. Conducting constitution-disease correlation research with a high level of evidence and establishing an evaluation system for the effectiveness of constitution conditioning interventions

A number of major diseases could be selected from the known spectrum of Yang-deficiency-related diseases, and prospective cohort studies could be conducted to observe and analyze the associations between Yang-deficiency constitution and the occurrence, development, and prognosis of these diseases. Further clinical studies should be conducted on the prevention and treatment of diseases by regulating the Yang-deficiency constitution, and the outcomes of the constitution conditioning interventions should be evaluated, thus providing reliable, evidence-based research to support disease prevention in TCM. Meanwhile, it is necessary to establish an evaluation system for the intervention outcome of the Yang-deficiency constitution and improve clinical study design. Based on literature research, experimental research, and clinical research, we tried to include the subjective indexes that reflect the characteristics of the Yang-deficiency constitution, as well as the objective indexes in modern medicine that reflect the effect of Yang-deficiency constitution intervention and have preliminarily constructed an evaluation system for the intervention outcome of Yang-deficiency constitution conditioning, which can provide a reference for further research.⁴⁷

4. CONCLUSIONS

The research on the nine TCM constitutions represented by the Yang-deficiency constitution should be an open system, and future research could complement and perfect this system. At present, the biggest problem related to the TCM constitution is the lack of research covering the full chain of nine constitutions. For example, the research on Yang-deficiency constitution is only limited to the Yang-deficiency constitution itself or the comparison between Yang-deficiency constitution and neutral constitution or a few types of constitutions; the research results are inevitably biased, and it is still impossible to determine with absolute certainty that the different genes, microbiota, and metabolic markers obtained from the research are unique to Yang-deficiency constitution. Due to the lack of exclusive verification covering all constitutions, the current research still cannot explain the problem, which should be further addressed by future TCM constitution research.