Abstract

The concept of cold and hot nature in Traditional Chinese Medicine is not only found in Chinese Materia Medica, but also in the Chinese folk diet. Each food has its property, ranging from cold to neutral to hot, which subsequently determines their effects on individual bodies. To better understand the underlying scientific evidence supporting this classification, this paper reviews publications on the functional and molecular basis for each category and extends discussion on the relationship of the nature of foods to inflammatory responses. The paper also summarizes chemical components that have been found to associate with the nature of foods, including carbohydrate, protein, fat, water, mineral contents, and vitamins. In addition, the anti-inflammatory role of cold food and pro-inflammatory role of hot food have also been discussed. This review aims to provide a more scientific understanding on TCM philosophies and pose suggestions on elucidating the controversial classification of foods and their functional roles.

Key words: TCM; Cold Food; Hot Food; Inflammation

Introduction

In Chinese culture, there is a famous folk saying that goes, “Eating radish in winter and ginger in summer.” It is believed that such a dietetic practice will benefit one’s health and play a crucial role in preventive health care. The aphorism has its theoretical root from the theories of Traditional Chinese Medicine (TCM), an ancient Chinese medical practice that has been commonly used in China nowadays. Although labeled as pseudoscience in many western societies, TCM has been gradually accepted as a form of complementary and alternative medicine (CAM) all around the world in recent years (Xue et al., 2010). The philosophies of TCM assign four properties–hot, warm, cold, and cool–not only to Chinese Materia Medica (CMM), but also to each food, which accordingly determines their effects on individuals (Zhou & Xu, 2021). Among the four properties, hot and warm can be fundamentally categorize as the same nature but with different level, so do cold and cool (Long et al., 2011). As a result, three characteristics–cold, hot, and neutral–can be used to summarize the traditional four properties. Coupled with the basic duality concept of ZHENG (syndrome), TCM has been widely implemented in treating diseases, applying hot CMMs or food on cold ZHENG, and vice versa (Zhou & Xu, 2021). As described in Shang Han Lun (Discussions on Cold Damage), the stomach is cold in the month of May (summer month), and hot in November (winter month); thus, based on homology of medicine and food in TCM (Zhou & Xu, 2021), consuming hot-natured ginger in summer and cold-natured radish in winter is beneficial to the digestive system.

ZHENG has been serving as a fundamental TCM concept for ancient Chinese physicians to diagnose disease for thousands of years (Su et al., 2014). Traditionally, Zheng can also be categorized to hot and cold status, directing the use of CMMs on different symptoms. Hot syndromes, including any heat-caused disorder such as fever, diaphoresis, thirst, flushed face, dark urine, and constipation, can be remedied by CMMs with cold property; cold syndromes, including coldness, no sweat, tastelessness, clear urine, diarrhea, can be instead remedied by hot-natured CMMs (Zhou et al., 2019). Based on this principle, consuming foods with certain hot/cold properties–thus certain therapeutic effects­–in different conditions can also be practical in maintaining a healthy state of individuals (Xie et al., 2020). In essence, the ultimate goal in TCM is to maintain the Yin-Yang balance (homeostasis) in which Yin refers coldness and Yang refers heat in one’s body. A disequilibrium of either one may lead to disease. Hot food can be used to alleviate cold-deficient symptoms like cold limbs and waists, chills, and nocturia, while cold food can clear heat and purge fire (anti-inflammation), detoxify, and expel dryness (Xu, 2019). Generally, cold nature has depressive and inhibitory functions while hot nature is active and aggressive (Pan et al., 2020).

This review will focus on discussing the molecular basis of cold/hot nature foods, as well as their roles in eliciting or inhibiting inflammatory responses.

Classification of Cold/Hot Nature Food

In general, the energy level of food plays a significant role in determining the cold-hot nature, with higher energy constituents correlate with hot property and lower energy with the coldness (Han et al., 2020). Lee and Shen (2008) discussed in their paper that cold food tends to reduce body temperature while hot food have the tendency to increase it. Foods that are rich in carbohydrate and protein can provide a great amount of energy for individuals and can heat up the body through a process called thermogenesis. Westerterp (2004) specifically looked into how dietary contents can contribute to the generation of heat in one’s body via diet-induced thermogenesis and found that protein contribute the most to such a process. This finding further supports the theory that the hot foods, which contains more energy, increase body temperature.

Examining at a microscopic level, previous studies have investigated the relationship of foods’ nutritional contents to their natures. It is revealed that macromolecules like protein, carbohydrates, and lipids are closely associated with the hot nature of food, whereas water and dietary fiber contributes to the cold property. For instance, as listed in Table 2, oats, which is high in carbohydrate, is classified as a hot food; watermelon, which is succulent, is categorize as cold. In terms of mineral elements, titanium, chromium, molybdenum, zinc, calcium, phosphorous, potassium, and selenium are associated with hot nature, and iron, copper, cobalt, and nickel are associated with cold nature (LIU et al., 2012; Ormsby, 2021; Xie et al., 2020). Additionally, vitamins, essential nutrients obtained mostly from food, also showed close association with cold-hot nature. In the study of Xie et al. (2020), it is found that niacin, vitamin C, vitamin E, and vitamin A are related to hot nature, and folate, B6, and B12 are instead related to cold property. Yet, the degree to which these individual components contribute to each property requires further research as some of the findings are conflicting when it comes to labelling, which often leads to controversy. For example, as one of the best sources for zinc (associated with hot nature), oyster is nonetheless labelled as cold rather than hot, which is contrary to the findings above. Tomatoes, which are high in VC content (associated with hot nature) as well as water (associated with cold nature), are grouped under the cold category. Further investigation to clarify the guideline is thus necessary to eliminate the discrepancies. In terms of neutral property, although one study showed that vitamin A, vitamin E, and aromatic amino acid can contribute to the neutral property (Xu, 2019), no other relevant researches have been done to investigate and support the finding. As a result, the nutritional components that determine the neutrality of a given food remains unclear.

Physiological Effects of Cold and Hot Food

Anti-inflammatory Effect of Cold Food

In the concept of TCM, the effects of cold food are described as depressive and inhibitory. When such a vague interpretation is translated into a more modern term that can be accepted by the Western society, cold food is then associated with anti-oxidation as well as anti-inflammation. Serafini and Peluso (2016) studied the effect of selected fruits and vegetables on modulating the oxidative and inflammatory stress in humans. The study subjects include extracts of tomatoes, carrots, cranberries, and strawberries, which are all listed as cold food in Table 2. Their results revealed that some fruits and vegetables had an anti-oxidant and anti-inflammatory functions in humans (Serafini & Peluso, 2016). Zhu et al. (2018) reported in their review paper that Chinese pears (a cold fruit) and bitter melon (a cold vegetable) respectively have potent suppressive function on inflammation. In the first study of Li et al. (2012) on Chinese pears (a cold fruit), fractions of pears with ethyl acetate were observed to inhibit the formation of carrageenan-induced edema on the rat’s paw as well as, potentially, xylene-induced ear edema. Edema is a type of acute inflammation caused by excessive water accumulation in the cell tissues (Scallan et al., 2010), and the inhibitory function of the cold-natured pears on edema supports the TCM theory of cold property food. In general, fruits have anti-oxidative potential via indirect redox and anti-inflammatory mechanism (Burton-Freeman, 2010). In the other study of Hsu et al. (2012) on bitter melon, it was revealed that the ethyl acetate extract of wild bitter melon suppressed cytokine production in the Propionibacterium acnes-induced inflammation. A group of European researchers has studied the effects of food intake on inflammation among European children, and their results suggested that diets of high vegetable consumption have an inverse correlation with inflammation (Gonzalez-Gil et al., 2016). As vegetables are generally cold, their findings further support the findings that cold foods are generally anti-inflammatory.

Although there are currently no relevant papers explaining why all the dairy products are mostly cold foods, there are studies trying to investigate the anti-inflammatory effects of dairy products. A systematic review written by Hess et al. (2021) incorporates a variety of studies about the effects of dairy products intake on biomarkers of inflammation. These inflammatory markers include CRP (C-reactive protein), IL-1β, IL-6, IL-8, TNF-α, MCP (monocyte chemoattractant protein)-1, adiponectin, and leukocyte numbers (Hess et al., 2021). The research done by Bordoni et al. (2017) tested the effect of dairy products on 98 biomarkers and indicated that they have weak association with anti-inflammatory property. Another research by Burton et al. (2017) found that postprandial inflammatory markers are reduced by probiotic yogurt and acidified milk in healthy young men. However, although it seems like plentiful studies provide enough evidence for the anti-inflammatory property of dairy products, Hess et al. (2021), after analyzing a series of other studies in a systematic manner, concluded that additional researches are needed to further reinforce the concept of dairy foods’ anti-inflammatory characteristics as current studies are insufficient to do so.

Pro-inflammatory Effect of Hot Food

Various studies showed the oxidative and pro-inflammatory effects of hot. Some evidence has provided an insight that saturated fatty acids with even numbers (lauric, myristic, and palmitic) can increase inflammation (Calder, 2015). One way of how dietary fat, a nutritional constituent contributes significantly to hot type food, stimulates pro-inflammatory activity is promoting the translocation of gut microbiomes into the blood stream. Specifically, it is the endotoxin(LPS) within the microbiome that has potent ability to stimulate pro-inflammatory responses (Fritsche, 2015). Another way of pro-inflammatory reaction is increasing the production of PGE2, which was reported by Ormsby (2021) that a previous study has made such an observation on mice peritoneal macrophage. Ethyl acetate extracts of hot-natured logan and lichi have also been found to enhance PGE2 production in the LPS activated macrophage model and a murine macrophage cell line, respectively (Ormsby, 2021).

Foods that are rich in protein and fatty acids such as meats, that are categorized under the hot nature, are found to have high AGE (advanced glycation end product) concentration. AGEs are thought to stimulate pro-inflammatory cytokines to arouse systemic inflammation and oxidative stress in the body. It is interesting to note that AGE contents are lower in foods that are in the cold category (Ormsby, 2021; Uribarri et al., 2010), which may explain why cold foods are less likely to cause anti-inflammatory effects.

Conclusion

In this paper, the chemical composition used to classify cold and hot foods in TCM is provided. Hot food with the potential to increase body temperature and to elicit pro-inflammatory response is closely associated with macronutrients like protein, carbohydrate, and lipids. On the other hand, cold food that has reverse effect on body temperature and inflammatory response is found to be closely linked to water content and dietary fiber. Mineral and vitamin constituents of foods have also been revealed to have certain contribution to the properties of foods. In the processing of incorporating data from different research to compose Table 2, discrepancies in food labelling have been found, which shows the need to standardize the categorization in the field. The role that each food play in inflammation is investigated via discussing the association of their respective nutritional contents to inflammation. A comprehensive review on the anti-inflammatory potential of cold food is provided with substantial evidence, whereas limited studies are found on the pro-inflammatory function of hot food. Future research may focus on exploring how hot-contributing nutrients, from carbohydrate to VC, are associated with pro-inflammatory response. This review will provide a clear picture for understanding TCM theory on food property and put forward the ambiguity in the field that needs to be elucidated in the future.

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