Academic literature on the hot–cold system in Mexico: history
The hot–cold system has been studied mainly by two related scientific disciplines: anthropology and ethnobotany. As the system is a cultural trait, most academic publications can be expected to be anthropological, particularly from social and cultural anthropology. Given the proximity, it is also unsurprising that much early ethnographic research in Mexico was conducted by scientists from the United States, including the first works on the hot–cold system [47,48,49]. Various anthropologists [12, 13, 31,32,33, 35, 87, 88] became interested in the topic and studied it in detail, while others noted this cultural trait in their ethnographic studies [84, 89, 90], and somewhat later Mexican anthropologists also took up the subject [34, 78, 83].
Ethnobotanical research on the hot–cold system in Mexico took a different path. According to Gómez-Pompa , modern Mexican ethnobotany initiated in the late 1950s. Since then, a considerable part of the research has focused on the study of medicinal plants [51, 92], an area where the hot–cold classification system is highly notable. However, few studies addressed this phenomenon during the first three decades (1960–1989).
Nationality and training of the researchers may have caused the omission. During the early decades of Mexican ethnobotany, researchers and teachers were mainly biologists . Their knowledge is undoubtedly necessary for the identification of botanical specimens and vegetation analysis. However, the nature of their biological training, combined with their Mexican background, may have limited their perception of cultural aspects related to the medicinal flora, such as this popular classification.
Cultural anthropologists are trained to perceive distinctive social and cultural phenomena in their own culture and in others. Thanks to foreign and Mexican anthropologists, we now have detailed descriptions of the hot–cold system that allows us to understand its expressions, functions, and symbolism in various modern human groups in Mexico. This anthropological research set the stage for other researchers to study the same topic from a different perspective.
The decline in anthropological research on the hot–cold system since the 1990s was likely due to the perception that the topic had been exhausted or sufficiently studied, along with the emergence of new research areas. Ethnobotanical publications addressing the subject have increased since that decade, coinciding with the growing number of ethnobotanical texts on Mexico .
Academic literature on the hot–cold system in Mexico: main themes
The hot–cold system is prominent in two large domains: traditional medicine and food; the anthropological and ethnobotanical literature reflect this. Both domains are directly related to health, and drawing a clear boundary is difficult. However, to study them, they have to be separated, preferably with clear criteria.
We identified some themes that should be explored further, though there is incipient information. For example, are plants classified in the same way by all human groups? Are there biological characteristics of plants associated with their hot–cold properties? Are there cultural patterns of classification of the useful flora? Patterns of traditional knowledge by regions or linguistic families should be a particular focus in the future, especially for the medicinal flora. As the hot–cold system plays an underappreciated role in food preparation, composition and preferences, food is another promising subject.
The classification of human beings and landscape is not well documented, perhaps because the system is less obvious in these domains; this subject should be amenable to ethnobotanical research. The other understudied domains, such as colors, seasons, soil types, and everyday utensils, can probably be captured best through ethnographic studies.
Here, we separated the domains for analytical purposes. However, all domains form a complex framework to structure the world. Things or events that may seem unrelated have a real or symbolic relationship under the hot–cold classification system.
Finally, the hot–cold classification can have concrete applications for health professionals who deal with patients immersed in the system [94, 95]. Without adequate knowledge, they may propose treatments and dietary changes that may be rejected. For instance, women avoid consuming cold food and bathing during menstruation (a “hot” stage) for fear that the “coldness” of the food and water will damage their bodies, especially their wombs, which is believed to lead to sterility [12, 34]. For a menstruating patient, a health professional knowledgeable about these practices could then suggest foods of the appropriate property (generally “hot”) and to clean her body with as little water as possible until the end of her menstruation.
The distribution of the hot–cold system in Mexico
The analysis shows that the system is widespread among people of different linguistic groupings, including all Indo-American language families, as well as Mestizo and Afro-Mexican groups, at least in the domain of traditional medicine. The panorama presented here is more comprehensive than the results of a previous publication , largely as a result of the additional information obtained from the Indigenous Medicinal Flora of Mexico  and Traditional Medicine of Indigenous Peoples of Mexico .
The apparent absence of sources on the use of the hot–cold system in some groups may be due to lack of documentation rather than its absence. Indigenous groups without records generally live very close to other ethnic groups for which the phenomenon has been documented. The Matlatzincas of San Francisco Oxtotilpan, State of Mexico, live 35 km away from San José Villa de Allende, inhabited by Mazahuas that use the system. The microregion formed by Sayula, Oluta, and Texistepec Popoluca territories in southern Veracruz is only 40 km away from the territorial core of the Sierra Popolucas, and 45 km from the Nahua community of Pajapan, both of which were recorded in this study. Kanjobals, Acatecs, Quichés, and Tekos share their area with Chujs, Mams, Kaqchikels, Jakalteks, Mochos, and Tojolabals in the southeastern part of the state of Chiapas, near Guatemala, and the hot–cold system has been documented for the latter. Kanjobals, Acatecs, Awakateks, Quichés, Kekchis, and Ixils live in the states of Campeche and Quintana Roo near Mam populations that use the system. The Chontal Mayas occupy approximately one-third of the Tabasco state’s territory and are situated very close to the only community inhabited by Ayapanecs recorded in this study.
One example of the lack of documentation is the Quichés. We found no sources from Mexico for this group; however, several studies in Guatemala [96,97,98] confirm that they do indeed use the system. According to Felger and Moser , the Seri do not classify their plants or remedies as “hot” or “cold”. Indeed, neither of the two sources on this ethnic group [28, 29] contained records of the classification of medicinal plants. However, they do classify “kidney disease” as “hot,” and there are various related concepts and beliefs in their nosology [39, 40]. They seem to classify some ailments but not medicinal plants and other types of remedies. Some other ethnic groups do not explicitly classify their diseases as “hot” or “cold” despite having concepts related to the hot–cold system in their etiology or treatment [16, 34, 100]. More research is needed in these cases.
It is possible that the Lacandons are an exception, even though they live close to other groups that use the system. It was the only ethnic group for which the website Traditional Medicine of the Indigenous Peoples of Mexico  makes no reference on the hot–cold system, and none was found elsewhere. Lacandons believe their deities send them diseases for failing to comply with social norms or ritual duties; these deities may use illnesses to communicate with the sick person and express a desire [40, 101]. The absence of the system makes sense under these etiological explanations of illnesses.
According to Foster , non-Western medical systems mostly explain the origin of diseases based on two basic principles: personalistic and naturalistic. Personalistic systems attribute diseases to external agents, whether human (witches, sorcerers), nonhuman (ghosts, spirits, ancestors), or supernatural (deities and similar entities), often in response to human misbehavior. In contrast, naturalistic systems essentially conceive diseases as a result of bodily imbalance or an imbalance between the sick individual and their social or natural environment; that is, they are attributed to natural causes. Explanations of diseases under the hot–cold system belong to a naturalistic system. Foster  specified that both naturalistic and personalistic systems can coexist in a society, but one usually predominates. Lacandons appear to have a predominantly personalistic etiology, which could explain the apparent absence of the system.
There are indications that the system has varying levels of relevance in the traditional medicine of Mexico. The system is undoubtedly important for Mixtecs [62, 68, 71, 100], Chochos , Zapotecs [13, 15, 32], Otomis , Nahuas [35, 103, 104], and Zoques [36, 63]. In these cases, the hot–cold system is applied to both diseases and medicinal plants. In Yucatec Maya communities, the system is relevant for explaining their diseases, but it has less importance in their medicinal flora . In traditional Mixe medicine, the system has little relevance in both the etiology of diseases and medicinal plants [52, 53].
The documentation of the hot–cold system has concentrated on the Nahua, Yucatec Maya, Zapotec and Mixtec ethnic groups, the most numerous , widespread and studied peoples. However, there are some large geographical gaps: parts of western and northern Mexico, the Gulf coast and some regions of the Yucatán peninsula. The first three regions have relatively few indigenous peoples and therefore have been less interesting to anthropologists and ethnobotanists. However, the lack of information from some areas of the Yucatán Peninsula is striking, and we have no explanation.
This study provides a general overview of the use of the hot–cold system by different ethnic groups in Mexico. To interpret it correctly, some issues related to the ethnicity of the groups and the temporality of the sources must be considered.
First, language was used as a somewhat flexible ethnic marker in this study. For example, Mixe language speakers were considered Mixes, but self-identification was also considered. “Mestizo” essentially refers to individuals whose primary or sole language is Spanish, without self-identifying as indigenous. But in most cases, mestizos have indigenous ancestry, resulting in indigenous cultural influences on mestizo culture even in urban populations, as indicated in section B of the Additional file 1.
Second, our sources span approximately a century. Language and local culture today may differ from the original descriptions. However, it appears that despite sociocultural changes, certain characteristics, such as the hot–cold system, may persist. George Foster visited Tzintzuntzan repeatedly for over 3 decades (approximately from 1958 to 1992) and observed that, despite the passage of time, the hot–cold system remained in effect:
“…In Tzintzuntzan I find that, in spite of the general acceptance of modern biomedicine for most medical problems, an astonishingly high percentage of illness episodes, including most of those treated by physicians, is explained pos facto in terms of hot and cold experiences” .
Even in present-day large cities, people widely apply the hot–cold system, as is obvious even to a casual observer. Recent studies of patients treated in healthcare centers in major cities in Mexico have shown that they apply the hot–cold classification system to both diseases and medicinal plants [106, 107]. These studies do not specify the ethnicity of the patients, but presumably the majority are assimilated into urban lifestyles.
The hot–cold system is spread across an ethnic and cultural continuum, represented on one end by monolingual speakers of indigenous languages living in rural communities and on the other by urban individuals who use Spanish as their only or main language. Historian Alfredo López Austin aptly illustrated this matter:
“La herencia cultural no se limita a los descendientes indígenas de los mesoamericanos, pues en los más diversos segmentos de la sociedad mexicana puede encontrarse, entre muchas otras creencias y prácticas, la división de enfermedades, medicinas y alimentos separados en fríos y calientes. Y no son sólo pautas clasificatorias sino guías de acciones dirigidas a conservar o recuperar la salud” .
[“Cultural heritage is not limited to the indigenous descendants of Mesoamericans, as among the most diverse segments of Mexican society, one can find, among many other beliefs and practices, the division of illnesses, medicines, and foods into cold and hot. And these are not just classificatory guidelines, but also guiding principles for actions aimed at preserving or restoring health” .]
The current hot–cold system is present in ethnic groups outside the Mesoamerican cultural area. This raises two questions: how and when did it spread? We suggest the possibility that the syncretism of European and Native American ideas took place in the central Mesoamerican area and then spread northward. This would explain the presence of hot–cold concepts of illnesses of the Seris but not in their remedies, indicating that perhaps only a part of the system reached them. It would also explain the observations of Latorre and Latorre , who apparently noted that the hot–cold system is not originally part of Kickapoo culture: “The Kickapoos have adopted many of the Mexican beliefs concerned with heat and cold, whether in food, beverages, medicine, or the state of the body.” Future studies of the cultural evolution and recent history (last 500 years) of living and extinct ethnic groups in Mexico could resolve these questions.