The New Gastronome
The Taste for Health
The Role Our Senses Play in What We Eat
by Gabriella Morini
by Gabriella Morini
Chemoreception: Taste, Smell and Chemesthesis
The senses allow us to pick up signals, then transmit them to the central nervous system where they’re interpreted. In simple words, they provide data for perception. Two of our senses respond to chemical compounds: smell and taste, and they are fundamental for identifying and driving us to the consumption of nutrients and avoiding the ingestion of toxic compounds. The sense of smell allows the identification of volatile compounds, so it is distal, while taste requires the contact in the oral cavity with what we have “to analyse”.
Chemoreception involves several phases: first is the molecular recognition, the interaction between a specific receptor and a substance able to bind to it; then, the generation of a signal transmitted to the brain, and (third), its translation into hedonic terms. And the hedonic answer “I like/I do not like it” is extremely important as it determines whether a certain food is swallowed or not.
If terms to rationalize various tastes are already found in ancient philosophies, the identification of specific receptors is much more recent (around the year 2000). There are five so-called fundamental tastes: sweet, umami, salty, bitter and sour, to which the fat taste is on its way to being added (following the identification of receptors that are able to bind long chain fatty acids).
Also, chemesthetic sensations contribute to chemoreception: hotness, pungency and coolness are chemically induced sensations, but they involve receptors that can be activated, both by temperature (physical stimulus) and by specific substances contained in some foods. Chemesthesis can therefore be defined as the chemical activation of temperature sensing sensors, which indicate a non-existent increase (hotness or pungent) or decrease (cooling) of temperature.
The Code of Taste
The code we developed during our long and slow evolution, millions of years ago, characterized by the scarcity of food, and therefore, by the difficulty in finding nutrients is quite simple: sweet substances are generally appreciated and include carbohydrates, an important source of energy; amino acids, some of which have an umami taste (which is very pleasant to humans), constitute proteins and are fundamental for our metabolism because, in addition to making calories, they are the only source of nitrogen.
The appreciation of the salty taste has been developed by mammals, during their evolution far from the sea, to maintain the concentration of the sodium ion (rare in nature) at acceptable levels. Fats are an important source of calories, bringing about twice the calories/g of other macronutrients, as well as being the means to introduce essential fat-soluble nutrients, such as some vitamins. The sour taste, which can be a sign of unripe fruits and spoiled food due to uncontrolled fermentations, can be tolerated and appreciated only to a certain extent.
Even bitter substances can be accepted, but only in low concentrations and must never to given to babies; most of them are produced by plants, which have developed the strategy of accumulating bitter or irritating secondary metabolites (such as polyphenols, flavonoids, isoflavones, terpenes, glucosinolates, etc.) to defend themselves against herbivores and pathogens. This does not mean that what is bitter is always toxic, and therefore, totally rejected. On the contrary, many of these bitter or chemesthetic secondary metabolites, once ingested, have positive effects on health.
All chemoreceptors allow the recognition of a compound without it entering the cell, so the substances present in food, once identified and if swallowed, continue their path in the digestive tract.
“There are different levels of complexity to prove the relevance of taste (including taste active compounds -tastants- and taste receptors) on health.”
Not Only in the Mouth
Until a few years ago, taste sensors were known (and imagined) only in the buccal cavity. Recently, gustatory chemoreceptors (especially sweet, bitter and umami receptors) have been identified in other locations of the body, including the gastrointestinal tract (GI) and the respiratory systems; so, chemoreception has to be considered a broad chemosensory system which recognizes exogenous (food) and endogenous agonists.
In the GI, the activation and modulation of taste receptors, by the same compounds that have taste present in food (and this is very important!), have very different functions ranging from absorption to the metabolization of nutrients through the release of satiety hormones, the modulation of glucose transportation, gastric emptying and gut motility regulation.
Activation of airways, bitter receptors by compounds produced by pathogenic bacteria, causes mechanical and chemical defenses against pathogens, contributing to natural immune response to infections.
Taste & Heath
To study the health effects of food and diet is a complex operation, because food is a mixture of compounds and it is difficult to identify the several targets and mechanisms of action of individual bioactive molecules, which often only act in very low concentrations and/or thanks to synergistic effects.
There are different levels of complexity to prove the relevance of taste (including taste active compounds -tastants- and taste receptors) on health. The simplest is that of taste as a tool for food choice, another is that of tasting compounds with pharmacological activity. One more is the modulation of taste receptors in the gut as a new therapeutic approach to regulate intestinal absorption and glucose metabolism. Therefore, also the study of the variability of taste receptors between individuals becomes relevant, as it can be one of the causes of the different responses to similar diets.
One of the more intriguing frontiers of bioactive compounds in food is the action they may exert as gene modulators (epigenetic effect). For foods such as garlic, onion, broccoli, many spices – but the list is long and growing – it is proven that they are the same compounds that give bitter taste and/or chemesthetic sensations to have beneficial activities in our body, also via epigenetic mechanisms.
Taste: a Form of Intelligence
There is no doubt that the ability to perceive the different tastes has been shaped by evolution. Chimpanzees, living organisms that are phylogenetically and genetically closer to humans, do not seem to particularly appreciate the umami compounds, which are not abundant in their diet since they are produced mainly after long cooking and fermentation, not performed by chimps.
Hence, the interpretation that man would have developed a strong preference for umami tasting compounds like monosodium glutamate and ribonucleotides as indicators of proteins are easy to digest, as well as markers of fermentations that provide probiotic bacteria with positive effects on health.
Furthermore, we are able to detect bitter substances at concentrations in the micromolar range, a thousand times smaller than those to which we detect the sweet taste of sugars (millimolar concentrations): the system that allows the perception of different tastes have evolved in such a way to respond to very low levels of potentially toxic stimuli, but to consistent doses of high-energy molecules.
The gustatory stimulus generated in the mouth is then transmitted through the nervous tissue to the brain, which also receives information about body temperature, the concentration of substances in the blood and, from the digestive system, the quantity of food ingested and the products of its transformation. It is important to note that to define how good or bad a food is, what you like or dislike and therefore what you decide to eat or not is extremely important to define nutritional status, and therefore health, but neither the sensors nor the brain are enough.
It depends on the processing of information, resulting from the previous ingestion of a certain food, that leads to two simple results: wellness or malaise, which we fix in our memory; nothing else than a learning process! So we can say that it is a form of intelligence that altogether makes sense of taste.
“Food allows us to fulfill the vital function of nutrition, but it also represents the deepest connection we have with the outside world and with the community in which we live.”
From Intelligence to Wisdom Through Cuisine
Wanting to do a simple experiment to understand what a man’s gustative baggage is, we can observe newborns and their facial expressions when they are offered different stimuli, to conclude that there is a general innate preference for sweet, umami and fat while bitter and sour are unpleasant. All newborns respond in this way, regardless of their ethnicity, because our biological evolution is in large part shared by all humans. The innate responses to the taste stimuli of newborns do not differ appreciably even from those of other non-human primates.
From birth, however, innate preferences are molded by culture through group and individual experiences, strongly dependent on the natural and social environment. And food, both in the sense of what we eat, its combination and preparation, is the final result of this puzzle of stimuli and responses. Food allows us to fulfill the vital function of nutrition, but it also represents the deepest connection we have with the outside world and with the community in which we live. This is why his search and preparation are the most important connections between biology and culture. In a certain environment, only a few potential sources of nourishment can grow.
The gastronomic preparations and the foods we call traditional owe their particularity both to the combination of the ingredients, but above all to the use of plants, non-domesticated plants, aromatic herbs, spices that are typical of a particular environment due to the geographical position and to the labor of those who have lived there. These are the elements that characterize the different sensory profiles that make them unique, and in some cases, appreciated and tolerated (often, also in a physiological sense) only by those used to eating them.
Cuisine, as a process of cultural assimilation following body assimilation, is a way to incorporate experience and awareness in procedures and from this point of view, cuisine can be considered a form of wisdom, the result of a long and complex bio-cultural process. Even more: cuisine is the bio-cultural process that differentiates us from all other living beings, but also among humans.
Healthiness, Biodiversity and Pleasure: The Mismatch
The panorama drastically changed a few decades ago when needs, availability and food cultures didn’t have a concerted adjustment. The increase in the availability of industrial food, very dense in terms of macronutrients (calories) but poor in bioactive ones, together with the speed with which these changes have occurred, are certainly contributing to the epidemic of food-related diseases. Human bodies have evolved to tolerate long periods of food scarcity, but they are not able to handle its excesses because it is a new scenario.
That’s why it’s very easy to put on weight while losing it is so difficult, being one of the least physiological phenomena in nature associated to severe illnesses or prolonged famines. Food industrialization and globalization have caused a reduction both of the consumed species and of differences in their use, leading to a shrinkage of the type and quantity of bioactive compounds we introduce in our bodies.
And taste, a tool developed by evolutionary mechanisms that played such an important role in our adaptation and evolution, is now the one responsible for our mismatch, continuing to attract us to what we needed in times of scarcity.
“This knowledge gives back value to the taste of food and to what is connected to it that goes well beyond gastronomic pleasure…”
Taste Education and Food Culture as Buffers to Changes in the Diet
This is why taste education is one of the tools that should be used to counter obesity, the main cause of many, if not all, food related diseases. And it does not mean we have to become gourmet: to educate taste is to know what is the function of the sense of taste and how its language works; that the compounds that have a positive effect in our body could be identified in a culinary preparation through their taste; that we have to seek bitter and chemesthetic sensations to find healthiness. To know that different tastes or varieties of fruits or vegetables correspond to different bioactive compounds; that is why biodiversity is a value for our health.
This knowledge gives back value to the taste of food and to what is connected to it that goes well beyond gastronomic pleasure: tastants can have a pharmacological relevance, and therefore, of great interest is their identification and interaction with taste receptors whose variability among individuals can play a role; it may contribute to understand the relationship between us and the microbiota, making chemoreception studies part of one of the most rapidly developing areas of science: molecular nutrition.
But it also brings us to understand the value of biodiversity, seasonality and of all the factors that have brought into practice in agriculture, transformation, preparation and consumption–that defined cuisines and traditional diets. In other words, to give value to food and not just to eating.