Why NIDCD Supports Taste and Smell Research
The chemical senses—more commonly known as taste, smell, and chemesthesis (the “feel” of a chemical; chemically provoked irritation)—enable us to use chemical signals to communicate with the environment and each other. For people, memories of taste and smell experiences are vivid and long lasting, and play an important role in our enjoyment of life. The chemical senses accomplish three major purposes:
- Nutrition: Seeking out safe and nourishing food.
- Protection: Helping us to avoid spoiled food and toxic chemicals.
- Communication: Conveying important information to others.
Specialized cells in the human oral cavity can detect at least five basic taste qualities: sweet, sour, bitter, salty, and savory (umami). Taste cells may also respond to components of fat, to calcium, and perhaps to other chemical substances found in foods and beverages. Together with the nose and oral cavity, the tongue also plays a role in chemesthesis, a multimodal chemical sensitivity whose burning sensations signal the presence of chemical irritants such as capsaicin in hot peppers and toxic chemicals in the air.
Olfactory sensory neurons in the nose can detect a wide array of odors and olfaction (smell) plays an important role in the perception of food flavor as well. In 1991, Linda Buck and Richard Axel described a very large family of about 1,000 mouse genes that give rise to an equivalent number of olfactory receptor types.48 These receptors are located on olfactory sensory neurons that occupy a small area in the upper part of the nasal epithelium. Drs. Buck and Axel received the 2004 Nobel Prize in Physiology or Medicine for this groundbreaking research, which established a foundation for understanding how odorant molecules interact with their odor receptors.
The prevalence of an impaired sense of smell increases with age, and is generally more prevalent in men than in women. From ages 53-59, approximately four percent of women and nine percent of men demonstrated an impaired sense of smell. By ages 70-79, the incidence is nearly 21 percent in women and 41 percent in men, and is even higher in people ages 80-97: approximately 59 percent in women and 70 percent in men.49
Evidence of taste and smell disorders in association with other health problems is increasing. People with early stage Alzheimer’s disease and idiopathic (not genetic) Parkinson’s disease report a reduced sense of smell, as do people with polycystic kidney disease (PKD). In the United States, about 600,000 people have PKD.50 Scientists have identified a link between gestational diabetes and an altered preference for sweet foods. Based on an international, multicenter study, gestational diabetes may affect as many as 18 percent of pregnancies.51
The chemical senses are important for regulating food preferences and intake. They evolved to help humans and other animals survive in environments in which required nutrients were scarce and many plants contained poisonous, bitter compounds. Consequently, we seek out sweet, fatty foods and tend to reject the bitterness that characterizes many nutritious vegetables. Although this behavior made sense as humans were evolving, an almost limitless availability of high-calorie foods today can cause the normal function of taste and smell to lead to overconsumption. Over two-thirds of American adults are overweight, and one-third are obese.52 Individuals who are overweight or obese are at risk of numerous serious conditions, including:
- type 2 diabetes
- coronary heart disease and stroke n metabolic syndrome
- certain types of cancer
- sleep apnea
- gallbladder disease
- fatty liver disease
- pregnancy complications53
People with smell disorders often have problems appreciating the smell of foods, and claim that food is less enjoyable. They may change their eating habits. Some may eat too little and lose weight, while others may eat too much and gain weight. In either case, there may be a long-term impact on overall health. Loss of the sense of smell may also cause a person to add too much sugar or salt to make food taste better. This can be a problem for people with certain medical conditions such as diabetes or high blood pressure. In severe cases, loss of smell can lead to depression.
Humans seek out their preferred flavors in foods. Flavor involves interactions between the sensors that detect taste, smell, and chemesthesis in our foods and the parts of the brain that interpret, remember, or think about them. Flavor plays an important role in determining whether someone accepts a particular food, and how much of it they choose to eat.54 Scientists studying the chemical senses are interested in learning more about the molecular and developmental bases for how flavors influence food intake and overall health.
Overconsumption of salt has become an area of particular concern due to the high levels of salt found in the processed foods that comprise the typical modern diet. Historical evidence suggests that human beings have consumed more salt than
is physiologically necessary for a long time.55 Scientists are interested in learning whether there is another undetermined reason for this high salt intake. Too much salt raises blood pressure, and high blood pressure56 is related to numerous health conditions, including heart disease, kidney failure, and stroke.56
Scientists are interested in learning more about how the body detects and responds to salt, fats, and other food characteristics that humans seek out. Data gained from these studies can help us determine new strategies to control overconsumption and improve health without reducing our enjoyment of food. Ongoing research is studying the structure and function of discrete taste, smell, and chemesthetic receptors, as well as their targets within the brain.
The chemical senses evolved to help us avoid environmental dangers. Bitter tastes warn of potential toxins. Odors associated with spoiled food, toxic volatiles, and dangerous organisms protect us against ingesting or contacting dangerous substances. Odors can even be used to label certain dangerous substances, such as the addition of smelly sulfur compounds to natural gas, which otherwise has no detectable smell. Chemesthesis primarily serves a defensive function, triggering a coughing or gagging reaction that allows us to avoid chemical irritants that cause tissue damage. Loss of chemesthesis results in the inability to detect toxic chemicals in our environment, possibly leading to increased exposure and greater risk of serious health effects. This loss of detection ability persists in people involved in the early rescue, recovery, demolition, or cleanup efforts after the collapse of the World Trade Center towers.57 Cancer treatments such as radiation and chemotherapy may also result in taste and smell loss.
Many animals, including mammals, detect chemical cues (some of which are called pheromones) given off by animals of the same species. These chemicals communicate a variety of messages, including fertility, social rank, health status, and individual identity. Pheromones can also inhibit or induce sexual maturation or mark territory via urination or spraying. Since so many animals use pheromones to communicate information through chemical signals, it seems reasonable to propose that humans do the same. However, the study of chemical communication and pheromones in humans is fraught with controversy. Scientists do not yet agree whether and how humans may use pheromones to communicate. However, other types of odors also affect the way humans interact. For example, people with smell loss may exhibit poor hygiene because they cannot detect their own body odor, thus affecting their normal interactions with others.
The cells that detect chemical signals show a remarkable capacity for regeneration. Their locations (in the nose, on the tongue, in the oral cavity) make them susceptible to damage from the environment, so regeneration is required if these cells are to continue to function throughout life. Scientists are interested in learning what enables these tissues to regrow and to re-establish the appropriate connections with the brain. What they learn could be applicable to other human systems and could lead to new treatments for not only taste and smell disorders but also for tissues damaged by stroke or neurodegenerative diseases.
The Taste and Smell Program
The NIDCD Taste and Smell Program supports the study of the chemical senses (taste and smell) to enhance our understanding of how individuals communicate with their environment and how chemosensory disorders can be identified and treated. NIDCD-supported research on molecular and cellular biology, animal models, biophysics, and biochemistry of the olfactory and gustatory systems is paving the way for improved diagnosis, prevention, and treatment of chemosensory disorders.