Taste Disorders

Taste disorders often go without treatment since taste is not needed to live. However the sense of taste is very important. Without it, the quality of life can be affected. The most common true taste complaint is phantom taste perceptions. Additionally, testing may demonstrate a reduced ability to taste sweet, sour, bitter, salty, and umami, which is called hypogeusia. Some people can detect no tastes, called ageusia. True taste loss is rare; perceived loss usually reflects a smell loss, which is often confused with a taste loss.

In other disorders of the chemical senses, the system may misread and or distort an odor, a taste, or a flavor. Or a person may detect a foul taste from a substance that is normally pleasant tasting.

Overview

Taste belongs to our chemical sensing system, or the chemosenses. The complex process of tasting begins when tiny molecules released by the substances around us stimulate special cells in the nose, mouth, or throat. These special sensory cells transmit messages through nerves to the brain, where specific tastes are identified.

Gustatory or taste cells react to food and beverages. These surface cells in the mouth send taste information to their nerve fibers. The taste cells are clustered in the taste buds of the mouth, tongue, and throat. Many of the small bumps that can be seen on the tongue contain taste buds.

Another chemosensory mechanism, called the common chemical sense, contributes to appreciation of food flavor. In this system, thousands of nerve endings--especially on the moist surfaces of the eyes, nose, mouth, and throat--give rise to sensations like the sting of ammonia, the coolness of menthol, and the irritation of chili peppers.

We can commonly identify at least five different taste sensations: sweet, sour, bitter, salty, and umami (the taste elicited by glutamate, which is found in chicken broth, meat extracts, and some cheeses). In the mouth, these tastes, along with texture, temperature, and the sensations from the common chemical sense, combine with odors to produce a perception of flavor. It is flavor that lets us know whether we are eating a pear or an apple. Some people are surprised to learn that flavors are recognized mainly through the sense of smell. If you hold your nose while eating chocolate, for example, you will have trouble identifying the chocolate flavor--even though you can distinguish the food's sweetness or bitterness. That is because the distinguishing characteristic of chocolate, for example, what differentiates it from caramel, is sensed largely by its odor.

Causes

Some people are born with chemosensory disorders, but most develop them after an injury or illness. Upper respiratory infections are blamed for some chemosensory losses, and injury to the head can also cause taste problems.

Loss of taste can also be caused by exposure to certain chemicals such as insecticides and by some medicines. Taste disorders may result from oral health problems and some surgeries (e.g. third molar extraction and middle ear surgery). Many patients who receive radiation therapy for cancers of the head and neck develop chemosensory disorders.

Diagnosis

The extent of a chemosensory disorder can be determined by measuring the lowest concentration of a chemical that a person can detect or recognize. A patient may also be asked to compare the tastes of different chemicals or to note how the intensity of a taste grows when a chemical's concentration is increased.

Scientists have developed taste testing in which the patient responds to different chemical concentrations. This may involve a simple "sip, spit, and rinse" test, or chemicals may be applied directly to specific areas of the tongue.

A person with a taste disorder is challenged not only by quality-of-life issues, but also deprived of an early warning system that most of us take for granted. Taste helps us detect spoiled food or beverages and, for some, the presence of food to which we're allergic. Perhaps more serious, loss of the sense of taste can also lead to depression and a reduced desire to eat.

Abnormalities in chemosensory function may accompany and even signal the existence of several diseases or unhealthy conditions, including obesity, diabetes, hypertension, malnutrition, and some degenerative diseases of the nervous system such as Parkinson's disease, Alzheimer's disease, and Korsakoff's psychosis.

Treatment

If a certain medication is the cause of a taste disorder, stopping or changing the medicine may help eliminate the problem. Some patients, notably those with respiratory infections or allergies, regain their sense of taste when the illness resolves. Often the correction of a general medical problem can also correct the loss of taste. Occasionally, recovery of the chemosenses occurs spontaneously.

Clinical Trials

• Taste Disorder Trials
• Study of Taste Deficits
• Orexigenic Therapy With Delta-9-Tetrahydrocannabinol in Advanced Cancer Patients With Chemosensory Abnormalities - a Pilot Study

Research

The NIDCD supports basic and clinical investigations of chemosensory disorders at institutions across the Nation. Some of these studies are conducted at several chemosensory research centers, where scientists work together to unravel the secrets of taste disorders.

Some of the most recent research on our sense of taste focuses on identifying the key receptors in our taste cells and how they work in order to form a more complete understanding of the gustatory system, particularly how the protein mechanisms in G-protein-coupled receptors work. Advances in this area may have great practical uses, such as the creation of medicines and artificial food products that allow older adults with taste disorders to enjoy food again. Future research may examine how tastes change in both humans and animals. Some of this research will focus on adaptive taste changes over long periods in different animal species, while other research will examine why we accept or have an aversion to different tastes. Beyond this, scientists feel future gustatory research may also investigate how taste affects various processing activities in the brain. Specifically, how taste interacts with memory, influences hormonal feedback systems, and its role in the eating decisions and behavior.

Already, remarkable progress has been made in establishing the nature of changes that occur in taste senses with age. It is now known that age takes a much greater toll on smell than on taste. Also, taste cells (along with smell cells) are the only sensory cells that are regularly replaced throughout a person's life span--taste cells usually last about 10 days. Scientists are examining these phenomena which may provide ways to replace damaged sensory and nerve cells.

NIDCD's research program goals for chemosensory sciences include

• Promoting the regeneration of sensory and nerve cells
• Appreciating the effects of the environment (such as gasoline fumes, chemicals, and extremes of relative humidity and temperature) on taste.
• Preventing the effects of aging.
• Preventing infectious agents and toxins from reaching the brain through the olfactory nerve.
• Developing new diagnostic tests.
• Understanding associations between chemosensory disorders and altered food intake in aging as well as in various chronic illnesses.
• Improving treatment methods and rehabilitation strategies.

Other Resources

• Massachusetts Eye and Ear Infirmary

References

• Dodd J, Castellucci VF. Smell and taste: the chemical senses. In: Kandel ER, Schwartz JH, eds. Principles of Neural Science. NY:. Elsevier Science;1991:512-519
• Schiffman SS. Taste and smell losses in normal aging and disease. JAMA. Oct 22-29 1997;278(16):1357-62