Introduction

Ageusia is the condition where someone experiences the complete loss of their sense of taste.¹ The causes of ageusia vary. Ageusia can arise from health conditions affecting the taste buds or nerves, lifestyle-related factors, or even medications. The sense of taste has an important role in nutrition, appetite, and quality of life, so it is understandable that ageusia can negatively impact these aspects of one’s health. 

Neurodegenerative disorders are a group of chronic health conditions that cause damage to the nervous system. These conditions involve degeneration of the brain and neurons, which are the functional units (specialised cells) that make up the nervous system and send signals throughout the body. Symptoms of neurodegenerative disorders typically manifest later in life. Examples of neurodegenerative disorders are Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease, and amyotrophic lateral sclerosis (ALS).

Figure 1. Illustration of a healthy brain in comparison to a brain affected by Alzheimer’s disease. National Institutes of Health. Wikimedia Commons [Internet]. Available from: Wikimedia Commons

There is emerging evidence linking sensory loss or decline with neurodegenerative disorders.^2-5 While ageusia itself is rare, the presence of gustatory (taste) issues may be an indicator of certain neurodegenerative conditions.^1,4,5 This may impact the diagnosis of neurodegenerative disorders and subsequent disease management.⁶ In this article, we will explore whether ageusia could serve as an early indicator of neurodegeneration.

Ageusia

Physiology of taste

Taste (aka gustation) is one of the five basic senses alongside hearing, vision, smell, and touch. It is the ability to sense different flavours in food and beverages and is distinguished by the taste buds on the tongue, mouth, throat, and other areas. The five basic tastes we can sense are sweet, salty, sour, bitter, and savoury (aka umami).⁷

Cranial nerves VII, IX, and X are the nerves involved with taste. They send information between the brain and areas of your body that sense the varying flavours and coordinate how your body will react.⁸ The gustatory cortex is the area of the brain that allows us to perceive taste. The sense of taste is also tied to the sense of smell, or olfaction.^6,9

Causes of ageusia

The cause of ageusia can be localised damage to an area of the body that receives taste stimuli or to the central nervous system, which includes damage to the brain and nerves that coordinate your response to taste. The following have been reported to cause ageusia:

• Brain or nerve injuries, particularly those affecting the cranial nerves linked to taste
• Neurological conditions such as Alzheimer’s disease and Parkinson’s disease 
• Having cancer in the head or neck and undergoing radiation therapy for it
• Viral infections like COVID-19
• Zinc deficiency
• Crohn’s disease
• Sjögren’s syndrome
• Hypothyroidism
• Diabetes mellitus
• Medications¹

Medications that have caused ageusia as a side effect include: antibiotics, chemotherapy drugs, antivirals, antifungals, antihistamines, thyroid medications, anti-Parkinsonism drugs, cardiovascular drugs, tranquilisers, and antipsychotics.¹

Ageusia in neurodegenerative disorders

Ageusia and changes in taste are rare compared to sensory disorders related to smell, hearing, or vision.¹⁰ Loss or decline of taste sensitivity is common in ageing, though elderly people with declining taste are usually not aware they have lost their sense of taste. Additionally, sometimes a person will perceive a loss of taste when they actually have problems with their sense of smell, since taste and smell are linked in perceiving flavours of food and drinks.⁶ 

There has been growing evidence that loss of taste is an early indicator of certain neurodegenerative disorders, such as AD.^3,11,12 Early diagnosis for neurodegenerative disorders is important because treatments can slow the progression of these conditions if the disease is detected in its early stages.

Ageusia in Alzheimer's disease 

Complete ageusia is rare in people with AD who experience a declining sense of taste, but possible. People with AD can have problems eating and have a lower sensitivity to certain flavours.³ For example, some AD patients may not recognise the umami flavour.¹³ 

One possible explanation for the rarity of ageusia in AD patients is that the loss of taste in people with dementia happens gradually over a long period of time, so a decline in taste may not be noticeable. Secondly, loss of taste is less likely because three cranial nerves relay sensory information from the tongue and mouth to the brain; therefore, all three need to be damaged for complete ageusia.¹³

A decline in one’s sense of taste can impact their appetite and nutrition. One study found that people with early AD had less sensitivity to taste but did not lose the ability to taste.³ Additionally, some people with AD showed a preference for sweet foods.³ Care for AD patients with a decline in taste can be improved by increasing the taste of meals while controlling the intake of calories and minerals.³

Biomarkers (naturally occurring measurements to test for disease) for early AD diagnosis are needed to improve the detection of AD in the ageing population. Some studies suggest that a decline in taste could be a potential biomarker for detecting early AD.^14,15 There are already at-home smell tests for detecting early AD, and research is underway to develop a similar taste test to detect early AD.

Ageusia in Parkinson’s disease

PD is a disorder of the central nervous system that affects movement. Both ageusia and anosmia, the loss of smell, have been documented in people with PD.⁴ People with PD can experience problems with tasting all 5 basic flavours (sweet, sour, salty, bitter, and umami), but most are not aware they have these issues.¹⁶ Those with PD report having a preference for foods with sweet, salty, and umami tastes.¹⁶

The early signs of PD are often motor symptoms. Anosmia is an early non-motor indicator of PD, while ageusia in PD is less studied but can impact the quality of life of people who have both conditions.⁴ Problems in taste can start early in PD progression, but are more frequent in advanced stages of the disease.^16,17

Ageusia in multiple sclerosis 

Multiple sclerosis (MS) is a chronic neurodegenerative disease characterised by the loss of myelin, which is a fatty protein that sheaths neurons to facilitate the movement of electrical signals around the brain and body. Ageusia is a rare symptom of MS, but it can occur due to demyelination in areas of the brain that control the sense of taste.^18,19 One MS patient who had ageusia also had hyposmia, dizziness, and tinnitus as symptoms.¹⁸ 

Figure 2. Illustration of a normal nerve cell compared to a nerve cell with multiple sclerosis. Wikimedia Commons [Internet]. Available from: Wikimedia Commons

Ageusia in amyotrophic lateral sclerosis 

ALS or Lou Gehrig’s disease is a neurodegenerative disease affecting the nerve cells in the brain and spinal cord. ALS leads to problems in controlling voluntary muscle movements. Change in the senses is not usually a symptom of ALS, but some people with ALS have reported a decline in taste.²⁰ There are a limited number of studies about ageusia in ALS, and there is not enough consistent evidence to show that neural pathways involving taste are major contributors to ALS progression.²¹

Clinical impact of ageusia

Taste disorders in elderly people are commonly overlooked because loss of taste is not thought to be life-threatening.⁶ However, sensory loss can pose problems for the safety and quality of life of an individual. Loss of taste can prevent you from tasting harmful substances in food or beverages. Ageusia can contribute to a lower appetite, weight loss, malnutrition, and lowered immunity, resulting in a lower quality of life overall.⁶ Ageusia and loss of other senses can lead to anxiety and depression in older people.⁶ Many ageusia patients develop depression because of their concern about their condition.¹

Nutrition

Problems with nutrition can occur in elderly people, who are more likely to have a neurodegenerative condition. Because sensory decline or loss can occur as we age, it is important to consider the role of taste in nutritional health. An older person needs a higher concentration of salt in their food in order to detect the taste of salt. An elderly person with a diet higher in salt and sugar is more likely to experience adverse health conditions.⁶ 

Future research

Screening and early detection: Biomarker potential

Anosmia, the loss of smell, is more common in neurodegenerative disorders than ageusia. Overall, while there are patients with various neurodegenerative disorders who report ageusia or decline of their sense of taste, loss of taste in the early stages of certain neurodegenerative conditions is not as extensively studied as the loss of other senses, particularly smell. 

While there is emerging research on the decline in taste as a biomarker for Alzheimer’s disease, more research is needed about taste as a possible early indicator of AD and other neurodegenerative conditions.

Challenges to ageusia as a biomarker 

There are challenges to investigating ageusia further as a biomarker for the early detection of neurodegenerative diseases due to:

The subjective nature of taste loss 

 Someone’s declining sense of taste may not be noticeable to them. The decline of taste can happen over a long period of time, so an individual may not flag their loss of taste as a major concern. Issues with tasting one specific taste may also occur, rather than a total loss of taste.¹

Overlap with ageing and other conditions 

As people get older, their sense of taste can decline naturally without neurodegenerative effects. Additionally, ageusia may arise from other conditions unrelated to neurodegenerative diseases. It may be difficult to find a direct link between ageusia and the early stages of neurodegenerative conditions if they exist due to comorbidities and other lifestyle factors.¹

Summary

Ageusia is the complete loss of taste. Ageusia itself is a rare condition, but changes or loss of sensations, including taste, are common symptoms of neurodegenerative disorders. There have been reports of ageusia in people with Alzheimer’s disease, Parkinson’s disease, multiple sclerosis, and amyotrophic lateral sclerosis. However, ageusia in these conditions is less common than anosmia, the loss of smell, and there are fewer clinical studies focusing on ageusia in the context of ageing and neurodegenerative conditions. Ageusia’s impact on nutritional health and quality of life can be overlooked and understudied.

Ageusia has the potential as an early indicator of certain neurodegenerative disorders, but more research is needed. Loss of taste in neurodegenerative disorders is rare and less robustly studied compared to the loss of other senses. With improved technology and research, it may be possible to integrate testing for taste decline into the diagnosis of early-stage Alzheimer’s disease in the future.