Many people receiving cancer treatment report that food does not taste the same. These alterations are commonly reported as an absence of taste or smell, reduced or increased sensitivity, distortion of taste or smell, phantom tastes or odours and metallic sensations. Changes to taste and smell experienced as a result of cancer treatment can have adverse effects on morbidity, mortality and quality of life. This is due to associations with inadequate energy and nutrient intake, weight loss, malnutrition, reduced compliance with treatment regimens and altered food relationships. This document focuses on taste and smell changes, and also refers to other components of flavour which are intimately connected to a person's eating and drinking experience during cancer treatment.
Taste: is one of the five senses and refers only to a process that occurs in the oral cavity when food molecules interact with saliva to stimulate taste buds. Each taste bud hosts cells equipped with receptors and transduction mechanisms so that five basic taste qualities can be detected. These basic taste qualities are sweet, salty, sour, bitter and umami (savoury). Taste interacts with smell and touch to allow the experience of flavour.
Smell: the sense of smell is referred to as olfaction. The smell of food is experienced via both the mouth and the nostrils. Orthonasal olfaction is the process of active sniffing and is considered an external process. Retronasal olfaction is considered an internal process. It refers to the act of passive smelling during breathing or to referral of odours into the olfactory receptors from the mouth, as occurs during chewing and swallowing.
Touch: encompasses oral touch, chemesthesis and oral sensation. Oral touch is the perception arising in the mouth when food interacts with teeth, saliva and touch receptors. This includes dry mouth and oral astringency, which can alter the perceived texture of food. Chemesthesis or oral irritation is the chemical stimulation of nerve fibres in response to thermal or pain sensations such as menthol, chilli burn or carbonation. Oral sensations are perceived in the oral cavity. An example of a commonly reported oral sensation during chemotherapy treatment is a metallic 'taste'.
Flavour: is much broader than taste, although the words taste and flavour are often used interchangeably. Flavour includes the sense of taste but also encompasses other sensory inputs such as smell and touch, and is influenced by food hedonics.
Food hedonics: refers to the extent to which eating and drinking is pleasurable. It encompasses food liking and appetite. Food liking is the experience or anticipation of pleasure from eating and drinking. Food liking can be influenced by appetite, which is the sensation related to the maintenance of eating and a desire for certain food types. Food aversions can occur as a result of unpleasant eating and drinking experiences; this generally occurs at a time of illness. Commonly, in the cancer treatment context, learned food aversions are associated with nausea or anxiety linked to treatment or anticipation of treatment. Additionally food aversions can develop to previously favoured food. This may manifest as alterations in food preferences or a reduced desire for consuming food and drinks.
Distinguishing between factors affecting the eating and drinking experience
It is important to distinguish between factors affecting the eating and drinking experience because the clinical strategies employed to improve symptoms will depend on the exact problem experienced. For example, food may still taste the same (i.e. taste receptors functioning as expected) but that taste may no longer be enjoyed (change in food preferences or liking of a particular food). Regardless of which particular component of flavour is affected during cancer treatment, similar impacts may ensue. For example, adverse alterations in taste or touch perception (oral dryness) can both result in reduced food enjoyment and a subsequent change in oral intake, although the recommended management strategies differ.
All cancer treatment modalities have the potential to influence components of flavour. Specific studies have been conducted in radiation therapy, chemotherapy and blood and marrow transplantation, demonstrating various consequences.
Each type of cancer treatment may affect taste and smell through different mechanisms.rSensory causes of taste and smell changes during cancer treatment are described in Table 1. Because each of the five basic taste qualities is mediated by a single physiological pathway, and these pathways can be affected in different ways, some basic tastes might be affected by cancer treatment when others are not. For example, a reduced perception of saltiness could occur in the presence of a heightened perception of sweetness.
Table 1: Sensory causes of taste and smell changes*
||Cause and effect
Integrity of the tongue, oral cavity wall and nasopharyngeal region
- Xerostomia from high cell death results in compromised lubrication of oral tissues and impaired buffering capacity with increased risk of oral infections.
- This can lead to increased oral puckering, rough or dry mouthfeel.
- May contribute to food 'tasting like cardboard'.
- Sensitivity to by-products of oxidation of lipids in foods or of epithelial cells generating a metallic sensation.
- Radiation therapy
- Blood and marrow transplant
- Surgical removal of structures involved in taste and smell can interfere with function.
Saliva function and presence of taste-active compounds
- Salivary dysfunction (viscosity and flow rate) secondary to mode of action and anatomical targets of cancer treatment e.g. salivary glands in head and neck radiation therapy.
- Chemotherapy drugs providing metallic or bitter sensation.
- Radiation therapy
- Blood and marrow transplant
Damage to taste and smell transduction nerves:
- facial nerve (CNVII) chorda tympani branch from the anterior tongue
- glossopharyngeal (CNIX) from posterior tongue
- vagus (CNX) from taste receptors in pharyngeal space
- Surgical damage or chemotherapy-induced neurotoxicity includes damage to sensory neuronal cell bodies, transport pathway, mitochondrial operation, calcium regulation systems and axonal membrane ion channel.
- Abnormal chemical stimulation of nerve fibres that alter thermal or pain sensations such as menthol, chilli burn or carbonation.
- Radiation therapy
*Reproduced in part from Boltong and Campbell, 2013 (invited commentary)r
Up to 76% of people receiving cancer treatment report alterations to the way flavour (taste, smell and touch) is perceived.r
In people receiving cancer treatment:
- True taste dysfunction, reduced appetite and reduced liking of sweet foods have been shown to be cyclical in chemotherapy only.rr
- Adverse changes are at their greatest 4-6 days following administration of chemotherapy, gradually returning toward pre-chemotherapy levels across the duration of a chemotherapy cycle until the next infusion.r
- Symptoms usually resolve completely within 2 months after completion of chemotherapy.r
- Most pronounced is a decrease in the ability to identify the basic tastes salt, sour and umami.r
- The ability to identify sweet and bitter tastes is largely retained and in fact these taste qualities can be perceived as more intense during chemotherapy.r
Sensory and hedonic changes are associated with lower energy (kJ) intake as well as negative nutritional, emotional and social consequences at similar cyclical time points during and after chemotherapy treatment.r
Taste changes in people who have received surgery or radiation therapy for the treatment of head and neck cancer may be more enduring.r Although some patients show full recovery by 6-12 months after completion of radiation therapy, others show incomplete or no recovery several years after completing treatment.r
Contributing risk factors to changes in flavour perception may include:rrr
- some chemotherapy agents
- some hormonal agents
- blood and marrow transplant
- radiation therapy to the head and neck region
- surgery to the head and neck region
- malignancy in the nasopharynx
- oral mucositis
- poor oral hygiene or other oral problems
- drugs other than chemotherapeutic agents such as some antibiotics, analgesics, bisphosphonates, antihypertensives, bronchodilators, muscle relaxants, antidepressants and anticonvulsants.