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Taste Changes in Renal Failure
Karen Manley
Why do so many renal patients complain that food just does not taste the
same as it used to?
Issues with taste and gastro-intestinal problems in renal failure are common,
ranging from 31 to 81%. Taste and smell disturbances may relate to issues
presented with fluid imbalance, toxin accumulation and metabolic derangements
characteristic of kidney disease and this may be intensified by specific taste genetics
sensitive to increased saliva urea concentrations found in CKD. 1
Most patients will experience some gastric symptoms when GFR > 15 but can
range from minor to quiet severe Patients generally commence renal replacement
therapy when biochemistry cannot be controlled or when very symptomatic or a
combination of both.
The symptoms many renal failure suffer Includes
• Dry mouth
• A metallic taste in the mouth or ammonia breath
• Anorexia
• Nausea and dry retching
• Protein aversion
• Food cravings
Saliva critical to oral health and important in taste stimulation with 500ml to 1
litre is produced daily and mainly swallowed. Saliva is >90% water containing
electrolytes, proteins, enzymes and nitrogenous products and minerals. Compared
with serum, normal saliva is hypotonic, low in urea nitrogen and sodium and high in
potassium and phosphorus and has greater similarity to urine than blood. 2
In renal failure saliva flow rate can be altered especially if fluids are restricted.
There is an increased rate of oral cell death with changes in constituents with higher
urea, sodium, potassium, and phosphate and generally a lower calcium. High
sodium, potassium and urea levels in saliva fall post haemodialysis treatment to
levels similar to healthy controls. Saliva’s pH is also changed in renal disease as
urea is metabolised by bacterial urease to CO2 and ammonium which generally
alkalinises the saliva
Individual taste buds have multiple receptor cells in each bud which mediate
taste perception and any bud may contain receptors to identify each different taste.
Taste buds arise continuously from underlying basal cell layer and have a life span
of about 10days. Taste receptors are also scattered within the alimentary tract and
respiratory passageways.
Taste and flavour are not the same and not interchangable. Taste involves the
chemical stimulation of taste receptor cells in the oral cavity to perceive the primary
tastes – sweet, sour, salty, bitter and unami. Flavour includes the senses of taste,
smell and touch which gives greater complexity to the five basic tastes and
influences the hedonistic response.The sense of smell is impaired with aging while
taste is more robust. Any alteration to taste sensitivity influences the hedonic
response to those tastes.
Taste substances must pass through the salivary fluid layer to reach receptor
sites. Saliva electrolytes provide the ionic environment for taste signal transduction
to occur. Taste receptors are constantly exposed to low concentrations of
electrolytes including sodium, potassium, bicarbonate ions and adapt to the salivary
environment with an increase or decrease in sensitivity.
Taste is responsible for the detection of nutrients in food. Sodium ions elicit
salty taste receptors can warn of excessive dietary sodium. Sweet stimuli indicate
energy dense carbohydrate foods. Umami taste indicates protein rich foods. Sour
excess will signal spoiled foods. Bitter taste indicates the presence of toxins
Taste is a trait controlled by many genes. Most of the genetic taste traits
concern the bitter perception as it is particularly important in protecting against the
ingestion of naturally toxic substances. Individual tolerance varies more widely for
bitter than for any of the other basic tastes. Bitter perception occurs through the bitter
taste receptors encoded by the T2R genes which identify between 25 different
members of this gene family. These differences may allow a wide variety of chemical
shapes, sizes and functionalities to be bound by these receptors and be perceived
as bitter. Even among the tasters, these chemicals can only be detected if it is
dissolved in the tasters own saliva. Compounds such as amino acids, peptides,
esters, organic and inorganic salts contribute to the bitter tastes in products such as
coffee, tea, chocolate, saccharin, wine and some fruits and vegetables. 3
Super-tasters have an increased number of taste buds on their tongue which
intensifies all tastes.
The common genetic taste tests are:
• Thiourea
• Phenylthiocarbamide(PTC)
• Sodium benzoate
Thiourea is an organosulphur compound structurally similar to urea, but their
properties differ significantly. The ability to taste thiourea is dominantly inherited and
is related to taste bud density and genetic expression of the taste receptor gene.
People are classified as non-tasters, tasters or hyper-tasters. About 70% of people
can taste thiourea with approximately 30% finding it incredibly nauseating or bitter
and tasters will tend to avoid any bitter foods like dark-green leafy vegetables, coffee
or dark chocolate.
Phenylthiocarbamide (PTC) is an organosulphur thiourea containing a phenyl
ring. The ability to taste PTC mainly as bitter is also inherited. Thiourea is closely
related chemically to PTC, but the ability to taste either is inherited independently so
the taster and the non-taster groups for the two substances need not be the same.
Hyper-tasters of PTC tend to be more sensitive to spicy and sweet foods and find
fatty foods, broccoli and grapefruit juice less appealing. Sodium benzoate is a
common food additive tastes differently to different people and can be perceived as
salty, sour, bitter or sweet or maybe not tasted at all. 4
Other chemicals can also affect taste. Zinc depletion has been implicated in
hypogeusia but zinc salts alters smell and can cause taste loss especially for sweet
and increases oral astringency. Urea is a known bitter stimulus while potassium
causes a metallic taste in sensitive people. Salt taste is detected only when above
the sodium-chloride concentration in the saliva and enhances the sweet taste
Nausea typically accompanies toxin-induced illnesses. As most toxins taste
bitter, nausea and dry retching often results even before the ingestion of the toxin.
People who are the most sensitive to the bitter stimuli are more prone to motion
sickness and nausea in pregnancy. 5 The bitter taste also slows gastric emptying and
is strongly sensed by the glossopharyngeal and vagus nerves which innervate the
posterior oral cavity and the gastrointestinal tract respectively causing dry retching. It
has been shown that bitter taste stimulation, but not sweet, salty, or umami taste
induces nausea. 6
Suggestions to improve G.I. Symptoms
Anorexia
• avoid cooking smells especially meat
• eat little and often
• don’t miss meals
• eat the foods that are tolerated
• eat in relaxed environment and with others
• have cold foods
Taste changes
• rinse mouth out or clean teeth before meals
• suck mints or chew gum
• use herbs and spices to add flavour
Dry Mouth - Keep lips and mouth moist
• Use lip balm daily
• Rinse mouth, spit out the water - don’t swallow
• Use mouth spray (not alcohol based)
- Stimulate saliva production
• Add a squirt lemon drinking water
• Suck on frozen fruit eg grapes strawberries
• Suck on mints or sour lollies or chew gum
• Suck on re-usuable plastic ice-cubes
Nausea and dry retching
• Rinse mouth out with a mild salt solution or sodium bicarbonate in warm water
• Sip soda water or dry ginger ale
• Small regular meals
• Chew gum or mint lollies
• Nibble dry biscuits
• Avoid cooking smells
• Eat or drink before arising
Saliva flows from the acini into the collecting ducts where sodium is actively reabsorbed
while potassium and phosphate are secreted
1.
Boltong A, Campbell K 'Taste' changes: A problem for patients and their dietitians. Nutr and
Diet 2013;70:262-269
2.
Humphrey SP, Williamson RT. A review of saliva: normal composition, flow, and function. J
Prosthetic Dent. 2001;85(2):162-9.
3.
Bartoshuk LM. Psychophysical advances aid the study of genetic variation in taste. Appetite
2000;34(1):105
4.
Tepper BJ. 6-n-Propylthiouracil: a genetic marker for taste, with implications for food
preference and dietary habits. Am J Hum Genet. 1998;63:1271-1276
5.
Sipiora ML, Murtaugh MA, Gregoire MB, Duffy VB. Bitter taste perception and severe
vomiting in pregnancy Physiol. Behav.2000;69:259-267
6.
Peyrot des Gachons C, Beauchamp GK, Stern RM, Koch KL, Breslin PAS. Bitter taste induces
nausea. Current Biology. 2011;21(7), R247-248