3) The electrical signal must be transmitted by neurons to
processing regions of the brain.
Upon depolarization, taste bud cells (of which there are
four major types with different taste receptors) can release
various neurotransmitters such as acetylcholine, serotonin, and
norepinephrine (Besnard, P., et al., http://dx.doi.org/10.1152/
physrev.00002.2015, 2016). These neurotransmitters reach
afferent nerve fibers of gustatory neurons that carry the
electrical signal to the brain.
Binding of CD36 to LCFA triggers the release of serotonin
and norepinephrine from type III taste bud cells. Denervation of
the peripheral gustatory nerves in mice suppresses their preference for fats.
4) The fat taste should be distinguishable from other tastes.
The perception of a fatty “taste” was long thought to arise
solely from the texture, smell, or appearance of a fatty food.
Although a fat taste is not as readily defined as, for example,
a sweet or salty taste, a recent study has shown that people
can distinguish fat from other tastes in the absence of textural,
olfactory, or visual cues (Running, C. A., et al., http://dx.doi.
org/10.1093/chemse/bjv036, 2015). Researchers gave study participants multiple cups of solutions, each containing a compound
corresponding to sweet, salty, bitter, sour, umami, or fatty. They
asked the participants to sort the cups into groups of similar
“quality or type” (researchers carefully avoided use of the word
“taste”). All of the solutions, which were served in opaque cups
with lids, had similar viscosities, and participants wore nose clips
to keep them from smelling the liquids.
The participants could easily separate sweet, salty, and sour
stimuli. They consistently grouped short-chain fatty acids with
sour stimuli, which could be expected since acetic acid is also
a short-chain fatty acid. There was a large overlap among
bitter compounds, medium-chain fatty acids (MCFA), and LCFA,
possibly because all were considered unpleasant stimuli. There
was also some overlap between MCFA and umami stimuli, but
the researchers attributed this result to the participants’ lack of
experience with umami sensations, rather than true perceptual
overlap. When the participants were given new cups containing
only fatty, bitter, and blank compounds, they could clearly
distinguish the three groups.
Among fatty acids, LCFA elicited the most unique, perceptible
sensations, consistent with the known preference of the CD36
receptor for LCFA.
5) There must be physiological effects following activation of
the taste bud cells.
The interactions between fatty acids and specific receptors
in taste bud cells have physiological consequences that affect
both food intake and digestion (Besnard, P., et al., http://dx.doi.
org/10.1152/physrev.00002.2015, 2016). The electric signal sent
to the brain by the taste buds increases digestive secretion in
preparation for the coming food. In addition, the signal stimu-
lates the brain to evaluate the hedonic (pleasure or palatabil-
ity) value of the food and the metabolic (energy) needs of the
organism, which together determine eating behavior. In addition
to releasing neurotransmitters, taste bud cells release gastroin-
testinal hormones that regulate digestive tract function, glucose
homeostasis, and appetite/satiety.
A brief oral exposure to fats (sipping and spitting, without
actually ingesting the fat) causes an immediate rise in plasma
triglyceride levels. The physiological significance of this increase
is unknown, but it might signal the body to prepare for fat
digestion and absorption (Keast, R. S. J., and Costanzo, A., http://
dx.doi.org/10.1186/2044-7248-4-5, 2015). Other physiological
effects of oral fat stimulation include increases in lipase secre-
tion, stimulation of gastrointestinal hormones, and fluctuations
in glucose and insulin levels.
So is fat really the sixth taste? Mounting evidence indicates
a strong possibility. However, several questions remain to be
answered before a definitive conclusion can be made. For
example, what are the precise roles of CD36, GPR120, and other
receptors proposed to detect or modulate the fat taste? How,
exactly, does fatty acid chain length influence taste perception?
And are there specialized types of taste buds that detect the fat
taste, as there are for other basic tastes?
Interestingly, lean mice and humans appear to have more
fatty acid receptors on their taste buds and are therefore more
sensitive to fatty acids than obese individuals, which reduces
their propensity to overeat. “In other words, the more you taste
fat, the less fat you eat,” write Keast and Costanzo ( http://dx.doi.
org/10.1186/2044-7248-4-5, 2015). In addition, diet appears to
modulate the perception of fat: A high-fat diet decreases sensi-
tivity, whereas a low-fat diet has the opposite effect. “The next 5
to 10 years should reveal, conclusively, whether fat can be clas-
sified as the sixth taste, but no matter what, there appears to be
a functional significance to oral chemosensing of fats,” say the