The role of the sense of taste is to act as gatekeeper of ingestion, if a potential food is deemed suitable for consumption it may be swallowed, if not rejected. To guide the decision making we have five taste qualities: sweet, sour, salty, bitter and umami. Sweet, salty and umami are all appetitive and signal the food contains essential nutrient, while excessive sour and bitter signal aversion and potential harm.
Over the past few years there has been considerable attention given to fat as an additional taste; it seems logical given that we have taste responses to the breakdown products of carbohydrate (sugars) that elicit sweet, and protein (amino acid) that elicit umami.
For fat to be considered a taste a chain of events must take place. There must be a class of stimuli (fats or the breakdown products fatty acids) that activate receptors on taste cells that are specific to the stimuli. A signal must be sent from the taste cell to taste processing regions of the brain. The signal that is decoded as a perception must be independent of the other tastes (not a combination of sweet and salt or any other possible combinations).
The first evidence of a fat taste came out of a rat model in 1998, with Dr Timothy Gilbertson from Utah University showing a taste response to fatty acids. Professor Richard Mattes from Purdue showed similar receptors may occur in humans when he looked at sham feeding butter or non-fat butter substitute in humans, and seeing that butter caused an increase in blood triglycerides. The implication being that the fatty acids were activating a taste receptor system and preparing the body for fat digestion.
The concentrations of fatty acid required to activate fat taste is very low, and in the range found in common fatty foods. In addition, we also have lingual lipase that can cleave fatty acids from triacylglycerols, albeit with low activity. But taken together human limits of detection for fatty acids are well within the range we find in common foods.
Various researchers have identified fatty acid receptors on taste cells with the most likely candidates for fat taste being CD36 and GPR120. Further evidence supporting fat taste was the discovery of fat sensitive neurons in taste processing region of the brain. Finally, using our taste methodology we have established perceptual independence from the other tastes at detection threshold level.
We started our research in 2007 and published our first paper in 2010 showing a link between fat taste and BMI, with subjects who were insensitive to fat having a higher BMI. Since then we have published papers on method development, reliability of fat taste measures, links with overweight and obesity, links with gastrointestinal tract sensing of fat, and mechanisms that link fat taste with overconsumption of fatty foods.
The one characteristic of fat taste that is different from the other 5 tastes is a conscious quality. For example, we place sucrose on our tongue and experience sweetness, or NaCl on our tongue and experience saltiness. For fat taste we present 3 solutions, one of which contains a fatty acid. The task is to identify which solution contains the fatty acid. If the subject is incorrect the concentration of fatty acid is increased and the test rerun. This continues until the correct solution is identified multiple times. Participants can correctly identify the fatty acid solution but cannot provide an adjective that describes any taste; they know it is different but cannot articulate why.
Questions will remain. Does no taste quality exclude fat from being classified as a taste? Or is there a piece of information that would exclude fat from being a taste? However, with the advances in scientific techniques, our growing understanding of the taste system and its role as the first part of the alimentary canal, emerging evidence for Kokumi and other non-traditional tastes on the horizon, it may be time to broaden the scope of how we define taste.
Keast, R. and A. Costanzo (2015). "Is fat the sixth taste? Evidence and implications." Flavour 4(5). doi:10.1186/2044-7248-4-5