More evidence that fat is the sixth primary sense of taste

Until fairly recently we thought our taste buds could only distinguish sweet, salt, sour and bitter although when combined with other senses such as smell these allowed an almost infinite number of flavours to be experienced. Umami, from the Japanese for ‘pleasant savoury taste’, was first described as a taste in 1908 by Kikunae Ikeda who identified the glutamate responsible for the flavour of a seaweed broth. It took almost 100 years for umami to be recognised as the fifth taste by the wider world and this was possibly because of the increasing popularity of monosodium glutamate-containing Asian foods.

Researchers now claim there is growing evidence for fat to be classified as the sixth sense of taste. Fat was described as a taste as long ago as 330BC by Aristotle but we have always associated it more with texture, flavour release and thermal properties in food. Three years ago a team in the USA described a chemical receptor in the taste buds that could recognise fat molecules and noted that its sensitivity varied between individuals. Up to 20% of people are believed to have a variant of the CD36 gene that is associated with producing lower levels of the receptor which could mean they are less sensitive to the presence of fat in food.The study also found that in animals a high fat diet could lead to lower production of CD36 which in turn could make a person less sensitive to fat. This suggests that the level of CD36 can be modified both by a person’s genetics and by their diet.

In February 2015, researchers from Deakin University in Melbourne reported that just as sugars and amino acids (the breakdown products of carbohydrates and proteins respectively) are responsible for activating sweet and umami tastes, fatty acids of varying chain length and saturation (the breakdown products of fats) can be detected orally by humans. They suggest the most likely candidates as fatty acid transporters and receptors on the taste buds are CD36 and G protein-coupled receptor 120.  

The criteria that must be met in considering what is a primary taste includes; the presence of a class of affective stimuli, receptors specific for the class of stimuli on taste bud cells, afferent fibres from those cells to taste-processing regions of the brain, perception independent of other taste qualities and downstream physiological effects. 

There is still some debate on whether fatty acids elicit a direct perception independent of other elements such as aroma but there is evidence that the presence of fatty acids on our tongues signals to the brain and digestive system that we should eat less. Incidentally, this could help to explain the lack of success of low fat diets.

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Citation
The Pharmaceutical Journal, February 2015;Online:DOI:10.1211/PJ.2015.20067959