How would you like to call the sixth basic taste—fat or oleogustus? Back in grade school, we were only taught four basic tastes, namely sweet, bitter, sour, and salty. It was until 1990 when umami (or savory) became an official basic taste. Although unlike the other tastes, the Japanese-discovered umami taste went through hardship before being widely recognized. This is because umami was initially thought to be just a flavor, not a taste. Today, scientists believe a sixth taste exists—and is called oleogustus. Difficult to pronounce? You could say fat as well because oleogustus is the latin word for “a taste of fat” (“oleo” means fatty or oily + “gustus” means taste).

To be considered a primary taste, oleogustus, like the other basic tastes, has to satisfy several criteria. A chemical stimulus has to be there that activates the taste buds. From the receptor, the certain taste signal is sent to the brain, which furthers identifies it. And lastly, a physiological response occurs in the body, such as increase salivation.

The discovery of taste receptors for fat in rodents’ tongue

Taste Receptors or “Taste Buds” are cells found in the oral cavity, but are more prominent in the upper surface of the tongue. Traditionally, there were only 4 tastes—sweet, bitter, sour, and salty—that activate the taste receptors in the tongue. In 2000, a team of neuscientists discovered a modified mGluR4, the first tastebud receptor for umami. This solidified umami’s status as the official fifth taste. What about oleogustus?

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Most of us believe that the sensation of fat has been in the texture (like creaminess and smoothness). This happens because of triglycerides, the form of fat our diet consists majority of. But triglycerides do not have any effect on our taste buds. Which in this case does not make fat a basic taste. While taste is based on one sensation, flavor is a combination of sensation (smell, texture, and expectation).

However, in 2005, a group of researchers led by Philippe Besnard from the University of Burgundy in France discovered that the tongue of rodents has taste receptors for fat. That receptor is CD36 or platelet glycoprotein 4. To confirm that CD36 in the tongue really does detect fat, the team studied rodents that were normal or had the gene for CD36 “knocked out”, shutting down or inactivating the taste receptor.

They found that the normal rats and mice showed preference for fatty food when offered. The “knocked out” or CD36-deficient rats and mice did not, indicating that CD36 doees detect fat.

Are fat receptors in the human tongue?

There are many people who love eating fatty foods. In the US, over a third of the population is obese, partly because of the natural attraction to fatty foods. Well, this is despite the well-known fact that excessive fat intake may affect our health in a bad way.

Like that of rodents’, the human tongue apparently has that same receptor for fat. In 2011, researchers at Washington University School of Medicine in St. Louis confirmed the existence of CD36, a potential fat receptor in humans. As first reported on Journal of Lipid Research, the main goal of the research was to understand how fat perception in food might influence what foods to eat and the quantities of fat consumption. The study found individual variability in how people sense fat. Specifically, obese people with a specific CD36 gene have a specific ability to detect fat in a solution. The team studied 21 individuals considered obese (with a BMI of 30 or more). Some of them had a genetic variant that allowed them to produce more CD36. While some produce less, some are in between.

According to Dr. Nada A. Abumrad, one of the authors, one potential reason for individual variability is that as people consume more fat, they become less sensitive to it, requiring more intake for the same satisfaction, as confirmed through a series of taste tests. Abumrad first identified CD36 as the protein responsible for the uptake of free fatty acids. The researchers concluded that people can taste fat, the same way we taste saltiness and sweetness.

Scientists confirm oleogustus (fat) as the sixth basic taste

So far, the three identified free fatty acid receptors are CD36, GPR120, and GPR40. While all these three are present in rodents, the human tongue produces GPR120 and GPR40 at relatively lower levels. With that said, CD36 is implicated as the leading candidate for taste receptor signaling in humans.

In 2015, a study was done at Purdue University in Indiana wherein 102 panelists were given cups containing one of the basic tastes. During the tests, panelists wore nose clips to prevent discrimination of long chain fatty acids from blank solutions. They grouped the cups according to taste— sweet, bitter, sour, salty, umami and fatty. Other sensory attributes including appearance, odor, and texture were controlled for. At first, they easily identified sweet, sour, and salty; but identified the cups of fatty mixture as bitter due to their unpleasant bitter-like taste. This unpalatable taste is similar to oleic and linoleic acid, in terms of intensity. But this suggested that fat taste receptors interact predominantly with long chain fatty acids.

With a new experiment, the panelists grouped together cups of bitter, umami and fatty mixtures. This time, they grouped the cups of fatty mixtures and distinguished them as separate from the rest. This just shows that the fatty acids evoke a unique sensation that satisfies one of the criteria required for the basic tastes.

With all the date gathered, the study concluded that:

These data added to the totality of evidence on “fat taste” now provide a comprehensive body of evidence supporting the existence of another basic or primary taste quality for selected fatty acids (fat taste), whose oral activity should thus be considered when examining the health consequences of fatty acid signaling.

The sixth taste or not (yet)?

To end, the researchers suggested that the new basic taste be named oleogustus or “taste of fat.” Treating fat as a separate and unique taste will help in food product development (such as low-fat foods), clinical practice (such as management of appetite and taste disorder), and public health policy (proposing dietary recommendations).

Although scientists say there is indeed a sixth taste, in terms of acceptance, oleogustus may actually need more time to be accepted widely. Yes, sure the people of science themselves are pushing it. But more experts believe that further research is necessary for wider recognition. It is much similar to umami, which was first identified in the early 1900s in Japan, but only accepted as a basic taste nearly a century later.

Other reference:

J. J. Provost, K. I. Colabroy, B.S. Kelly, M. A. Wallert, The Science of Cooking: Understanding the Biology and Chemistry Behind Food and Cooking, John Wiley & Sons, Inc., 2016

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