In elementary school science class, you may remember learning about the basic flavors a human tongue can taste: sweet, salty, sour and bitter. In 2002, scientists declared a fifth, called umami (“yummy” in Japanese), which accounts for the savory flavor in cheese, meat and soy sauce.Now, scientists are getting closer to figuring out whether there is a sixth recognizable taste: fat.MORE: Why You Crave Fatty FoodsLast month, researchers at Washington University in St. Louis identified the first known human taste receptor—a molecule in the tongue that responds to flavor—that detects fat. An analogous molecule is located in the digestive tract and may help with the digestion of fat.What’s more, a genetic variation in some people could affect how sensitive they are to the taste of fat, according to the study. The research gets us one more step closer towards understanding whether or not there is a link between fat sensitivity and body mass index (BMI).“This receptor is important to detect fat, and nature put it in your mouth and put it also in your gut,” says Yanina Pepino, Ph.D., an assistant professor at Washington University’s Center for Human Nutrition and the lead author on the study. “Now we know that having a genetic variation affects how sensitive you are. But we don’t know the implications. Certainly, it could be related to fat preferences and also how you handle fat once it reaches your gut.”A Taste (and Gut) SensationWhen you eat, the chemicals in your food interact with specialized receptors on your tongue, sending a message to your brain to tell you what it is you’re tasting.  These receptors are clumped together into taste buds, with 50 to 150 receptors per bud. Your tongue has between 2,000 and 8,000 taste buds, which works out to hundreds of thousands of taste receptors.All taste buds include a mix of receptors for all the categories of taste. But some areas of the mouth are more sensitive to specific tastes than others—something you may also remember from that elementary school science class, when a Q-tip dipped in a sugary solution tasted sweeter on the tip of your tongue than it did near the back. Animal studies suggest that the so-called fat taste receptors may reside at the back and sides of the tongue, but this hasn’t been definitively proven yet in humans.QUIZ: How Much Fat Are You Eating?A growing body of evidence suggests that the molecules that form the taste receptors in your mouth are also present in the stomach and intestines. Here, the molecules help with digestion. Proteins that form the receptors on the tongue that detect sweetness, for example, may also help regulate blood sugar levels as carbohydrates break down in the gut.  And the protein that Dr. Pepino’s team identified as a potential fat taste receptor may help digest fats.From an evolutionary perspective, taste receptors help animals recognize foods that will provide them with essential nutrients, like the sweet taste of the carbohydrates that give you energy or the savory proteins that are the building blocks for your body and most of its functions.  Taste can also warn of potentially dangerous foods, like the bitterness of some toxins or the rancidness of rotten meat.Even fat has its purpose, which is why we’re able to sense and digest it. “Fats are always demonized—that we’re obese because of the fat and fats are bad for you and we shouldn’t eat fat,” notes Pepino. “But fats are very important for life. There are some fats that, if you don’t get them in your diet, you just don’t get it. Your body cannot synthesize them. So we depend on getting fats that are essential and critical for life through our diet.”A Fat Receptor?While researchers have long recognized our ability to identify the presence of fat in food, it was mainly attributed to senses other than taste, such as smell, sight and touch or the sensation of the smooth texture of fat on the tongue.However, scientists identified potential fat taste receptors in animals, including mice and rats and theorized that similar receptors existed in humans. One such receptor is a protein called CD36. Mice that have been genetically altered to stop producing CD36 are unable to detect fat at all (normal mice show a preference for fatty food over non-fat food; mice without the gene show no preference). The genetically altered mice also had difficulty digesting fat, likely because they lack CD36 in their digestive tracks, which helps break down fat.MORE: Are Low-Fat Foods Making You Fat?The Washington University study aimed to find out whether the same thing happens in humans, as well as to figure out if varying levels of the protein affect the ability to recognize fat. In the study, 21 test subjects tasted three different solutions—one containing fatty oil and two that were fat-free—in a red-lit room to mask visual cues while wearing nose clamps to mask the smell. The fat-free solutions included a thickening agent to give them a similar texture to oil.The researchers also analyzed the test subjects’ DNA for variants of the CD36 gene, which controls the production of the CD36 protein. Those whose bodies produced high levels of the protein were the most sensitive to fat, while those who produced little had difficulty detecting the presence of fat, which means they may have a harder time knowing when they’re satiated.More than GeneticsBut genes aren’t the only factor affecting how easily we detect fat. Previous research led by Russell Keast, Ph.D., a nutrition scientist at Deakin University in Victoria, Australia, indicates that people of all body weights on a low-fat diet have a heightened sensitivity to even low concentrations of fat. When the same people are put on a high fat diet, those with a normal BMI are less able to detect fat, while those who are overweight or obese show no change in sensitivity.QUIZ: Are You an Emotional Eater?According to Dr. Keast, when we eat fat, it initiates mechanisms that signal we’re full. In other words, a certain amount of fat will tell us when to stop eating. It will also signal to our brains that we’re still full between meals.“These findings have implications for obesity,” says Keast. “Those who are insensitive to fat do not get the ‘fullness’ signals it provides. Therefore, if you consume a fatty meal, a healthy weight subject would start to feel full and stop eating. The gap between eating events would also be extended. Those who are insensitive to fat do not feel full and therefore keep eating.”But whether or not there is a real connection between BMI and the ability to detect fat remains to be seen, argues Richard Mattes Ph.D., a nutrition scientist at Purdue University. The hypothesis is compelling, he says, but equally compelling is the opposite claim: that people who are very sensitive to fat receive a higher reward when they eat it, which could increase their intake of fat.More tests have to be done to show how an increase in sensitivity to fat changes the diet, if at all. But sussing out the details is no easy task. “We shouldn’t blame our genes,” cautions Pepino, “What we eat is so complicated, and food preference is influenced by ethnic background, age, lifestyle, the kinds of foods we ate during childhood. Even if we find this link in the lab between fat and genes, it is very difficult to apply to a real-life situation.”It’s ComplicatedThe tongue’s ability to sense fat is complex, and the CD36 protein isn’t the full story, adds Dr. Mattes. While the new research identifies one component of a potential fat taste receptor, that receptor—the CD36 protein—only detects one component of fat, long-chain fatty acids. “There is a role for CD36 in human fat detection, but we’re not sure it’s taste,” says Mattes.Still, Mattes is optimistic. If scientists are able to unravel how we perceive the taste of fat—and whether that influences how much fat we eat—the findings could lead to new ways to fight obesity, including fat replacements and medications.That breakthrough research would also change our understanding of biology. “This is a direct challenge to the very strongly-held belief that taste is defined by four to five primary qualities: sweet, salty, sour, bitter and maybe umami,” notes Mattes. “The question is, is ‘fatty’ also a taste? This shakes the roots of basic biology—of one of our fundamental sensory systems.”MORE: Outsmart Your Eating Instincts