We are a month into 2015, and a lot of us are probably struggling with our New Year's diet resolutions. But if you find yourself staring at the half eaten donut in your hand saying, "Why can't I quit you?" don't beat yourself up too much - new research out this week suggests that our brains are hard wired to love that donut.

Writing in the publication Cell, scientists at MIT say that they've discovered the neural circuit that controls sugar and food addictions. It's called the LH-VTA Loop and it's like a highway between the lateral hypothalamus, or "LH", which controls how hungry you feel, and the ventral tegmental area, or "VTA", which is the center of the brain's reward circuit. Scientists knew that the LH-VTA Loop existed, problems in this area have been linked to some sexual and drug addictions. But they didn't know if it was responsible to food addiction as well. 

So to test its role in eating behavior, they used a technique called optogenetics on mice. They genetically modified certain neurons in mice's brains so that those cells can basically be turned on or off by exposing them in the light. By delivering a yellow light through a small implanted fiber optic, the scientists could turn those neurons on and activate the LH-VTA Loop. They could also turn those same neurons off by delivering a blue light.

With these modifications in place, healthy, well fed mice were put into two stations. The first had a cup full of food pellets and the second had a sugar dispenser. The scientists then activated the yellow light. With the reward circuit stuck in the "on" position, the mice ate for longer periods of time in the first station and kept going back to the sugar dispenser repeatedly at the second station. The mice at the second station would even walk across a platform that delivered electrical shocks just to get more of that sweet stuff. But when the scientists used the blue light to turn off the LH-VTA Loop, the mice wouldn't walk across the electrified platform and they wouldn't eat if they were full.

Now, we humans also have that same loop in our brains and it's likely there for a reason. Many scientists believe that our taste for what we now think of as junk food evolved as a way to reward us for finding pallatable, high-energy food when food was scarce. But because we now live in a world with Krispy Kreme on every corner, our desire for sugar has become more of a hindrance than a help. So the scientists say that finding the part of our brain that regulates these cravings can help in developing treatments for often debilitating food addictions.

But, beside our brain's rewards system, what else makes us love food? Well, taste of course.  There's bitter, sweet, salty, sour and what's sometimes called the fifth taste, known as umami. It's best described as a savory, but not salty flavor that you can't quite put your finger on. 

Umami flavor comes predominantly from high levels of the amino acid glutamate and it was discovered by a Japanese scientist in 1908. It's found in cheeses, shiitake mushrooms, ham, and mono-sodium glutamate, a food additive that was developed in 1909 to enhance the umami flavor of food.

Now, according to a new study in Japan, tasting umami might be important to our health. Scientists performed what's known as a "paper filter disk test" on 44 elderly patients. The test uses a small piece of paper soaked in different concentrations of a tasty solution placed on parts of the tongue responsible for each taste. And 16% of the people tested turned out to have unusually high thresholds for umami, meaning they could barely taste it. And those same patients were also the ones who stated that food in general just wasn't palatable to them anymore. As a result, they had suffered from loss of appetite and weight loss.

Part of their problem, it turned out, was hypo-salivation, or the inability to produce enough saliva. You have to produce saliva in order to taste anything because food needs to be partially dissolved by saliva for our taste buds to register them. And you know what actually stimulates saliva production? Food with umami in it. So, in a weird kind of catch-22, the patients needed to eat more umami in order to taste umami to get their appetites back.

So the scientists prescribed a daily regime of konbu-cha, a tea made from kelp that's rich in glutamate. The tea began stimulating their umami receptors, which caused them to slowly increase saliva production and as they started to produce more saliva, they began to taste foods more strongly. Eventually, food became more palatable and they regained their appetite.

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