Crossword, Sudoku, and Jeopardy! enthusiasts beware: Every french fry and candy bar you consume might be throwing off your game.
A study conducted by researchers at Oregon State University and published in the journal Neuroscience suggests that high-fat, high-sugar diets have a detrimental effect on what they refer to as "cognitive flexibility," or the power to adapt and adjust to changing situations. 

The research was performed using laboratory mice that consumed diets with varying levels of fat and sugar before facing a gamut of tests—primarily mazes and basic puzzles—to monitor changes in their mental and physical function. The researchers paid specific attention to the types of gut bacteria present in each control group.

 So, Why Should You Care? “Bacteria can release compounds that act as neurotransmitters, stimulate sensory nerves or the immune system, and affect a wide range of biological functions,” Kathy Magnusson, a professor in the OSU College of Veterinary Medicine and principal investigator with the Linus Pauling Institute, said in a statement. In about four weeks, the mental and physical performances of the mice on a high-fat, high-sugar diet started to drop significantly, especially when one or more variables in the test changed. One of the most disparate physiological factors within the groups of mice, and the suspected reason for the decreased brain function, was gut bacteria.
People have been paying more attention to their guts than ever. Yogurt companies are using the term “probiotic” to peddle their sugary treats to would-be health junkies, and celebrities like sportscaster Erin Andrews are now digestive health spokespeople for hire, trying to finally make gut bacteria the hip and cool subject it deserved to be all along. But the scientific community has also started paying closer attention to those trillions of stomach-dwelling microflora.

One of the first studies to link gut bacteria to brain function was performed by researchers at the Gail and Gerald Oppenheimer Family Center for Neurobiology and Stress and appeared in the peer-reviewed journal Gastroenterology. They took three groups of women and had one group consume yogurt loaded with supplemental probiotics daily for four weeks, had another group eat a substance that looked and tasted like yogurt but had no probiotics, and gave the third group nothing specific to eat. Not only did the sans-probiotic group suffer in cognitive tests, just as the mice in the recent Oregon State study did, but it also faltered in emotion-based tests, linking poor gut health to stress.

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“This work suggests that fat and sugar are altering your healthy bacterial systems, and that’s one of the reasons those foods aren’t good for you.” Magnusson said. “It’s not just the food that could be influencing your brain but an interaction between the food and microbial changes.”
Slowly but surely, people are starting to go with their gut.