The colour and taste of a placebo mouth rinse alters sweetness perception, neural responses and performance in trained cyclists
This study demonstrates that the color of a non-caloric mouth rinse alters sweetness perception, modulates prefrontal cortical activity in trained cyclists, and paradoxically enhances cycling performance when the color is incongruent with sweetness expectations.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
Imagine your brain is a super-smart detective that is constantly trying to guess what's going to happen next. It doesn't just wait for information; it uses clues from the past—like how a red apple usually tastes sweet or how a green lemon might taste sour—to build a "prediction" about the world. This is called expectancy. When your brain's prediction matches reality, everything feels smooth. But when the prediction clashes with reality—like seeing a red drink that tastes like vinegar—your brain has to work harder to fix the mistake. This whole process of guessing, checking, and adjusting is a huge part of how we experience taste and even how we feel about our own bodies.
Scientists have long known that if you trick your brain into thinking you're getting a sugary boost (even if you aren't), your body sometimes reacts as if you actually got it. This is the placebo effect: a real change in performance or feeling caused by belief rather than a chemical ingredient. But here's the mystery: does the color of a drink matter? Does a red liquid make your brain expect sweetness more than a green one? And if your brain gets tricked by a color, does it actually make you run or ride a bike faster? This is the question a team of researchers from Brazil set out to solve, diving into the weird and wonderful world where what we see changes what we taste and how we perform.
The Colorful Experiment: How a Mouth Rinse Can Hack Your Brain
A team of scientists decided to test if the color of a drink could act like a "brain hack" for athletes. They didn't use real sugar or energy drinks. Instead, they used a placebo mouth rinse—a liquid that tasted sweet because of an artificial sweetener but had zero calories and no energy-boosting chemicals. The only thing that changed was the color.
They ran three different experiments to see how this worked, moving from simple taste tests to high-tech brain scans and finally to a real cycling race.
Experiment 1: The "What Color is Sweet?" Test
First, the researchers needed to know which colors people naturally associate with sweetness. They showed 119 people (a mix of regular folks and trained cyclists) cups of water colored red, green, pink, purple, blue, yellow, and brown. They asked, "Which of these looks the sweetest?"
The results were clear: Red was the winner. People consistently ranked red as the color that meant "sweet." Green, on the other hand, was the loser; it was consistently ranked as the least sweet (often associated with sour or bitter things). Interestingly, the trained cyclists actually rated all the drinks as less sweet than the non-athletes did, suggesting that drinking lots of sports drinks might have made their taste buds a bit more skeptical.
Based on this, the team decided to use Red as the "Sweet-Congruent" color (the one that matches our expectation) and Green as the "Sweet-Incongruent" color (the one that tricks us).
Experiment 2: The Brain Scan Mystery
Next, the researchers wanted to see what was happening inside the brain. They hooked up 20 people (10 cyclists and 10 non-cyclists) to an EEG, a machine that reads electrical activity in the brain like a radio picking up signals. They focused on the prefrontal cortex, the part of the brain responsible for attention and decision-making.
They measured brain waves in two bands: Theta (slow waves, often linked to daydreaming or mental effort) and Beta (fast waves, linked to focus and alertness). They calculated a ratio called the Theta-Beta Ratio (TBR). A lower TBR usually means the brain is more engaged and focused.
Here is where it got fascinating:
- Before the taste: When cyclists saw the Red (Congruent) liquid, their brains lit up with fast Beta waves, and their TBR dropped. It was as if their brains were saying, "Ooh, red! Sweetness is coming! Get ready!" This happened before they even tasted it.
- During the taste: When they actually tasted the liquid, both the Red and Green drinks changed the brain activity compared to a clear, colorless drink.
- The Cyclist Difference: This brain magic only happened in the trained cyclists. The regular people's brains didn't react to the colors in the same way. This suggests that being an athlete changes how your brain processes these visual cues, perhaps because they are used to associating specific drink colors with performance.
Experiment 3: The 4-Kilometer Race
Finally, the team put the cyclists to the test. They had 29 trained cyclists do a 4-kilometer time trial (a race against the clock) on a stationary bike. Before the race, they rinsed their mouths with one of three things:
- Red liquid (The "Sweet" expectation).
- Green liquid (The "Not Sweet" expectation).
- Clear liquid (Control).
- No liquid (Control).
The results were surprising. You might think the Red liquid, which promised sweetness, would make them faster. But it didn't. Instead, the cyclists who rinsed with the Green (Incongruent) liquid finished the race 11.2 seconds faster than when they used the clear liquid.
This is a huge deal because the liquid had no sugar and no calories. The blood tests confirmed this: their blood sugar and lactate levels didn't change based on the color. The cyclists didn't even feel like the green drink was sweeter or better during the race. They didn't consciously think, "This green drink is making me faster."
What Does This Mean?
The study suggests that the color of a drink can act as a powerful signal to the brain, changing how we perceive sweetness and even how our brain waves behave. But the biggest twist is that the Green drink, which should have been the "wrong" choice for sweetness, actually helped the cyclists go faster.
Why? The researchers have a few playful theories. Maybe the green color created a "prediction error." The brain saw green, expected something bitter or sour, but then tasted something sweet. This mismatch might have forced the brain to pay extra attention or switch gears, giving the cyclist a tiny boost in focus. Or, perhaps athletes have learned to associate green drinks (like citrus-flavored sports drinks) with hydration and energy, even if they aren't sweet.
The paper concludes that color is a real, physical tool that can change how we feel and perform, even without any actual fuel in the drink. It shows that our brains are constantly running a simulation of the world, and if we change the visual cues, we can change the outcome. While the study doesn't prove that green drinks will make everyone a world champion, it does suggest that the next time you grab a sports drink, the color might be doing more work than you think.
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