BY THE OPTIMIST DAILY EDITORIAL TEAM

In a small experiment, researchers found that only participants who drank decaffeinated coffee showed significant improvements on memory tests. Those who drank caffeinated coffee did not. The findings, published in Nature Communications, suggest that caffeine may not be the main thing going on when coffee affects the brain.

A small experiment with a surprising split

The experiment enrolled 31 regular coffee drinkers, who were asked to stop drinking their usual three to five cups a day for two weeks. The withdrawal period produced measurable shifts in gut microbe profiles. Participants were then split: half received decaffeinated instant coffee, half received caffeinated, with neither group knowing which they had. Both groups consumed four servings daily for three weeks.

Both groups reported lower perceived stress, depression, and impulsivity after reintroducing coffee, regardless of caffeine content. Only the caffeinated group showed reductions in anxiety and improvements in attention and vigilance, consistent with decades of caffeine research. But on two memory tests, one involving word learning and one involving spatial location recall, only the decaf group showed significant improvement. The caffeinated group showed none.

“But I guess the most surprising thing was that when we put coffee back, we also got really exciting effects by putting decaffeinated coffee back,” said lead researcher John Cryan, a stress neurobiologist at University College Cork. “So it wasn’t just the caffeine.”

The polyphenol pathway: gut microbes, the vagus nerve, and memory

Cryan’s lab focuses on the gut-brain axis: the communication network between the gut and the brain, running through the vagus nerve, the immune system, and the microbiome. His group coined the term “psychobiotics” in 2013 to describe how probiotics can shape gut bacteria and potentially affect mental health through that pathway.

Coffee doesn’t contain probiotics, but it contains polyphenols, plant compounds that feed gut microbes and can alter the composition and activity of the microbiome. With caffeine removed, polyphenols are the most plausible candidate for what the decaf group experienced. The working hypothesis is that they reshape which microbes are present, change what those microbes produce, and that some of those downstream changes affect how the brain processes and retains information. Several of those steps remain unconfirmed.

“Coffee is more than just caffeine — it’s a complex dietary factor that interacts with our gut microbes, our metabolism, and even our emotional well-being,” Cryan said. “Our findings suggest that coffee, whether caffeinated or decaffeinated, can influence health in distinct but complementary ways.”

Strong signal, narrow scope

Thirty-one participants is a small sample. The coffee used was instant, which may not reflect the polyphenol profile of other brewing methods. Causality hasn’t been established: the study shows association, not a confirmed mechanism. “It’s a small study,” Cryan says. “It asks a lot of questions.”

Earlier research has found that decaf can improve cognitive function and alertness without any caffeine to account for it. This study offers a candidate explanation, even if it doesn’t nail one down. Coffee contains more than a thousand compounds beyond caffeine, most of which have barely been studied in a neurological context. Caffeine gets the attention because it’s the obvious active ingredient. This study suggests the obvious ingredient may not be the most interesting one.

Source study: Nature Communications- Habitual coffee intake shapes the gut microbiome and modifies host physiology and cognition

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