For many clients, coffee is not just a beverage. It is a ritual, a performance aid, a mood booster, a digestive trigger, a social habit and, sometimes, the only thing standing between them and the morning workout they promised themselves they would do.

As a health and exercise professional, you hear about coffee all the time, even when the conversation is not technically about coffee. A client mentions needing a second cup to get through the afternoon. Another says they cannot train without it. Someone else has switched to decaf to help with sleep, while another is worried coffee is “bad for the gut.” These are practical, everyday questions, but the science behind them is surprisingly complex.

A new study published in Nature Communications adds an interesting layer to the conversation. Researchers examined how habitual coffee intake may shape the gut microbiome and influence physiology, mood and cognition in healthy adults. The study is especially useful for professionals because it moves beyond the familiar question of “Is caffeine good or bad?” and toward a more nuanced perspective: Coffee is a complex food-like beverage, and caffeine is only one part of the story.

As John Cryan, professor and principal investigator at APC Microbiome Ireland at University College Cork, put it: “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.”

This perspective is a helpful starting point for client conversations. Coffee may not be something to automatically praise or demonize; rather, it’s something worth trying to understand in context.

What the Researchers Studied

The study included 62 healthy adults between ages 30 and 50 living in Ireland. Thirty-one were non-coffee drinkers, meaning they never consumed coffee, and 31 were moderate coffee drinkers, defined in the study as people who usually consumed three to five cups per day.

The researchers first compared coffee drinkers and non-coffee drinkers at the start of the study. Then, the coffee-drinking group abstained from coffee and caffeine for two weeks. After that washout period, those habitual coffee drinkers were randomly assigned to drink either caffeinated or decaffeinated instant coffee for three weeks. The intervention was blinded, meaning participants did not know which version they were receiving.

This design allowed researchers to ask several useful questions: How do coffee drinkers differ from non-coffee drinkers? What changes when habitual coffee drinkers stop consuming coffee? And what happens when coffee is reintroduced, with or without caffeine?

The study was unusually broad in what it measured. Participants completed psychological questionnaires, cognitive tests, caffeine and food diaries, and assessments of stress, mood, sleep, physical activity and gastrointestinal symptoms. Researchers also collected stool, urine, saliva and blood samples to evaluate gut microbiome composition, metabolites, cortisol and inflammatory markers.

It’s significant that the study did not look at coffee through just one lens. Instead, it examined coffee as something that may impact several systems at once: the gut, the brain, the immune system, stress physiology and behavior.

Coffee and the Gut-Brain Axis

The gut-brain axis refers to the two-way communication between the gastrointestinal tract and the central nervous system. This communication involves nerves, hormones, immune signaling, microbial metabolites and other pathways. In plain language, the gut and brain are constantly exchanging information.

How does this show up in real life? Think about how stress can affect digestion, the way poor sleep can change a person’s appetite or how anxiety can alter bowel habits. And, of course, diet changes often seem to affect mood and energy. The science is still evolving, but the practical relevance is clear.

Coffee plays an interesting role in this system because it contains caffeine, but also many other bioactive compounds, including polyphenols, phenolic acids and other plant-derived compounds. Some of these compounds may interact with gut microbes, while gut microbes may help determine how individuals metabolize and respond to them.

In this study, coffee drinkers showed a distinct gut microbiome profile compared with non-coffee drinkers. The researchers observed higher levels of certain bacterial species in coffee drinkers (we’ll spare you the technical names). Importantly, coffee did not appear to simply increase or decrease overall microbiome diversity or abundance. Rather, it seemed to influence specific microbial strains.

In other words, the takeaway is not “coffee improves the microbiome” in some universal way. The more accurate conclusion is that habitual coffee intake was associated with specific microbiome differences, and some of those differences changed when coffee was removed or reintroduced.

Caffeinated Coffee vs. Decaf

One of the most practical findings for client conversations is that caffeinated and decaffeinated coffee appeared to have distinct but overlapping effects.

Both caffeinated and decaffeinated coffee were associated with improvements in some self-reported mood and behavioral measures after reintroduction. Participants in both groups reported lower perceived stress, lower depression scores and lower impulsivity scores. That suggests some effects may be related to coffee compounds beyond caffeine, such as polyphenols.

Caffeinated coffee, however, had some caffeine-specific associations. Participants consuming caffeinated coffee showed reduced anxiety and psychological distress, along with improvements in attention and vigilance. This aligns with what many people experience subjectively: Caffeine can make some people feel more alert, focused and ready to engage.

But that doesn’t mean more caffeine is always better. The study also found that, at baseline, habitual coffee drinkers had higher impulsivity and emotional reactivity scores than non-coffee drinkers. After the washout period, those measures decreased. That’s why caffeine can’t really be presented as a simple mood or performance solution. For some people, caffeine may support alertness and perceived well-being; for others, it may contribute to feeling wired, reactive or dependent.

Decaffeinated coffee also produced interesting findings. Participants who consumed decaf showed improvements in several memory-related measures, along with reported improvements in sleep quality and physical activity. The researchers noted that some of the memory findings could be influenced by learning effects from repeated testing, so this should not be interpreted as proof that decaf coffee improves memory. Still, it does support the broader idea that non-caffeine components of coffee deserve attention.

In a coaching setting, this can be a useful message: Clients who are trying to reduce caffeine do not necessarily have to give up coffee entirely. Decaf may still provide some of coffee’s bioactive compounds and may be a reasonable option for clients who enjoy the ritual or taste but struggle with caffeine-related sleep disruption, jitters or anxiety.

What Changed in the Body?

The researchers also found that coffee intake was linked to changes in metabolites, which are small molecules produced through digestion, microbial activity and normal metabolism.

As expected, coffee drinkers had higher levels of caffeine-related compounds, but they also showed differences in phenolic metabolites, which are related to coffee’s plant compounds. After the coffee washout, some metabolite profiles shifted toward those seen in non-coffee drinkers. When coffee was reintroduced, many phenolic metabolites increased again, including in those consuming decaf.

So, while decaf coffee may contain far less caffeine, it still contains other coffee-derived compounds that can interact with the gut and metabolism.

The study also found changes in inflammatory markers. At baseline, coffee drinkers had lower levels of C-reactive protein, or CRP, and higher levels of the anti-inflammatory cytokine IL-10 compared with non-coffee drinkers. After avoiding caffeine, some inflammatory markers increased. After coffee was reintroduced, both caffeinated and decaffeinated coffee influenced immune responses in different ways.

These findings are intriguing, but they aren’t enough to say with confidence that coffee “reduces inflammation.” This was a relatively small study in healthy adults, and inflammatory markers are not the same thing as disease outcomes. A better way to frame it is that coffee may influence immune-related pathways, and that this is one reason researchers are increasingly interested in coffee as part of the broader diet-health picture.

What Coffee Did Not Clearly Do

Not every measure changed, and that is just as important as what did.

At baseline, the coffee drinkers and non-coffee drinkers did not differ significantly in body mass index, blood pressure, stress, anxiety, depression, gastrointestinal symptoms, sleep quality or physical activity. Nor did cortisol responses differ meaningfully between groups during the stress testing. In other words, coffee appeared to influence some mood, behavior, microbiome, metabolite and immune markers, but it did not dramatically reshape every aspect of health in these participants.

The study also did not show that coffee improves fitness, weight management, recovery or exercise performance. Those outcomes were not the focus. Therefore, it is appropriate to discuss how coffee habits may relate to energy, sleep, training readiness, digestion and stress. However, it is not appropriate to overstate the study as evidence that coffee will improve workouts or health outcomes for every client.

How to Apply These Findings to Your Work With Clients

The practical value of this study is not that every client should drink coffee. It is that coffee may offer measurable benefits for those who choose to consume it.

When you are discussing sleep, energy, stress, hydration, pre-workout habits, digestive comfort or nutrition routines, ask about coffee with the same curiosity you bring to exercise history. How much does the client drink? What time of day? Is it caffeinated, decaf or both? Do they use it before exercise? Do they feel better, worse or no different when they skip it? Does it affect sleep, anxiety, bowel habits, appetite or afternoon energy?

These questions are within scope for most health coaches and exercise professionals because they are about awareness, education and behavior patterns. They are not about diagnosing a gut disorder, treating anxiety or prescribing caffeine as medicine.

For clients who tolerate coffee well, enjoy it and sleep normally, moderate intake may fit comfortably within a healthy lifestyle. For clients who are caffeine sensitive, anxious, hypertensive, pregnant, taking certain medications or struggling with insomnia, referral to a physician or registered dietitian nutritionist may be appropriate before making major changes. For clients who rely heavily on caffeine to function, the conversation might shift toward sleep quality, recovery, total workload and whether coffee is masking fatigue rather than solving it.

Decaf deserves a more prominent place in these discussions. Many clients think of decaf only as a less-appealing compromise. This study suggests it may be more useful to think of decaf as a lower-caffeine way to retain some of coffee’s non-caffeine compounds and rituals. That may be especially helpful for clients who enjoy coffee but are trying to reduce late-day caffeine or manage sleep.

Finally, keep the microbiome findings in perspective. You can explain that emerging research suggests coffee interacts with gut microbes and metabolites. You should not interpret microbiome tests, promise “gut healing,” recommend coffee as a treatment for digestive symptoms or suggest that clients use coffee to manage inflammation or mood disorders. Those claims go beyond what this study can support and beyond the scope of a health and exercise professional.

The best application is simple and client-centered: Coffee is not automatically good or bad. It is a variable. Like training volume, sleep timing or meal patterns, its effects depend on the person, the dose, the timing and the larger lifestyle context.

For the client who loves a morning cup, this research may offer reassurance that coffee is more than a jolt of caffeine. For the client who feels jittery, sleeps poorly or depends on coffee to push through exhaustion, it may be a prompt to experiment thoughtfully. And for health and exercise professionals, it is a reminder that even ordinary habits can reveal a lot about how a client feels, functions and recovers.

Coffee may be common, but it is not simple, which is why it is worth asking about.