The research team, drawn from institutions including Kastamonu University and Karabuk University, set out to fill a gap in the sports nutrition literature. Caffeine is arguably the best-studied ergogenic aid on the planet, working largely by blocking adenosine receptors in the brain, which blunts the perception of fatigue and sharpens alertness and motor control. Rhodiola rosea, by contrast, is an adaptogen — a plant extract traditionally used to improve resilience to physical and mental stress — with proposed anti-fatigue and anti-hypoxic properties. Despite decades of separate research on each, direct comparisons of the two, and of their combination, under controlled laboratory conditions have been scarce, particularly in soccer players whose sport demands repeated explosive sprints.

The experimental design was a single-blind, randomized, four-condition crossover. Each participant completed every condition in a randomized order separated by 72-hour washout periods: a cellulose placebo, caffeine at 3 milligrams per kilogram of body weight, standardized Rhodiola rosea extract at the same 3 milligrams per kilogram dose, and the two together. The Rhodiola product was a commercial standardized extract supplying salidroside and rosavins, the compounds most often credited with the herb’s adaptogenic activity. Sixty minutes after swallowing the capsules, the players performed a Wingate anaerobic test on a mechanically braked cycle ergometer, with resistance set at 7.5 percent of body mass — the classic laboratory proxy for the explosive, anaerobic efforts that decide soccer matches.

The performance results centered on the combined condition. Peak power output averaged 867.7 watts after the caffeine-plus-Rhodiola dose, against 755.7 watts after placebo — a difference of roughly 112 watts that reached statistical significance. Relative to body mass, the combined condition produced about 12.12 watts per kilogram versus 10.91 on placebo. Average power, a measure of anaerobic capacity sustained across the full 30 seconds, was also highest after the combined dose, and significantly exceeded the Rhodiola-only condition by about 50 watts. Notably, caffeine alone and Rhodiola alone did not significantly beat placebo on any power metric, which makes the pattern — apparent benefit only when the two are paired — the most intriguing finding of the study.

Alongside the ergometer data, the researchers collected saliva immediately after each test and measured three biomarkers using enzyme-linked immunosorbent assays: cortisol, the body’s principal stress hormone; alpha-amylase, a marker of sympathetic nervous system activation; and secretory immunoglobulin A, a first-line antibody of the mucosal immune system. Cortisol concentrations were significantly higher in the combined condition than after placebo. Alpha-amylase, counterintuitively, was significantly lower after caffeine alone than after placebo. And IgA was significantly elevated after Rhodiola alone, a result the authors flag as relevant to athletes, since falling IgA during heavy training has been linked to increased susceptibility to upper respiratory infections.

The authors are careful — unusually so for this field — about what these numbers do and do not prove. Because the study used repeated-measures ANOVA with pairwise comparisons rather than a formal interaction analysis, the superior results in the combined arm cannot be statistically attributed to a true synergistic interaction between caffeine and Rhodiola. The paper repeatedly stresses that the findings represent condition-specific acute responses that require confirmation in larger, better-powered trials. The researchers also note that the biomarkers were sampled within roughly a minute after the Wingate test, capturing only an early snapshot of the neuroendocrine stress response rather than its peak, and that no baseline saliva samples were taken before supplementation in each session.

The limitations section is candid and extensive. With only ten participants, even a crossover design leaves little room for detecting small effects, and the a priori power calculation assumed a relatively large effect size that was not derived from any single prior study. Habitual caffeine intake was never quantified, so individual tolerance — a known modifier of caffeine’s ergogenic effects — could not be accounted for. Period and sequence effects were not modeled statistically. Salivary flow rate was not measured, meaning IgA was analyzed as a raw concentration rather than a secretion rate. And the sample was exclusively male, limiting generalizability to the growing population of female athletes.

One limitation stands out for its potential to explain the headline result: blinding. The placebo, caffeine, and Rhodiola conditions each involved a single capsule, while the combined condition required two. Participants may well have guessed when they had received the double dose — and the combined condition was precisely the one where positive findings emerged. The authors acknowledge that expectancy effects cannot be excluded and recommend a double-dummy design, in which every condition involves the same number of capsules, along with formal checks of blinding success in future work.

Mechanistically, the researchers offer a plausible but unproven rationale for why the pairing might help. Caffeine’s adenosine blockade could enhance central nervous system drive and neuromuscular recruitment during maximal effort, while Rhodiola’s proposed effects on cellular energy metabolism and stress regulation — possibly involving the hypothalamic–pituitary–adrenal axis — might complement that stimulation. The elevated cortisol seen after the combined dose, they caution, should not be read as a harmful stress response; transient cortisol spikes are part of the normal physiology of intense exercise, and the study captured only one early time point. The lower alpha-amylase after caffeine alone is similarly hard to interpret, since sympathetic activity was not directly measured and prior studies of caffeine’s effects on this marker have been inconsistent.

For now, the practical takeaway is modest. The study offers preliminary evidence that a single acute dose of caffeine plus Rhodiola rosea can raise peak anaerobic power in trained soccer players under laboratory conditions, while leaving open whether that advantage survives on the pitch, in women, or with chronic use. The authors call for larger randomized trials with both sexes, sport-specific performance measures, baseline biomarker sampling, and proper interaction statistics before any definitive recommendations can be made. Until then, the combination remains an intriguing hypothesis rather than a proven recipe — one that the growing market for adaptogenic supplements will no doubt watch closely.

Subject of Research: Acute effects of caffeine and Rhodiola rosea supplementation, alone and combined, on anaerobic performance and salivary stress biomarkers in soccer players

Article Title: Effects of Rhodiola rosea and caffeine, alone and in combination, on Wingate performance and physiological stress responses in soccer players

Article References: Yelken, M. E., Hepsert, S., Küçüksayan, H., Vural, E., Gülten, S., Bildirici, M. A., & Akçay, N. (2026). Effects of Rhodiola rosea and caffeine, alone and in combination, on Wingate performance and physiological stress responses in soccer players. Frontiers in Physiology, 17, Article 1964904. https://doi.org/10.3389/fphys.2026.1964904

Image Credits: AI Generated

DOI: 10.3389/fphys.2026.1964904

Keywords: caffeine, Rhodiola rosea, Wingate anaerobic test, soccer players, ergogenic aids, cortisol, alpha-amylase, immunoglobulin A, crossover trial, exercise physiology, sports nutrition, adaptogens