What is Caffeine?
Caffeine is a naturally occurring stimulant, most commonly found in coffee, tea, cacao, chocolate, and many energy drinks. For centuries, cultures around the world have brewed coffee and tea to unlock its energizing effects, so much so that some historians suggest caffeine has shaped societies by fueling productivity and creativity.
Today, caffeine is one of the most widely studied ingredients in the world, recognized for its ability to enhance both mental and physical performance. Research shows it can sharpen focus, improve memory and reaction time, and extend physical endurance by stimulating adrenaline release, making it a trusted ally for athletes, professionals, and anyone looking to perform at their best.
On a scientific level, caffeine works by blocking adenosine, a neurotransmitter that signals fatigue. This adenosine-receptor antagonism helps reduce drowsiness and increases alertness. It also influences dopamine and noradrenaline activity, supporting sharper focus, learning, and memory. Caffeine further affects brain energy metabolism, though it may reduce cerebral blood flow and cause relative hypoperfusion.
Physically, caffeine’s psychostimulant effects enhance motor coordination and vigilance (Nehlig et al., 1992). It also stimulates fat oxidation, the body’s ability to break down fat for fuel, especially during aerobic exercise (Acheson et al., 2004; Fernandez-Sanchez et al., 2024). The degree of this effect varies: lower to moderate doses (<6 mg/kg of bodyweight) may be more effective than higher doses, and untrained individuals may experience greater benefits than trained athletes.
Additional research suggests caffeine may improve aerobic performance through glycogen sparing (Fernandez-Sanchez et al., 2024) and could support body composition when paired with an energy-restricted, high-protein diet (Tabrizi et al., 2019), though findings remain debated (Antonio et al., 2024). In strength training, studies show caffeine can improve both upper- and lower-body peak power (Duncan et al., 2019).
In the sections that follow, we will explore caffeine’s impact on health, hydration, cognitive and physical performance, and sleep. We will also highlight natural sources, discuss evidence-based dosing strategies, and address some common myths and misconceptions.
Health Research on Caffeine
One important consideration with caffeine is its effect on mental health, particularly anxiety. A recent meta-analysis found that caffeine does increase the risk of anxiety, with the impact being dose-dependent. At low doses, caffeine may moderately increase anxiety risk, while higher intakes (>400 mg) are more strongly linked to heightened anxiety (Liu et al., 2024).
Although moderate caffeine intake can sometimes provide short-term mood improvements, through its effects on neurotransmitters like serotonin and dopamine, excessive consumption can worsen anxiety, disrupt sleep, and negatively affect overall mental wellbeing (Antonio et al., 2024).
This is where L-theanine comes into play. L-theanine is an amino acid naturally found in tea, known for its calming properties. It increases alpha brain wave activity, creating a state of relaxed alertness without drowsiness, and influences neurotransmitters such as dopamine and serotonin to help balance mood (Wang et al., 2022).
What makes this pairing particularly powerful is their synergistic effect. Research shows that while caffeine alone can improve focus and performance, it can also bring side effects such as jitteriness or increased stress. When combined with L-theanine, these drawbacks are reduced. In fact, studies suggest the combination provides sharper cognitive performance, sustained focus, and reduced mental fatigue compared with either compound on its own (Sohail et al., 2021). Importantly, L-theanine is recognized as both safe and effective, making it a natural complement to caffeine.
Caffeine’s Effects on Hydration
A common misconception is that caffeine dehydrates the body. While caffeine has mild diuretic properties, research consistently shows that moderate consumption does not cause dehydration. In fact, coffee and other caffeinated beverages can contribute to daily fluid intake in much the same way as water.
For example, Armstrong et al. (2005) found no significant difference in hydration markers between caffeine and non-caffeine conditions over a 24-hour period. Similarly, Burke et al. in Caffeine for Sports Performance concluded that regular caffeine intake does not compromise hydration, noting that tolerance to its diuretic effect develops within 4–5 days of consistent use.
Further evidence supports this: Killer et al. (2014) demonstrated that caffeine intake, even at moderate levels of up to 6 mg/kg, does not significantly alter total body water balance or fluid retention. While very high doses (≥6 mg/kg or over 500 mg) may increase urine output temporarily, these effects are minimal and short-lived (Antonio et al., 2024).
Caffeine’s Effects on Cognitive Function
Caffeine is one of the most reliable enhancers of cognitive performance, though the impact depends on the task and dosage. Research shows clear, measurable improvements in focus, alertness, and processing speed.
A meta-analysis of 13 studies found that low to moderate doses of caffeine taken before or during exercise significantly improved self-reported energy, mood, attention, accuracy, and cognitive processing speed in sporting contexts (Calvo et al., 2021).
Beyond sport, caffeine has also been shown to improve occupational performance. McLellan et al. (2016) reported enhanced alertness, vigilance, reaction time, and attention, though effects on memory and higher-level executive function were less consistent. The authors concluded that caffeine is an effective strategy for improving performance in occupations where both mental sharpness and physical readiness are critical.
