Caffeine

Caffeine is one of the most-researched substances in modern supplement science, with a literature spanning endurance exercise performance, muscle strength and power output, cognitive function (especially alertness and reaction time), mood and well-being, headache treatment (as an active ingredient in many over-the-counter analgesics), and metabolic effects. It works through adenosine receptor antagonism, blocking the dampening effect of adenosine on the central nervous system. Most caffeine research has been conducted on doses commonly delivered by coffee, tea, energy drinks, and pre-workout supplements (roughly 1-6 mg/kg body weight), which makes 'supplemental' caffeine and 'dietary' caffeine less methodologically distinct than for most other substances.

The outcome dimensions covered most heavily are endurance exercise performance (the most-studied indication, with trials across many designs and athletic populations), muscle strength and power output, cognitive function and alertness (especially under sleep deprivation or mental fatigue), mood, headache treatment, and weight management (smaller and more contested literature). Newer research streams include effects on perceived exertion specifically, sport-specific performance metrics in team sports, and habitual-vs-acute caffeine effects in chronic users.

Demographically, the literature concentrates heavily on young trained athletes (the dominant performance-research population), with substantial healthy-adult cognitive subsamples and growing older-adult literature. Genetic variation in caffeine metabolism (via CYP1A2 polymorphisms) is an actively expanding research area: fast and slow metabolizers respond differently to the same dose, which is one of the most-personalizable variables in supplement science. Timing of administration (typically 30-60 minutes before exercise for performance use) and habituation status both meaningfully modify response.