Caffeine increases catecholamine levels, promotes calcium mobilization in muscles, and antagonizes the adenosine A1 and A2a receptors. Via these mechanisms, preworkout caffeine decreases RPE and increases exercise performance.
In this crossover randomized controlled trial (RCT), 36 men who had been doing strength training for at least one year were divided into tiers of (self-reported) usual caffeine intake. In the “low” tier, the average daily intake was 20 mg; in the “moderate” tiers, 88 mg; and in the “high” tier, 281 mg.
The 36 men were then divided between three interventions: caffeine (6 mg per kilogram of body weight: 6 mg/kg), placebo, and nothing. About one hour after ingestion, they were tested for power (via a countermovement vertical jump) and strength endurance (via four sets of leg presses at 70% of 1RM, to failure). They underwent each intervention, in random order, after a washout of at least four days.
The outcomes were power (jumping performance), strength endurance (number of leg-press reps), RPE, and pain. RPE and pain were measured after both the power test and the strength-endurance test.
RPE and pain didn’t differ based on the intervention or the tier of usual caffeine intake. Power and strength endurance were greater in the caffeine intervention, with no difference based on the tier of usual caffeine intake.
Vigor and perception of performance were the most common of the various “side effects of caffeine supplementation” this trial tested for; they were greater in the caffeine intervention, with no difference based on tier of usual caffeine intake. Headaches, anxiety, insomnia, tachycardia, and gastrointestinal disturbances didn’t seem to differ between interventions. The authors, however, did not conduct a statistical analysis.
Performance improvements are typically observed at doses of 3–6 mg/kg. Someone weighing 80 kg (176 lb) would therefore consume 240–480 mg preworkout.
In contrast, the average intakes within the “low” and “moderate” usual intake tiers were very low: 20 and 88 mg/day. It is possible that daily caffeine intakes higher than 281 mg (the average intake within the highest tier of usual intakes) would cause a decrease of caffeine’s benefits to power, strength endurance, vigor, and perception of performance.
For reference, the average American consumes 300 mg of caffeine per day.
According to the FDA, 400 mg/day is “not generally associated with dangerous, negative effects”, but keep in mind that caffeine affects different people differently. Even the speed at which it is metabolized varies greatly from person to person.
Still according to the FDA, toxic effects can occur with rapid consumption of ≈1,200 mg, but much lower doses can already cause various adverse effects, such as headaches, anxiety, insomnia, tachycardia, nausea, and stomach upset. High caffeine intakes should be avoided by pregnant women, older people at risk of bone fracture, people with hypertension, and people taking medications known to interact with caffeine.
The big picture: Two RCTs found caffeine’s preworkout benefits to be greater in people who don’t usually consume caffeine. Three, including the present one, found no such difference. Those five RCTs all used a one-time dose of caffeine, but two other RCTs looked at the effect of daily caffeine supplementation in people who didn’t usually consume caffeine: 1.5–3.0 mg/kg/day lost all of its benefit to total workload over the 28 days of a 2017 RCTs, whereas 3 mg/kg/day lost some of its benefit to VO2 max and half-Wingate tests over the 20 days of a 2019 RCT.
Unfortunately, none of these RCTs tell us how often you can use caffeine without its benefits decreasing, or how long you must abstain from caffeine for its benefits to fully come back.
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