The JournalCompetition

Caffeine and L-theanine: what is proven, and what is not

CORE PROTOCOL TEAM6 MIN READ

Two overlapping halftone circles, gold for caffeine and purple for L-theanine, joined by a plus sign, beside the headline Caffeine and L-theanine

Caffeine is one of the best-supported performance supplements there is, and one of the easiest to overdo. Add L-theanine and the evidence gets thinner. Here is what is proven, what is not, and how to use caffeine without it costing you your sleep.

What caffeine does

Caffeine works mainly by blocking adenosine, a chemical that builds up through the day and makes you feel tired. The result is greater alertness and, for many people, a lower sense of effort at the same workload.

What the evidence says

The 2018 International Olympic Committee consensus statement on supplements puts caffeine in the small group with good evidence of benefit. The International Society of Sports Nutrition's 2021 position stand adds the detail:

  • doses of 3 to 6 mg per kilogram of body weight consistently improve performance
  • aerobic endurance shows the most consistent benefit, moderate to large
  • strength and power improve by small amounts: roughly 2% to 7% for strength and 2% to 4% for jumping
  • results in team and anaerobic sports are more mixed

Responses vary from person to person, which brings us to the dose.

How much, and when

The position stand puts the usual timing at about 60 minutes before the effort, and notes that the best timing depends on the source: chewing gum acts faster than capsules. The lowest effective dose may be as low as 2 mg/kg. Very high doses, around 9 mg/kg, bring a high rate of side effects without extra benefit.

Core Protocol's protocols aim for 3 mg/kg, the low end of the researched range, capped at 400 mg, and round to the nearest whole capsule so that the dose you are shown is exactly what you take. For a 75 kg athlete that is two capsules, 200 mg. It is taken 45 to 60 minutes before the event, and for long aerobic events the dose is split between before and during rather than taken in one hit.

The European Food Safety Authority judges that up to 400 mg a day from all sources is not a safety concern for adults, and that single doses of up to 200 mg, about 3 mg/kg for a 70 kg adult, are not either, including within two hours of intense exercise in normal conditions. The key words are all sources. Around 200 mg is roughly two and a half espressos or four cups of tea, so your coffee, tea and any pre-workout count towards the same total.

Where L-theanine fits

L-theanine is an amino acid found in tea. The case for pairing it with caffeine rests mostly on mental performance. A 2021 systematic review concluded that the pair is likely a safe and effective cognitive enhancer, with better reaction time, working memory and accuracy than caffeine alone, using about 200 mg of L-theanine with 150 to 160 mg of caffeine.

In sport the picture is mixed:

  • A 2023 trial in elite curlers, using 6 mg/kg of each, found the combination improved shooting accuracy and reaction-time test results more clearly than either on its own.
  • A 2025 crossover study of 20 competitive athletes, using 3 mg/kg of caffeine and 200 mg of L-theanine, found no added effect on maximal strength, and no differences in handgrip, aerobic endurance or hand-eye coordination. Caffeine on its own did not beat placebo for strength there either, and L-theanine on its own was associated with lower leg and back strength than caffeine or placebo.

Both are single trials, so they are a hint rather than a verdict. The honest summary is that caffeine is the proven part. L-theanine is plausible for focus and precision, and has not been shown to add raw physical output. Many people say it takes the edge off caffeine, and that is the idea behind the pairing, but the studies above did not measure it, so it is a reasonable expectation rather than a proven effect.

Sleep is the cost

A well-known study of 12 healthy sleepers found that 400 mg of caffeine taken six hours before bed cut objectively measured sleep by more than an hour, and the sleepers did not notice. That was a bigger dose than most athletes use, but it makes the point: for an evening event or a late session, count back at least six hours from bedtime before you decide when to take it.

Why people respond so differently

Genetics matter. The position stand notes that variants of the genes involved in how quickly you clear caffeine, and how sensitive you are to its effects on anxiety, change the response. In some studies, slow metabolisers have performed worse on caffeine rather than better. Being a regular coffee drinker is not a reliable guide either: the position stand found no consistent difference in the performance effect between habitual and non-habitual users, though the evidence on tolerance conflicts.

Who should be careful

Pregnancy is the clearest case. EFSA puts the safe level at up to 200 mg a day, and Core Protocol leaves the caffeine protocol out entirely during pregnancy. Take advice before using it if you have a heart condition, high blood pressure, anxiety or panic attacks, trouble sleeping, or take regular medicines.

Never try it for the first time on the day

Because response is so individual, the plan matters more than the product. Trial the dose in training, with the same timing and source you will use on the day. Notice your heart rate, your stomach, how jittery you feel and how you sleep that night. If it makes you shaky or anxious, go lower or leave it out.

Where this leaves you

Caffeine is a proven tool, scaled to your body weight, timed to the effort and counted across everything you drink and take. L-theanine may help with sharpness and calm for some people, and it is not something to expect extra strength or endurance from. Caffeine is also one of the ingredients that turns up in more than one product, which is why we wrote about why static stacks fail serious athletes.

Sources