Ingredient deep-dive

Caffeine And Exercise: What The Performance Trials Measured, And The Dose Gap

Caffeine has one of the best-supported performance records of any legal supplement ingredient in sports science. It also has a dose-response curve, and this capsule’s 138 mg sits at a very different point on that curve than the doses the trials actually used.

By SlimSet Team · Last updated September 27, 2026 · 11 min read · Ingredient deep-dive

What the exercise literature actually established

Unlike several other rows on this panel, caffeine as an ergogenic aid — a substance that improves physical performance — is one of the most extensively studied and most consistently positive areas of sports nutrition research. Multiple independent meta-analyses across decades converge on the same broad conclusion: caffeine, at the right dose, improves endurance performance, reduces how hard exercise feels, and can shift the body toward burning more fat during exercise. None of that is in serious dispute.

What is less often discussed on a product page is that almost all of this evidence is dose-dependent, and the doses that produced these effects were measured relative to body weight, in milligrams per kilogram, not as a flat number of milligrams. That distinction is the entire subject of this article.

The 2024 cycling meta-analysis and its dose cutoff

A 2024 meta-analysis in the Journal of the International Society of Sports Nutrition pooled fifteen placebo-controlled crossover studies testing caffeine’s effect on cycling time-trial performance, measuring both completion time and mean power output, and specifically compared a moderate-dose subgroup (4–6 mg/kg) against a low-dose subgroup (1–3 mg/kg).

Fifteen crossover studies pooled, random-effects model. The moderate-dose group is where caffeine’s ergogenic reputation in cycling actually comes from.

The authors’ own conclusion names the moderate dose range, 4–6 mg/kg, as the identified optimal range for cycling time-trial performance, explicitly noting the lower range did not produce a significant effect. This is the clearest possible statement, from the most recent pooled analysis available, that caffeine’s exercise-performance benefit has a floor.

Doing the kilogram math on this capsule

This capsule’s label prints 138 mg of caffeine anhydrous. Converting the trial dose ranges above into a flat number requires knowing a body weight, so here is that arithmetic for a few reference weights.

This capsule’s green tea and green coffee rows also carry some undeclared caffeine, covered in a separate article, which would push these figures up slightly — not enough to close the gap to the 4–6 mg/kg range at any of these weights.

At every reference weight above, one capsule lands inside or below the 1–3 mg/kg range the 2024 meta-analysis found did not produce a statistically significant time-trial benefit. This is simple arithmetic, not a criticism of the ingredient: caffeine works as an ergogenic aid, and 138 mg is a real, physiologically active dose for other purposes, but it is not the dose the cycling performance literature tested and found effective.

Fat oxidation has its own dose floor

A different outcome, more directly relevant to a weight-management product, is fat oxidation — how much of the energy used during exercise comes from burning fat rather than carbohydrate. A 2020 meta-analysis in Nutrients pooled nineteen studies published between 1978 and 2020, all using a fasted, crossover design comparing caffeine against placebo before submaximal aerobic exercise.

The pooled result found caffeine significantly increased the fat oxidation rate, alongside a corresponding drop in the respiratory exchange ratio and a rise in oxygen uptake — a coherent physiological signature of the body drawing more on fat for fuel. Critically, the paper also identified its own dose threshold: more than 3.0 mg/kg was necessary to obtain a statistically significant fat-oxidation effect. It also found the effect was larger in sedentary or untrained individuals than in trained athletes, meaning the people this capsule is most likely marketed to may actually see more of this specific effect than a trained cyclist would, dose permitting.

At 1.5–2.3 mg/kg for the reference weights above, this capsule again sits below the identified 3.0 mg/kg threshold for a significant fat-oxidation effect, taken as a single capsule before a workout.

The other mechanism: perceived effort

Not every benefit in the caffeine-and-exercise literature depends on power output or fat oxidation. A meta-analysis in the Scandinavian Journal of Medicine & Science in Sports pooled 21 studies and 109 separate effect sizes specifically on caffeine and rating of perceived exertion (RPE) — how hard a given workload feels, rather than how fast it is completed.

Caffeine reduced RPE during exercise by an average of 5.6% compared with placebo, a moderate but statistically robust effect, and regression analysis found this reduction in perceived effort explained roughly 29% of the variance in caffeine’s performance benefit across the pooled studies. This meta-analysis did not report its dose range broken out the way the two studies above did, so it is not possible to say precisely where this capsule’s 138 mg sits on this particular curve — it is included here because perceived effort is a mechanism separate from raw power output, and it is plausible that a workout feeling somewhat easier is a real effect at doses too low to move a stopwatch.

Does a daily coffee habit blunt the effect?

A 2022 systematic review in Sports Medicine specifically tested whether habitual caffeine consumption reduces caffeine’s acute ergogenic effect during exercise, pooling 60 studies with sufficient data on participants’ regular caffeine intake. The overall result found no influence of habitual consumption on the ergogenic effect — caffeine improved performance similarly whether the person was a light or heavy daily consumer, across endurance, power and strength exercise types, and regardless of how long they had abstained beforehand.

This is worth noting because it contrasts with caffeine’s well-documented cardiovascular tolerance — blood pressure and heart-rate responses to caffeine do tolerize with daily use, a separate topic covered in full here. The exercise-performance effect and the cardiovascular effect appear to follow different rules.

Timing matters as much as dose

None of the three meta-analyses above tested caffeine taken 20–30 minutes before a meal, which is this label’s own instruction. The cycling and fat-oxidation trials they pooled generally dosed caffeine 45–60 minutes before exercise, on an empty stomach or a standardized light meal, to line up with caffeine’s typical absorption curve. Peak blood caffeine levels are usually reached somewhere between 30 and 120 minutes after ingestion, with meaningful individual variation.

A capsule taken with breakfast, as this label instructs, and a workout that follows some unspecified time later is a different timing pattern than most of the trials cited above used deliberately. This is not a criticism of the label’s own instructions, which are written for a general weight-management routine rather than a pre-workout supplement protocol — it is a reason that anyone specifically timing this capsule around a workout for a performance reason should treat the label’s general instruction and a sports-science pre-workout protocol as two different things aimed at two different goals.

A morning workout, taken an hour or so after the capsule and its glass of water, lines up reasonably well with typical absorption timing. An evening workout, many hours after a morning capsule, is closer to catching whatever tail end of the caffeine curve remains, which is a smaller and less predictable dose than the 138 mg printed on the label.

What this means for one capsule before a workout

None of this means 138 mg does nothing before exercise — the perceived-effort literature above suggests a real, if modestly documented, benefit is plausible even at a sub-optimal dose, and the alertness effect of caffeine is well established regardless of exercise. What the dose-response literature does mean is that anyone taking this capsule specifically hoping to replicate the cycling time-trial or fat-oxidation results published in the sports science literature should understand those results were built on 4–6 mg/kg and 3+ mg/kg respectively, roughly double to triple what one capsule delivers for most adult body weights.

• For general alertness before a workout, this capsule’s dose is well within normal, well-tolerated ranges.
• For a specific performance or fat-oxidation target matching the published trials, the arithmetic above is worth doing against your own body weight before assuming the dose matches.
• Stacking with other caffeine sources before a workout changes this math — the article on this label’s total caffeine covers what else on this panel contributes.
• A second capsule is not a substitute for reading the dose-response curve. The label asks for one capsule a day and says not to exceed it; the trials above are about understanding what this capsule’s printed dose does and does not match in the literature, not a reason to take more than the label allows.

Related reading: how much caffeine is really in SlimSet totals every source on this panel, and does the metabolic boost from caffeine fade covers what happens to these effects with daily use.

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