Writing — Dose math

Why the Same Caffeine Dose Lands Differently in Two Bodies

The same 160 mg of caffeine that leaves your training partner sharp can leave you buzzing — restless, slightly wired, thoughts skidding sideways. You drank the identical thing. You had the opposite afternoon. Most people put that down to tolerance, or sleep, or their own constitution. The likelier explanation is arithmetic, and it is sitting on the label where nobody does the division.

Caffeine is dosed in research by milligrams per kilogram of body weight. It is sold in fixed milligrams per can. Those two facts do not meet anywhere, and the gap between them is the whole of this article.

The arithmetic nobody prints

One number, two bodies. A 220 lb frame is about 100 kg; a 130 lb frame is about 59 kg. Put the same fixed dose through both.

The lighter person is taking roughly 1.7 times the dose per kilogram from the identical product. That is not a rounding error, and it is not a matter of willpower. A single dose spread across 100 kg of body simply does not arrive the way the same dose arrives across 59 kg.

So one of you gets calm, locked-in attention. The other gets the anxious version, and blames the coffee — or worse, blames themselves. You were taking a dose that was never sized for you. Nobody told you, because the number on the pack is fixed and the body holding it is not.

Research doses caffeine per kilogram. Industry sells it per can. Nothing on the label bridges the two.

What the caffeine research actually measured

The effect itself is well established, and worth stating in the size the papers report rather than the size the marketing implies. A systematic review and meta-analysis of 31 randomised, double-blind, placebo-controlled trials across 1,455 participants concluded that caffeine acutely enhances attention through both reaction time and accuracy, with a Hedges' g of 0.28 for reaction time and 0.27 for accuracy [1]. Those are small-to-moderate effects in a study population, and doses at or above 200 mg produced larger effects than lower doses, with a linear dose-response for reaction time [1].

The safety numbers are equally specific, and neither of them is a per-serving cap. The FDA cites 400 mg a day as the amount not generally associated with dangerous negative effects in healthy adults (U.S. Food and Drug Administration) [4]. EFSA's review puts single doses up to 200 mg and daily intakes up to 400 mg in the no-safety-concern range for healthy adults (European Food Safety Authority) [5]. If you have seen a number like 160 mg described as a legal or safe single-dose limit, that is not a regulator's figure. It is a can size.

The partner, and the ratio that was trialled

The thing that changes the character of a caffeine dose is not fewer milligrams. It is what the caffeine arrives with. That partner is L-theanine, an amino acid found in tea.

Two trials define the useful range. In the first, 100 mg of L-theanine with 50 mg of caffeine improved the speed and accuracy of attention-switching at 60 minutes and reduced susceptibility to distraction at 60 and 90 minutes (Owen et al., Nutritional Neuroscience, 2008) [3]. In the second, 37 acutely sleep-deprived young adults took 200 mg of L-theanine with 160 mg of caffeine in a double-blind, placebo-controlled crossover design; the combination improved hit rate (P = 0.02) and target-distractor discriminability (P = 0.047), and cut reaction time to hazards by 38.1 ms against placebo (P = 0.003) (Nawarathna et al., British Journal of Nutrition, 2025) [2].

Read the second one for its ratio rather than its headline. 200 mg of theanine against 160 mg of caffeine is 1.25 mg of theanine for every 1 mg of caffeine. That is the pairing the measurements belong to. Honest caveats, since they are ours to state: 37 people is a small trial, they were sleep-deprived rather than rested, the 38.1 ms belongs to that group at that timepoint, and formal dose-response work on theanine with caffeine is still thin (Examine) [6]. The ratio is the best-evidenced anchor available, not a settled constant.

What we actually made, and what it is not

SCRUPLE Focus Base is one pouch with one fixed dose: 190 mg of L-theanine per 5.50 g serving, with the electrolytes, acids and flavour published in full. It contains no caffeine at all. You bring the caffeine you were already going to drink — coffee, tea, a capsule — and the pouch supplies the other half of the ratio at roughly 1.25 to 1 against a 150 mg caffeine dose.

We keep the dose fixed and say so. All 300 pouches in the founding batch are identical. No pouch is blended to your body, nothing adjusts at checkout, and any page that told you otherwise was wrong. What varies is not the pouch — it is what 190 mg means once it is inside you:

That is the same arithmetic we just used against the energy drink, turned on our own product. A fixed number can only ever be right for one body. Publishing the number and the division is the part most labels leave out.

The calculator is free, and it is not the pouch

Because the pouch is fixed, the sizing has to live somewhere else. So it does: a weight-based dose calculator, free and with no purchase. It asks for an email before it shows your reading, and that is the whole price. Put in your weight and it shows the milligrams per kilogram you are actually working with and how a fixed serving reads against your own body.

It is a reference tool, not a prescription, and it does not sell you caffeine — we do not make any. If what it tells you is that a 190 mg fixed dose is not sized for you, that is a useful answer and you should act on it.

Why the formula is short

Focus blends tend to grow: taurine, a B-complex, a proprietary botanical, a number on the front of the tub. We left those out for one reason, stated plainly — we could not find evidence for them at the timepoint this product is built around, the 60-minute window where the attention measurements were taken [3] [2]. An ingredient without a trial at the moment you need it is decoration, and decoration is what you end up paying for.

The full formulation, including the exact milligram of every component and the carbohydrate derivation, is published on the formula page. Nothing is proprietary, because a proprietary blend is a way of not telling you the dose.

The deposit, and what it is for

The founding batch is funded by a refundable $25 deposit rather than a pre-order. The deposit is refundable in full until the batch funds at 265 reserved, at which point it is committed to ingredients; the $20 balance is charged only when the batch is ready to ship. If the batch never funds, every deposit comes back in full.

This is a mechanism, not a reassurance. It forces the ingredient cost into the open and answers the only sensible objection to a first run: will this actually ship, or am I funding an idea. Either 265 people agree the arithmetic is worth a pouch, or nobody is charged for one.

The whole argument

Your dose is not a serving suggestion. The claim gets made with confidence, the label names a serving, and the division — the milligrams against your kilograms, the ratio, the timepoint — goes in a footnote nobody prints.

Work out your own number first. If you decide the pouch fits, the pouch is there. If you decide it does not, you still leave with the number, which is the more valuable of the two things on this page.