Caffeine

The hour-by-hour rise comes from single doses given in short test sessions, which may not match a regular drinker's morning cup. Only three trials, of about 2 weeks each, looked past the first day, too short to catch a change that builds more slowly. An older review found the rise strongest in people who already have high blood pressure, so it may be smaller in others.

Coffee habits were self-reported and chosen, not assigned. People worried about their blood pressure may cut back, which would hide a rise among heavier drinkers. An earlier pooling of six studies found 9% higher risk at 1 to 3 cups a day and no rise above 3. American studies found slightly lower risk and European ones did not, so the overall figure averages a real split.

The trials gave caffeine itself, not coffee, at doses and for lengths that varied from trial to trial. Eight trials is a small base to split by sex, dose and length, so those three patterns are leads rather than settled findings. The clearest dose effect, above 400 mg a day, was in diastolic pressure.

Everyone in these 45 studies was short on sleep, at six hours or under or awake for a full day, so nothing here says what caffeine does for someone who slept well. The benefit kept rising with dose without the review ever finding the point where it stops rising, so the top of that range is unmapped.

This is a precautionary ceiling drawn from a broad safety review, not a figure any trial in pregnant women produced. It describes a population rather than any particular pregnancy.

This is a threshold for a population of healthy, non-pregnant adults rather than a personal limit; the same 400mg lands differently on people who clear caffeine at different speeds. It is regulatory guidance rather than the result of a single trial. The 200mg single-serving figure rests on thinner data than the daily one.

That 9-hour figure is an average across many doses and timings, not a personal cut-off. How fast a person clears caffeine varies widely, partly for genetic reasons, so the same cup at the same hour lands differently on two people. Older adults' sleep tends to be more sensitive to it than younger adults'.

The 11 trials used a spread of doses and designs rather than one fixed ratio, so this is a pattern across mixtures rather than a test of any single blend. The authors call theanine's contribution comparatively modest rather than absent, which leaves room for a larger pool to move it. And every trial pooled here tested combinations, so none of it measures theanine by itself.

One trial of 48 people, and it was designed to study caffeine, with theanine as the modifier rather than the subject. For anyone taking theanine by itself for calm focus, the same trial is a null result: alertness, jitteriness and mood did not move.

Those timings are population ranges, and plenty of people stop and feel nothing at all. The 100mg figure is the lowest daily intake shown to produce withdrawal, not the usual one. This is a critical review of 57 experiments and 9 surveys rather than a pooled statistical analysis, though nothing since has replaced it.

Something else in cola, or about the people who drink it, may carry the risk, and the authors could not say what. Caffeine intake was not flat either: women in the middle fifth had 12 to 13% higher risk than the lowest. Both intake and diagnosis were reported by the women themselves.

With 17 men on coffee, the trial was sized to catch a difference of 220 ml of urine or more, so a smaller loss could have gone unseen. Larger doses were not tested, and the researchers note a measurable effect on urine is likely once a single dose passes about 300 mg of caffeine. The men were fully hydrated and had fasted overnight. The litre went down in half an hour, not as cups spread across a day.

Nobody was assigned to drink coffee, so whatever else separates a three-cup habit from none, in diet, smoking, exercise or income, is folded into that number. Coffee carries hundreds of compounds besides caffeine, and nothing here tells them apart. The size of the association differed by region, running stronger in Europe and Asia than in the US.

That coffee drinkers die less often is not in doubt; whether anything in the cup causes it is. Nobody was assigned to drink coffee, and nothing separates caffeine from the hundreds of other compounds in it.

The trial was built as a caffeine comparison, so the pairing was never its main question. Accuracy was one of several measures taken in that task. The advantage was over placebo, and caffeine alone was credited on a different measure, so nothing here sets the pair against caffeine by itself.

What this trial cannot show is whether the pair beats caffeine alone: its edge was over placebo, on one brief task. The edge is unconfirmed rather than contradicted.

A single two-hour task. Theanine's effect showed up in behaviour but not in the brain-wave marker much of this field leans on, so the two kinds of evidence do not line up here. And it disagrees with a similarly dosed trial that did find an advantage for the combination.

What is open is whether pairing theanine with caffeine helps attention generally or only on the brief task that found it. Counting errors over two hours in 27 people, the pair did no better than either alone.

Eleven people give only a rough picture of when the rise fades and how completely. One moderate dose was tested, about 210 mg for a 70 kg adult, not the range people drink.

In one small trial, daily caffeine's blood-pressure rise faded to near zero within three weeks; pooled trials found about 2 mmHg that was larger in longer trials. Whether the body adapts fully or only partly is unsettled.

One small crossover trial in young women only, so nothing here covers men, older adults or anyone especially sensitive to caffeine. The 30mg of caffeine is well under a standard cup of coffee at roughly 95mg, which leaves the everyday dose untested. The lead author is affiliated with a commercial tea company.

The trial reported which way each measure moved, not by how much. Nine people took each treatment, and three published papers all describe the same 27. The caffeine was a slow-release tablet, not coffee.