Caffeine

It is the most widely consumed psychoactive substance in the world. It occurs naturally in the coffee bean, the tea leaf, cacao and the guarana seed, and it is added to energy drinks, cola soft drinks, sports supplements and some painkillers. In Australia, where there is recent national data, around 90% of adults consume it regularly, with an average of about 175 mg a day: roughly two espressos.

At the doses at which it is consumed, only one mechanism has been demonstrated: occupying the site of adenosine, the molecule that builds up in the brain during waking hours and produces sleep pressure. Caffeine does not provide energy. It covers the tiredness signal for as long as its effect lasts.

It sustains alertness and shortens reaction time. The clearest benefit appears when sleep is lacking: in candidates for the US Navy SEALs, after some 72 hours without sleep, a single dose improved vigilance and reaction time against placebo. Working memory is not among what improved.

In someone who drinks coffee every day and has slept well, much of what they notice with the first coffee is not an improvement on their normal state but the return to it after a night without a dose.

The Australian Institute of Sport considers that there is solid evidence of improvement in endurance lasting more than an hour, in intense efforts of 1 to 60 minutes, in team and intermittent sports, and in single efforts of strength or power, which is where the measured effect turns out to be smallest. The same source warns that the individual response varies: there are people who do not respond and people who respond worse.

The proposed mechanism is not muscular but nervous: caffeine reduces the perception of effort, which would allow pace and technique to be held for longer. Trained cyclists cut their time trial by 4.2% with 3 milligrams per kilogram of body weight. The Australian Institute of Sport classifies it in Group A, its category of solid evidence for specific situations in sport.

It has been studied above all in people who have to perform while awake when they should be sleeping: night shifts, on-call duty, driving in the small hours. It does not replace sleep, and its effect on the sleep that follows is documented.

In team and racket sports the improvement is split between physical work and concentration.

On ultra-endurance there is no answer: no solid randomized trial has been located in ultramarathon or ultratrail.

The Australian Institute of Sport places the ceiling of the effect at 3 milligrams per kilogram, about 210 mg for someone weighing 70 kg, and proposes starting with 1.5 mg per kilogram, around 100 mg. In the reference trial, doubling the 3 mg/kg did not improve time trial performance and did increase adverse effects.

The timing allows for variants: up to an hour before, spread out during the effort, or well into it when fatigue appears. The three schedules have given equivalent results in trials. Effects have been observed with capsules, coffee, gel, bars, chewing gum and dissolvable oral strips, which dissolve in the mouth and are swallowed. Mouth rinsing is a different matter: it is spat out without being swallowed and is the route with the least backing, as explained further down.

The Australian Institute of Sport holds that abstaining from caffeine in the days before a competition does not increase the benefit.

To sustain alertness without sleep, 200 mg was the optimal dose of the three tested in the SEAL study. EFSA places its reference limits for healthy adults at 200 mg per dose and 400 mg a day. Above the ergogenic ceiling, the time trial measured no further performance, and the Australian Institute of Sport lists among the effects of high doses anxiety, digestive discomfort, lack of concentration and disturbed sleep.

Half the dose is still in the body about five hours after taking it. Four hundred milligrams taken four hours before bed took away 50 minutes of total sleep and 30 of deep sleep; at eight and at twelve hours before, deep sleep was still impaired. One hundred milligrams did not move any measure, not even at four hours.

Participants only said they had slept worse when they had taken it four hours before: at eight and at twelve hours, deep sleep degraded without their noticing.

“A cup of coffee” is not a quantity. When twenty espressos from twenty different coffee shops were measured, the caffeine per serving ran from 51 to 322 mg: a sixfold difference for ordering the same thing.

A 250 mL cup of filter coffee is around 80 mg, with a range of 40 to 110; instant, about 60. Decaf is not zero either: it contains from 0 to 13.9 mg per large serving in the samples analyzed.

Tea has less, though with an equally wide range: from 25 to 110 mg per cup in black tea and from 30 to 50 in green. Herbal infusions, such as chamomile or rooibos, contain none: they do not come from the tea plant. A 60-gram bar of dark chocolate provides 10 to 50 mg, and a 250 mL can of energy drink, about 80.

The studies that associate coffee with less diabetes, less liver disease or lower mortality do not allow that effect to be attributed to caffeine: decaf shows similar associations. They are also observational: they describe who drinks what and who falls ill, without demonstrating that the one causes the other.

There are situations in which the prudent course is to consult a healthcare professional.

For minors, the American Academy of Child and Adolescent Psychiatry places the maximum at 100 mg a day between the ages of 12 and 18, and sets no safe dose below that age. The societies that have taken a position agree that energy drinks are not appropriate for minors.

Pure caffeine powder is a case apart: a teaspoon contains about 3,200 mg, close to 28 cups of coffee. The doses described as lethal lie above 5 to 10 grams, with considerable individual variability.