The European Food Safety Authority (EFSA) in its Scientific Opinion on Caffeine noted ‘that caffeine at doses of 3 mg/kg bw (equivalent to 210 mg for a 70-kg adult) ingested about one hour prior to endurance exercise appear to induce only a modest increase in body temperature compared to placebo’. They also added that ‘higher doses of caffeine (5-6 mg/kg bw equivalent to 420 mg for a 70-kg adult) ingested about one hour prior to prolonged endurance exercise in a hot environment do not affect body temperature or hydration status beyond what could be expected from the testing conditions’14.
An additional comprehensive and critical review10 of 10 studies, which scrutinised the different protocols and the results obtained, concluded that:
- A daily intake of 300mg of caffeine (the amount found in approximately 3 regular cups of coffee) induces only a mild, short-term diuretic effect, similar to that of water, with no significant effect on overall fluid balance
- There is no evidence that caffeine is detrimental during exercise in hot climates when fluid losses are maximal
- The long-term effects of caffeine on fluid balance are not adequately reported. There are a number of methodological limitations. For example, urine should be collected over longer periods, at least 24 hours. Studies over a short time period suggest a mild, short-term diuretic effect of caffeine after 3 hours, though this effect ceases to be observed over longer time periods
- Statements suggesting the avoidance of caffeinated beverages before, and during, exercise are unfounded
In a more recent survey of the available literature on caffeine and exercise performance, the various components of hydration during sport were reviewed. As far as caffeine is concerned, ingestion of moderate to low levels (less than 300mg) in one shot did not contribute to dehydration either at rest or during exercise2. Similar conclusions can be drawn for the long-term ingestion of caffeine.
Caffeine and endurance exercise performance
A 2009 study systematically and critically reviewed the effect of caffeine on endurance performance11. The most relevant studies were selected for the review, excluding the studies with no time-trial component. A total of 19 studies were analysed. The authors concluded that, overall, caffeine ingestion in moderate quantities can be effective for endurance athletes.
In addition, in a paper aimed at developing a position statement on the effect of caffeine on sports performance, a team of experts reviewed the available literature in a systematic and sizable survey12. They concluded that caffeine is beneficial for sports performance. This literature review did not support caffeine-induced diuresis during exercise.
Caffeine and temperature regulation
Several factors can reduce heat tolerance during exercise in hot environments. A literature review assessed fluid balance, temperature regulation and exercise-heat tolerance during exercise. The results showed no evidence that caffeine induces chronic dehydration or negatively affects temperature regulation in a hot environment13.