At a fixed submaximal workload, 3-6 mg/kg of caffeine left heart rate unchanged (P =.99, n = 207) but lowered perceived exertion by 0.8 points and raised ventilation (+3.36 L/min), blood lactate (+0.69 mmol/L) and glucose (+0.42 mmol/L)
A meta-analysis of 26 double-blind randomised crossover trials
Glaister and Gissane pooled double-blind, randomised crossover trials to see what a standard caffeine dose does to physiological responses during steady submaximal exercise. Twenty-six studies qualified: each included a sustained bout of 5-30 min at a fixed intensity between 60% and 85% of VO2max, after 3-6 mg/kg of caffeine taken 30-90 min before.
With a random-effects model, caffeine against placebo raised minute ventilation (+3.36 L/min), blood lactate (+0.69 mmol/L) and blood glucose (+0.42 mmol/L), and lowered ratings of perceived exertion (−0.8). It had no effect on heart rate, respiratory exchange ratio or oxygen uptake.
The authors note that caffeine's benefit for sustained high-intensity performance is widely accepted while its mechanism is unresolved, and suggest these effects may help explain it.
Participants
Methods
Key Results
| Measure | Caffeine − placebo (95% CL) | P | n |
|---|---|---|---|
| Minute ventilation | +3.36 L/min (1.63 to 5.08) | .0001 | 73 |
| Blood lactate | +0.69 mmol/L (0.46 to 0.93) | < .00001 | 208 |
| Blood glucose | +0.42 mmol/L (0.29 to 0.55) | < .00001 | 129 |
| Perceived exertion | −0.8 (−1.1 to −0.6); scale not in the abstract | < .00001 | 147 |
| Heart rate | no effect (difference not given in the abstract) | .99 | 207 |
| Respiratory exchange ratio | no effect | .18 | 181 |
| Oxygen uptake | no effect | .92 | 203 |
Practical Application
Limitations
How this entry was checked and changed
risk of bias not read (abstract only; no risk-of-bias assessment is reported in the abstract); no marker step, because the responses themselves (heart rate, perceived exertion, ventilation, lactate) are what this entry is about (rule 3 exception)