Regular coffee drinkers get weaker caffeine boost

People who drink coffee on a regular basis may benefit from the caffeine kick in the first cup of the day but do not experience the extended stimulation of occasional drinkers, according to new research from Canada.

Scientists from the Operation Medicine Section, Defence R&D Canada sought to assess whether the ergogenic effect - the 'caffeine kick' sought by many people to help them through the say - was different for regular and occasional drinkers of coffee, and whether it was related to the concentration of caffeine in circulation.

Writing in the Journal of Applied Physiology​, Douglas G. Bell and Tom M. McLellan said they had focused on 19 civilian and two military subjects (15 males and six females) with an average age of 32. Thirteen were regular caffeine users (ingesting greater than or equal to 100 mg/day) and eight were considered non-users (ingesting less than or equal to 50 mg/day).

Caffeine was primarily ingested in the form of coffee. Subjects were asked to refrain from heavy exercise and alcohol for 24 hours before each trial, and caffeine consumption was halted 12 hours before the events.

The subjects completed six randomised exercise rides to exhaustion at 80 per cent of maximal oxygen consumption on a cycle ergometer after ingesting either a placebo or 5 mg/kg of caffeine. Exercise to exhaustion was completed once per week at either one, three or six hours after placebo or drug ingestion. Blood samples were taken from each subject.

Bell and McLellan found that caffeine improved the time to exhaustion from 24 minutes during the placebo trials to 28.8 minutes, although the improvement was greater in the non-users of caffeine. Non-users also benefited longer from the caffeine effect, which was detected six hours after ingestion.

Heart rates were higher for non-users throughout the trials, as were glucose levels prior to exercise. Caffeine consumption elevated the levels at a slight but significant rate. Caffeine also produced a small but significant increase in oxygen consumption after 15 minutes of exercise for both users and non-users.

The change of caffeine concentration in plasma above the baseline value was the same for users and non-users following caffeine intake. For the one-hour trial, however, caffeine concentration increased significantly throughout exercise, whereas it remained constant in trials conducted at three and six hours after ingestion. Generally, the concentration increase one and three hours after caffeine intake was greater than that after six hours.

The researchers claim that these findings suggest that occasional drinkers of coffee benefit more from the caffeine boost than regular users, although the latter group will still benefit from the first cup of coffee, However, neither group derives a significant boost from excessive consumption and less is definitely more when it comes to caffeine.