The study by Sophie Killer, Andrew Blannin and Asker Jeukendrup, from the University of Birmingham compares the effects of coffee consumption against water ingestion across validated hydration techniques.
Killer et al. explain that caffeine acts as a diuretic when consumed in large doses of ≥500mg, but say research suggests that low to moderate doses in ‘caffeine naïve’ individuals does not induce this effect.
“To our knowledge this is the first study to directly compare the chronic effects of coffee ingestion with water against a wide range of hydration assessment techniques,” they write.
“It is estimated that 1.6bn cups of coffee are consumed worldwide every day. Thus it is of interest to know whether coffee contributes to daily fluid requirement, or whether it causes low-level chronic dehydration.”
Dehydration science doesn’t stack up
Healthy adults are often advised to avoid caffeinated beverages due to dehydration risk, the authors note, but they insist such guidance is based on a “relatively small collection of caffeine studies” published over the past few decades (e.g. Neuhauser-Berthold et al. 1997).
“Our aim was to directly compare the effects of a moderate intake of coffee in caffeine-habituated adults against equal amounts of water across a wide range of hydration markets, including the gold standard TBW measure.”
50 male coffee drinkers (healthy non-smokers aged 18-46) habitually drinking 3-6 cups/day took part in two trials, each lasting three consecutive days.
Physical activity, food and fluid intake were controlled, and subjects drank either 4x200ml of Nescafé Original coffee (with 4mg/kg caffeine) or water, with a suitable gap left between each trial.
Total body water (TBW) was calculated before and after the trial by ingestion of deuterium oxide, while urinary and hematological hydration measures were checked daily, as was nude body mass measurement (BM).
‘Similar hydrating qualities to water’
No significant changes in TBW were found from beginning to end of either trial and between the caffeine and water trials; no differences were observed in hematological markers or in urine, osmolality or creatinine.
No significant differences in body measurement were found between the two trials.
“Our data shows no significant differences in the hydrating properties of coffee and water across a wide range of hydration indices,” Killer et al. write.
“These data suggest that coffee, when consumed in moderation by caffeine habituated males, provides similar hydrating qualities to water.”
But Killer et al. note the limitations of their study design, since a metabolic ward would have controlled the environment and its participants, and say a decaffeinated coffee condition would have been interesting.
Title: ‘No Evidence of Dehydration with Moderate Daily Coffee Intake: A Counterbalanced Cross-Over Study in a Free-Living Population’
Authors: Killer, S.C., Blannin, A.K., Jeukendrup, A.E.
Source:PLOS One, published online January 9 2014, doi: 10.1371/journal.pone.0084154