Dehydration does not influence cardiovascular reactivity to behavioural stress in young healthy humans.

Enhanced hydration increases the human cardiovascular reactivity to mental stress. If reduced water intake has the opposite effect, this would suggest controlling for water deprivation when studying such responses. Blood pressure, heart rate and parasympathetically dominated beat-to-beat heart rate fluctuations were assessed during resting baseline and mental stress. Two challenging cognitive-motor tasks, a 5-Choice Reaction Time Task (CRTT) and a Paced Auditory Serial Addition Task (PASAT), served as mental stress tests. Eight female and eight male volunteers were examined twice, after 24 h of water deprivation and after normal water intake (counterbalanced order, 7-day interval). Water deprivation resulted in moderate dehydration with a mean 2.6% decrease of total body weight. Dehydration did neither affect baseline blood pressure, heart rate, nor blood pressure reactivity to mental stress. However, dehydration slightly (-1.2 bpm) diminished heart rate reactivity to the PASAT (P = 0.03) and increased beat-to-beat heart rate fluctuations in response to the CRTT (P = 0.05). Dehydration intensified CRTT- and PASAT-induced reductions of beat-to-beat heart rate fluctuations in females (gender x dehydration interactions: P = 0.04-0.05). Moderate dehydration induced by water restriction has no effect on blood pressure reactivity to mental stress. The effects on heart rate reactivity are small. However, stress-induced parasympathetic withdrawal may be fortified during dehydration in females, which suggests controlling for water intake when studying such responses.