People who drink less water show a stronger cortisol response to stress, a Liverpool John Moores University study has found. Researchers compared 16 participants who drank under 1.5 litres of water a day with 16 who met daily guidelines for fluid intake. Hydration was monitored for seven days using urine and blood tests before a lab stress test. The stress test involved an impromptu job interview, followed by a mental arithmetic task, with cortisol measured from saliva samples before and after. The low-fluid group had higher cortisol levels, while both groups reported similar levels of anxiety and showed similar increases in heart rate.

Researchers used EFSA guidelines for reference: 2.5 litres a day for men and two litres for women. The UK Eatwell Guide suggests about 1.5 to two litres daily. The team noted that thirst did not differ between the groups. Professor Neil Walsh, from LJMU’s School of Sport and Exercise Sciences, described the test as ‘really flustering’. He noted that while low daily fluid intake is linked to poorer hydration, it was not known whether stressing those individuals would trigger a larger cortisol response.

Saliva samples were taken before and after the stress test to measure cortisol, the body\’s main stress hormone. The study found higher cortisol levels in the low-fluid group and concluded that further work is needed to confirm whether increasing water intake can blunt responses to everyday stressors. The research is published in the Journal of Applied Physiology.