The Influence of Fluid Intake on Mental Performance

Inclusion Criteria: * Individuals who… * …are free-living, who fully understand and agree to the objectives of the study, who gave, signed and dated informed consent, which includes compliance with the requirements and restrictions listed in the informed consent form and in this protocol. * …are 'healthy' men and current combined oral contraceptive (COC)-using women (having taken the COC for at least 3 months and should they be eligible and willing to participate, are happy to continue taking the same COC during the duration of their participation). * …are aged 18-35 years * …engage in \< 5 hours of vigorous exercise a week. This information will be obtained during the initial study brief and using the health screening questionnaire. * …have a habitual fluid consumption of ≤ 1.6 L/day OR ≥ 2.9 L/day for men and ≤ 1.5 L/day OR ≥ 2.5 L/day for women, verified by a 7-day fluid intake record. * …have a day-to-day variation in fluid intake of ≤ 25%. * …have a verified UOsm aligned with the following: * IF habitual low TFI, i.e., TFI ≤ 1.5 L/day for women OR ≤ 1.6 L/day for men have a UOsm ≥ 500 mOsm/kg. * IF habitual high TFI, i.e., TFI ≥ 2.5 L/day for women OR ≥ 2.9 L/day for men have a UOsm \< 500 mOsm/kg. * …have a day-to-day variation in UOsm ≤ 28%; UOsm screening will be determined during verification and intervention periods. * …have access to a domestic fridge at home so that urine samples can be stored prior to laboratory visits. * …have a body mass index (BMI) \< 30 kg/m². * …have an IOS or Android smart phone and therefore the ability to download the HidrateSpark mobile application. Exclusion Criteria: * Individuals who… * …are vulnerable, as defined by individuals whose willingness to volunteer in the study may be unduly influenced by the expectation, whether justified or not, of benefits associated with participation, or of a retaliatory response from senior members of a hierarchy in case of refusal to participate (for example, if members of a group with a hierarchical structure linked to the investigator or to the sponsor, subordinate laboratory personnel, and employees of the investigator or sponsor. * …are not able to answer questionnaires by writing whatever the reason. * …have a loss of personal liberty, by administrative or judicial decision. * …engage in \> 5 hours of vigorous exercise a week. * …engage in endurance exercise bouts lasting longer than 40 minutes. * …do not have access to a domestic fridge at home so that urine samples can be stored prior to laboratory visits. * …do not have an IOS or Android smart phone and therefore do not have the ability to download the HidrateSpark application. * …are currently participating in another relevant clinical study (e.g., likely to influence hydration status or stress responsivity). * …have completed the TSST before. * …are expected to be living in the same home as a current participant. Health screening Individuals who… * …have had any surgery or medical procedure requiring a general anaesthesia in the preceding 4 weeks, or who plan to have one during the study. * …have an ongoing clinically diagnosed medical condition. * …are currently taking prescribed medication for an ongoing medical condition. * …are clinically diagnosed with one of the following psychiatric disorders known to influence stress reactivity: melancholic or atypical depression; panic disorder; obsessive-compulsive disorder; schizophrenia; social or generalised anxiety disorder; autism spectrum disorder; post-traumatic stress disorder; psychosis. * …have a clinically diagnosed sleeping disorder. * …are a current smoker (including e-cigarettes and vapes). * …have had a respiratory or gastrointestinal infection within the last 2 weeks. * …have a clinically diagnosed eating disorder. * …have a special medicated diet (e.g., for obesity, anorexia, metabolic pathology). * …have changed their dietary habits within the last 4 weeks or plan to change their diet during the study (e.g., start of a high-fibre diet, intentionally reducing or increasing calorie intake). * …take dietary supplements known to influence stress reactivity (e.g., high-dose caffeine). * …are women not currently on COC (e.g., naturally menstruating, or on progestogen-only forms of contraception, such as IUS, mini-pill, implant or injection). * …are women currently using COC but for \< 3 months prior to reading the participant information sheet or not planning to continue taking COC during their potential participation in the study. * …are pregnant women or women planning to become pregnant during the study or are breast-feeding women. Total fluid intake screening Individuals who… * …have a daily total fluid intake of: * Men: between 1.6 and 2.9 L/day. * Women: between 1.5 and 2.5 L/day. * …have a day-to-day variation in fluid intake of \>25%. * …have a UOsm of: * Men: \< 500 mOsm/kg if habitual TFI is ≤ 1.6 L.day or ≥ 500 mOsm/kg if habitual TFI is ≥ 2.9 L/day. * Women: \< 500 mOsm/kg if habitual TFI is ≤ 1.5 L/day or ≥ 500 mOsm/kg if habitual TFI is ≥ 2.5 L/day. * …have a day-to-day variation in UOsm \> 28%. * …habitually consume \> 1.3 L/day of caffeinated hot beverages (e.g., tea, coffee; 1.3 L equates to 4 average sized mugs) as estimated by a fluid intake record questionnaire. * …consume ≥ 91 units of alcohol per month (equivalent to more than 3 pints of beer per day or more than 3 large glasses of red wine per day).