People live, work and exercise in different environments and climatic conditions, all of which can influence daily fluid losses and water requirements.
European dietary reference values for total water (foods plus fluids) are 2.0 litres for females (≥ 14 years) and 2.5 litres for males1. However, these Adequate Intakes (AI) apply only to moderate physical activity levels (PAL 1.6) and moderate environmental temperatures1. While the requirement for water is probably lower than the AI for some individuals, or in colder environments where sweat losses are low or absent, in most cases requirements will exceed the AI due to water losses from sweat and respiration. Here we will discuss the various circumstances where water needs may be higher than the AI.
Air temperature and humidity vary greatly depending on the region and season, even across Europe. Warm and/or humid climates raise body temperature and the normal response to this is sweating, which leads to cooling through evaporation. This influences water losses and thus, requirements.
Average sweat losses during physical activity are 1-2 litres per hour but can be as low as <0.5 litres per hour, or even as high as >3 litres per hour2.
Certain types of workers carry out their normal duties in hot environments and are, therefore, at risk of heat stress. Examples are industrial workers, building labourers, road workers and farmers.
In temperatures of 25-40°C (77-104°F), sweat losses can reach 0.5-1 litres per hour, resulting in fluid requirements of up to 12 litres per day3,4. However, it is not just water that is lost during heat exposure, this is because sweat contains body salt. Research suggests that salt losses over a typical working shift could be as high as 10-15g, potentially increasing the risk of hyponatraemia (low blood sodium) if the salt is not replaced via foods and fluids. Left untreated, hyponatraemia can have serious health consequences since sodium is vital for blood pressure regulation.
Several studies have found that a high proportion of industrial workers are inadequately hydrated at the start of their shifts4, 5 leading to concerns about their health and wellbeing. Experts have called for workers under heat stress to have access to education programmes that aim to promote good hydration throughout working shifts4, 5.
Key points for healthcare staff delivering these types of programs are:
The effect of cold temperatures on water needs is often underestimated. Although sweating may be expected to be minimal when, wearing heavy or impermeable clothing whilst exercising in the cold, such activity can lead to unexpected sweat losses6.
In cold and dry environments, increased water loss occurs through breathing, with the impact becoming significant for those exposed to such extreme conditions for several hours per day. If the body becomes cold, cold-induced diuresis may lead to an increased production of urine with a low urine specific gravity (USG ~1.009 g/ml)1. It should be noted that, under such conditions, a low USG could be misinterpreted as indicating euhydration (adequate hydration).
Water losses increase at high altitude because the air becomes drier. This stimulates hyperventilation, which tends to elevate respiratory water losses. Furthermore, exposure to the thinner air of high altitudes lowers blood oxygen levels, leading to hypoxia-induced diuresis7, 8, often occurring within a few days of arriving at higher altitudes. All of this means that workers or climbers at high altitude need significantly more water than those living closer to sea level.
Research shows that people climbing at heights of 1600m to 4393m have an average water turnover of 7.1±1.1 L (95±18 ml/kg) per day, which is substantially higher than in sedentary adults8. While, there is no consensus on the exact percentage increase in water requirements at altitude, there are useful assessment tools for identifying dehydration in those working or climbing in these environments, e.g. short-term weight loss, urine colour, urine specific gravity7.
In conclusion, temperature, humidity and altitude can increase water needs compared with general recommendations. Healthcare professionals, such as physicians and dietitians, should be aware that climate and environment can influence hydration status and should account for this when giving advice. Furthermore, individual variations in sweat losses make general recommendations difficult to implement, suggesting a need for individual hydration assessments and tailored programmes for those living, working or exercising under extreme conditions.
The EHI thanks to Dr. Hans Braun, EHI Science Advisory Board member, German Sport University Cologne and German Research Centre of Elite Sport (Germany), for providing the content used as a basis for the text in this section. His full scientific article entitled Effects of climate and Environment on water needs can be found in our section Selected Articles.