The 2-litre-a-day water rule challenged by a major Japanese study

The advice repeated for decades, suggesting that everyone should drink two litres of water per day, is being called into question by researchers. A study conducted by the National Institute of Biomedical Innovation, Health and Nutrition in Japan shows that hydration requirements vary considerably from person to person, depending on age, sex, level of physical activity, health status, and environmental conditions.

The research included over 5,600 people from 23 countries and tracked total water turnover in the body – that is, all the water that enters and leaves the body each day. This took into account water consumed directly, water obtained from food, water produced by internal metabolic processes, as well as losses through perspiration, breathing, and excretion. The researchers concluded that there is no single fixed figure that applies to everyone.

Significant differences between groups

According to the data, daily water turnover in adults can range from one litre to over six litres, and in elite athletes it can exceed ten litres. This refers to the total quantity of fluid processed by the body, not merely water consumed as such.

• Young men aged between 20 and 30 recorded the highest values, with an average of 4.3 litres per day.
• Women aged between 25 and 60 had an average water turnover of 3.4 litres per day.
• People over the age of 70 have a significantly lower water turnover, which researchers attribute to changes in metabolism and the reduction of muscle mass that occurs with age.

Physical activity and ambient temperature also influence fluid requirements. Physically active individuals need, on average, one litre of water more per day than those who are sedentary. In areas where temperatures exceed 30°C, the body processes an additional litre of water per day in order to regulate its temperature.

The role of altitude and the body's ability to self-regulate

The study also shows that altitude has an impact: at high altitudes, water loss through breathing increases, so for every 1,000 metres ascended above sea level, hydration needs rise by approximately 500 millilitres. Humidity, diet, and lifestyle also contribute to the way in which the body manages water.

The researchers further highlight that the body has a remarkable capacity to self-regulate its fluid balance: when fluid intake is low or perspiration is heavy, the kidneys reduce and concentrate the amount of urine produced. Conversely, when more water is consumed than necessary, the surplus is eliminated quickly and the urine becomes noticeably lighter in colour.

Rather than a rigid rule applicable to everyone, the researchers recommend an approach tailored to the body's own signals and to the environmental conditions in which each individual finds themselves.