Water is an essential nutrient to the organism, and we get the majority of our water from the food and drinks we consume. The minimal amount of water recommended per day is around 1.5 L.
What are the main roles of water?
In our organisms, water has numerous physiological roles. Water in particular, plays a vital role in the transport mechanisms within the body. For example blood, lymph and all other liquids are composed of mainly water. Another essential role is in the elimination of waste products, notably via the kidneys.
Moreover, water is largely involved in the digestive, thermoregulatory and reabsorption mechanisms, and is also an essential part of the environment of many biological activities within the body.
The human body is mainly composed of water, with water representing 75% to 80% of total body mass at birth. As we grow older, this proportion decreases to around 60% to 70% in adult men, and 50% to 60% in adult women. This notable difference is due to the fact that the female body holds onto more fat, leading to a higher fat mass in women. These percentages also tend to decrease with age. In the average adult weighing 70kg, the total volume of water in the body is at around 42 L.
Where is water stored in our bodies?
Water is present in all of the organs and tissues in our bodies, but in different concentrations. The tissues least rich in water are adipose tissue (body fat) at around 10% concentration, and bone tissue at around 22% concentration. Other tissues in the body are relatively similar, and the concentrations in water tend to remain between 70% and 80%. Water content of tissues and organs is relatively stable, but this stability is a facade of all of the mechanisms at play, as in reality, water is responsible for many of the vital exchanges which take place within these tissues and the body. Just think about how around 180 litres of water circulate through your kidneys every 24 hours !
What are our daily requirements in water?
The body requires around 2.5 to 2.6 litres of water per day. In reality, we drink less than this amount, as our requirements for water are divided into two parts. Firstly, we have exogenous intake. Which corresponds to the 1L of water we get from solid food and the 1.5L we get from drinking. Secondly, there is water of endogenous origin, which is produced by the oxidative metabolism, and represents around 700mL of water.
How to maintain the balance between water in and water out?
Physiologically, water loss in the body is balanced out by water intake. There are four main routes of water loss from the body :
– Losses linked to renal : the kidneys release around 1.5 mL of water per day, and this quantity is determined by the water ingested and the hormonal environment.
– Faecal losses, which represent around 200 – 300 mL. This rate is mainly constant, but can change and greatly increase, particularly during episodes of diarrhoea.
– Pulmonary losses represent around 400mL of water daily, and are relatively constant. This amount can change during intense physical exercise, increasing ventilation and therefore the volume of loss.
– The final route in which water can leave the body is through the skin, which is responsible for perspiration and sweating. Perspiration is the natural mechanism in which water moves from the inside to the outside of the body, at a rate of around 100 ml per day, and maintains the skin’s hydration. Perspiration is also linked to thermal regulation, which represents a loss of around 400mL of water. Of course these numbers vary depending on environmental conditions, such as physical activity and temperature.
What is dehydration ?
Dehydration is defined as a water deficit within the body, due to either water loss, a reduced intake of water or both. Dehydration is expressed in the body by an increase in plasma osmolarity.
The sensation of thirst, which is a warning sign, is triggered when the body is already dehydrated. In adults, when water deficit reaches 8% of body weight, the prognosis could be life threatening, and corresponds to an extreme state of dehydration.
Dehydration is most often progressive and is accompanied by a variety of alterations, symptoms and signals in humans. When water loss is in the order of around 1% of total body mass, we can observe a decrease in the body’s physical ability. When this loss is within the ranges of 1 – 2%, cognitive performance starts to get impaired. Between 2 – 4% results in a changing of the cardiovascular s, in particular increased heart rate and decreased blood pressure. Water loss ranging above 4% of total body mass results in mood disorders and the appearance of headaches.
Young infants under 1 year of age are the most at risk of dehydration compared to other age ranges. Similarly, seniors over the age of 65 are the most at risk of complications linked to dehydration, compared to the younger population.
The origin of dehydration can be linked to a decrease in intake which can be caused by anorexia or fasting for instance, as this causes intake deficiency. Dehydration can also be caused by increased losses, which can be linked to digestive disorders, particularly infectious diarrhoea or burns. Hyperventilation, particularly in children, can also lead to dehydration.
Nutritional engineer at Nutrimis