Dietary water
Water is the most abundant and most frequently overlooked component of food. The water content of solid foods is variable, ranging from ‘wet’ foods such as fruits and vegetables at 80–95% water content through raw lean meat at about 60% to ‘dry’ foods such as nuts at about 5%.
Balancing our daily water needs
The figures shown are for an average adult person living in a temperate climate who has an inactive lifestyle and a balanced diet. For an active New Zealand teenager in the summer months, the sweat and breathing output figures could increase markedly, pushing the water input ‘water we drink’ figure over 2.5L.
Consumption of solid foods makes a considerable contribution to the total water intake. For an average inactive adult male with a well balanced diet, the intake from solid food alone is estimated to be 1 litre per day.
Daily water requirements
Water is essential for the human body to function properly. Deprive the body of water and dehydration sets in.
Being dehydrated can degrade physical and mental functions:
2% dehydration can cause daytime fatigue, inability to concentrate on mental tasks, light-headedness and nausea.
4–5% dehydration can result in a 20–30% reduction in work and exercise performance.
10% dehydration will produce a significant health risk.
15% dehydration is likely to result in death.
Water is lost from the body through breathing, perspiration and waste removal. These losses are counterbalanced by consumption of water via beverages, water present in solid food and water production via respiration (metabolic water).
Importance of water
Water is essential to the functioning of every cell and organ system in the body. It plays a key role in all of the chemical processes that occur in the body. For example, in the digestion of foods such as carbohydrates and proteins, water is involved in the chemical breakdown of the bonds holding the basic building blocks of these macronutrients together. This process is called hydrolysis
Hydrolysis
The addition of water breaks down the bonds holding a starch molecule (a carbohydrate) together, producing single glucose molecules.
Water is an excellent solvent, and this allows it to act as a carrier fluid within the body for a wide range of substances. For example, glucose molecules produced from the digestion of starch weakly bond to water molecules that are part of the bloodstream, which distributes these energy-rich molecules to all parts of the body.
Water plays a key role in the thermoregulation of the body through perspiration. Its high specific heat capacity means that a relatively large amount of heat energy is needed to evaporate the thin layer of perspiration that forms on the skin in hot conditions or during exercise. This ‘loss’ of body heat energy helps to keep the core body temperature at a steady 37°C. The thermoregulation system in adolescents is still undergoing a maturation process, so they are at more risk of dehydration, especially when exercising.
Without water, the functioning of all organ systems within the body is compromised. Depending on conditions, the body usually cannot survive longer than 1 week without water.
Water’s special properties
The composition and shape of the water molecule are responsible for its unique properties that make it life’s essential ingredient.
Water molecules have a slight positive charge on one end and a slight negative charge on the other, which causes the hydrogen atoms in one water molecule to be attracted to the oxygen atom in another water molecule. This attraction is known as hydrogen bonding.
It is the polar nature of the water molecule that allows it to function in a multitude of chemical reactions, to bind to other polar molecules and to serve as the universal solvent in living systems.
Sugar dissolving in water
Polar water molecules are attracted to the hydroxyl groups present in the sugar molecule. The sugar molecules are surrounded by water molecules and then carried off into solution.
Water molecules can hydrogen bond to other polar molecules. For example, food macromolecules like carbohydrates (sugar) have polar hydroxyl (OH) groups that can hydrogen bond to water molecules. As weak hydrogen bonds form, energy is released and this is used to break up the sugar crystal allowing it to dissolve. The sugar molecules are surrounded by water molecules and then carried off into solution.
Related activity idea
Students quantitatively analyse carrot and banana for moisture content and total solids using and comparing two drying methods in the activity Water content of foods.
Useful link
More information from The New Zealand Nutrition Foundation on the role of water and fluids in our diet.
