Water is the single most essential nutrient in the human body. It supports thermoregulation, transport, metabolic reactions, and cellular function. In the new IB SEHS specification, students are required to understand the functions of water, the regulation of fluid balance, mechanisms of water loss during exercise, the consequences of dehydration and hyperhydration, and how hydration status can be assessed and maintained in sport. This knowledge is foundational to health, safety, and performance — particularly in endurance and heat-stressed environments.

Water makes up approximately 60–70% of body mass, distributed between intracellular and extracellular compartments. It serves multiple physiological roles: maintaining blood volume and pressure, enabling thermoregulation through sweating, facilitating digestion and nutrient absorption, and acting as a medium for biochemical reactions. Even mild dehydration can impair these functions and negatively affect both cognitive and physical performance.

During exercise, especially in hot or humid conditions, the primary mechanism of heat loss is evaporation through sweat. Sweat is produced by sweat glands and drawn from plasma, meaning excessive sweating can significantly reduce blood volume, disrupt electrolyte balance, and impair cardiovascular and thermoregulatory efficiency. The more intense or prolonged the activity, the greater the rate of water loss.

The body regulates water balance through a finely tuned system involving the hypothalamus, pituitary gland, and kidneys. The hormone antidiuretic hormone (ADH) is central to this process: when plasma osmolality rises (indicating dehydration), ADH is released, prompting the kidneys to reabsorb water and reduce urine output. Conversely, during periods of adequate or excess hydration, ADH secretion is suppressed, allowing more water to be excreted.

Dehydration occurs when water loss exceeds intake. Even a 2% drop in body mass from fluid loss can impair endurance, increase perceived exertion, slow reaction time, and heighten the risk of heat-related illness. Dehydration reduces plasma volume, increasing heart rate and decreasing stroke volume, which limits oxygen delivery and thermoregulation. Hyperthermia can result, especially in warm environments where sweating is the primary cooling mechanism.

Hyperhydration, while less common, can be equally dangerous. Excessive intake of hypotonic fluids (e.g. plain water) without adequate electrolyte replacement may dilute plasma sodium, leading to hyponatraemia — a potentially life-threatening condition characterised by confusion, headache, nausea, and in severe cases, seizures or death.

Monitoring hydration status is essential for athletes. Common strategies include urine colour, urine specific gravity, body mass changes pre- and post-exercise, and subjective thirst ratings. Best practice involves pre-hydrating before activity, drinking to thirst during, and replacing fluid and electrolytes after prolonged or intense exertion.