Biological hydration refers to the internal fluid balance, where water is absorbed into the bloodstream and distributed to cells. The idea that swimming hydrates the body via external absorption is a common misunderstanding. Despite being fully submerged, the surrounding water does not contribute to this internal state. The direct answer is no; swimming does not hydrate you internally.
The Skin Barrier and Water Absorption
The human body possesses a defense system against unwanted water influx, located in the outermost layer of the skin. This layer, known as the stratum corneum, functions as a nearly waterproof barrier to maintain internal homeostasis. It is structured like a “brick-and-mortar” wall, composed of dead skin cells (corneocytes) and a specialized lipid matrix (ceramides, cholesterol, and fatty acids).
This lipid-rich construction prevents excessive transepidermal water loss (TEWL) from the inside out, and also resists the absorption of external water inward. The body strictly controls its internal salt concentration, which differs from the concentration in pool or ocean water. If external water could pass freely, osmosis would cause massive fluid shifts, potentially leading to cell damage or dangerous swelling. While the outermost layer of skin can become saturated and temporarily soften, this external moisture does not reach the internal systems to provide systemic hydration.
Why Swimmers Still Experience Water Loss
Even while surrounded by water, the body actively loses fluids during a swimming session, leading to a risk of dehydration. This fluid loss occurs because the body’s temperature regulation mechanisms are still operating, and the water masks the most obvious signs of exertion. Swimmers sweat just as they would during a land-based workout, but the surrounding pool water instantly washes the sweat away, preventing the perception of dampness or heat.
Fluid loss also occurs through the respiratory system as water vapor when breathing heavily during intense exercise. The hydrostatic pressure of being immersed in water increases blood flow to the central organs, which stimulates the kidneys to increase urine production, a phenomenon known as immersion diuresis. The cooling effect of the water can trick the body’s sensory receptors, causing a delay in the feeling of thirst. This delay can lead to a greater fluid deficit before the swimmer recognizes the need to drink.
Essential Hydration Practices for Swimmers
Maintaining a proper internal fluid balance requires a conscious effort before, during, and after leaving the water. Swimmers should begin their session well-hydrated, consuming 16 to 20 ounces of water 60 to 90 minutes before getting in the pool. This pre-loading ensures the body starts with optimal fluid levels to manage the demands of the workout.
During the swim, take short breaks to consume 7 to 12 ounces of fluid every 15 to 20 minutes, especially during prolonged or high-intensity sessions. For workouts lasting longer than 60 minutes, replacing electrolytes is relevant to restore minerals like sodium and potassium lost through sweat. Following the session, replenish any lost volume by drinking 16 to 24 ounces of fluid for every pound of body weight lost. A simple way to monitor hydration status is to observe urine color, aiming for a pale or light yellow shade.