Summary
Highlights
Water and electrolytes are essential for brain, nerve, and muscle function. Neurons in the brain and nerves use electrolytes like sodium and potassium to generate electrical signals (action potentials), enabling communication and muscle contraction. Our bodies don't plug into an outlet; instead, we use charged atoms called ions (electrolytes) dissolved in water to conduct electrical currents. These electrolytes are pumped in and out of cells, creating a charge difference that facilitates electrical signaling.
Water acts as a universal solvent, facilitating chemical reactions, transporting substances through the blood, and regulating body temperature. It also provides moisture and cushioning for tissues and joints. Electrolytes influence water distribution throughout the body, dictating where water flows based on solute concentration. Water is found in three main spaces: intravascular (blood vessels, 7%), intracellular (inside cells, 66%), and interstitial (between vessels and cells, 26%). Imbalances can lead to issues like swelling (edema) or high blood pressure with excess fluid, or hypovolemic shock with insufficient fluid. Drinking too much water can also be dangerous, causing cells to swell.
The goal is to maintain 'euhydration', a normal state of water balance. 'Hyperhydration' is excessive water, while 'hypohydration' (or dehydration) is insufficient water. Water loss occurs in two ways: insensible (unnoticed, like breathing and skin moisture, about 1 liter/day) and sensible (noticed, like sweat, urine, and feces). Urine loss is about 1.5 liters/day, and fecal loss is minimal (100 ml/day). Sweat loss is highly variable, ranging from minimal to over 2 liters/hour during intense activity in hot environments.
A general strategy is to drink slightly more water than needed to ensure mild 'hyperhydration', allowing the kidneys to excrete excess. For someone not exercising or in a hot climate, daily water loss is around 2.7-3 liters. This can be easily replenished, especially since 20-25% of water comes from food. This means drinking just over 12.5 cups of water daily.
During exercise, many variables affect water loss, making a one-size-fits-all approach difficult. However, a key point is that most people can only absorb about 1 liter of water per hour. Therefore, pre- and post-exercise hydration strategies are crucial. Before exercise, consume 5-10 ml of fluid per kilogram of body weight, 2-4 hours prior, especially if already euhydrated. If hypohydrated, an additional 3-5 ml/kg may be needed. During exercise, approximately 1 liter of fluid per hour, taken periodically, is a safe and effective guideline, though individual needs vary based on intensity and environment. After exercise, drink 1.25 to 1.5 liters of fluid for every kilogram of body weight lost to replenish fluids.
For exercises less than 90-120 minutes, sodium replacement is often not necessary if dietary intake is typical, as replenishment can occur through food and fluids afterward. For activities longer than 90 minutes, especially for heavy sweaters or in hot/humid environments, ingesting 0.7 to 1 gram of sodium per hour, mixed with fluids, can be beneficial. Practical methods to monitor hydration include using a urine color chart (with caveats for lighting, food, and medications), and weighing yourself before and after a workout to gauge fluid loss. For greater accuracy, a combination of morning thirst, urine color, and weight can indicate hydration status: one condition (possibly hypohydrated), two (likely hypohydrated), and three (very likely hypohydrated).