How much water should I take in during the day?
You should take in 35mL/kg of body weight. However, things such as training environment, duration of training, and type of training can change this.
Fluid intake from ALL non-alcoholic sources counts towards daily fluid intake.
This section is adapted from The Institute of Medicine of The National Academies book: “Dietary Reference Intakes For Water, Potassium, Sodium, Chloride, and Sulfate” unless cited otherwise.
Please note that these are averages, you may need more or less.
- Adequate Intake (AI) for men: 3.7 liters/day – of which 3L should be from fluid.
- Adequate Intake (AI) for women: 2.7 liters/day – of which 2.2L should be from fluid.
Adequate intake does not mean maximum or minimum it is the amount of fluid you need to be adequately hydrated. If you are thirsty or consume more fluid than this, that is NOT a problem. Typically the kidneys can process 600-1000mL/hour. The average adult has an insensible diffusion (normal non-exercise sweating) rate of ~450mL/day (~18.75mL/hour). You also lose fluid via respiration and other vital functions. You can safely consume ~1L of fluid/hour without issue. The upper tolerable limit for water has not been set. However, toxicity has been reported in individuals exceeding 1.0L/hour
People who live or exercise in warm environments also need more fluid. During exercise you can lose up to ~0.72mL/hour. 4.9L of fluid lost via sweat has been documented in desert environments & 2.3L of fluid lost via sweat has been documented in tropical environments.
A good way to gage your hydration status is to monitor your output. Output of 100mL/hour typically equates to a well hydrated person. Output of 300-600mL/hour can indicate excess fluid (hyperhydration), but does not mean that you are have hyponutremia. Output of less than 30mL/hour indicates dehydration.
While monitoring your actual output isn’t the most practical thing, watching the color of your urine output is often the best indicator of hydration status. See attached chart. Note that excess vitamins and minerals can alter the color of urine output.
Drinking to thirst is a common way to stay hydrated. Thirst is triggered by fluid shifts and electrolyte imbalances. When thirst is felt, the average person is already dehydrated by ~2%. While this not an issue for the average person, for an athlete, it can cause performance issues. Especially if the dehydration was caused by fluid loss due to exercise. In studies comparing humans & animals drinking ad libitum, humans tend to under-replace their fluid needs over the short term. Thus, fluid intake evenly spaced throughout the day with a larger emphasis around and training, training may improve athletic performance. However, drinking to thirst is fine for the general population.