Estimating Hydration Needs:

When talking about health and diet, the topic of hydration often comes up, and rightly so if you think about the fact that the average human body is comprised of 55-70% water.  Water is so essential to human life that without it humans cannot survive more than a few days, so it’s no surprise that there would be questions about optimizing intake. But how much do we need?  

We have all heard the advice to drink 8 glasses of water per day, which converts to just under 2 liters.  Other proposed ways to estimate needs include the Holliday-Segar method which is a weight based calculation and, for example, works out to be approximately 2.7 L per day for a 180-pound individual.  Another popular method is calorie based with a starting point of 1 ml of fluid for every kcal consumed.  To investigate this further the Food and Nutrition Board of the Institute of Medicine (IOM) gathered a panel of experts to evaluate the data. In their 2004 consensus report, they concluded that the majority of healthy adults are able to meet hydration needs by responding to their thirst cues.  The panel also set an Adequate Intake of water for men and women of 3.7 L and 2.7 L respectively which they estimate would meet the needs of most healthy adults. The IOM report highlighted that fluid needs would be individualized to account for activity level, environmental temperature, physical fitness, body size, and health conditions.

If you’re looking at your own intake and thinking you are not likely meeting this don’t panic yet.  The AI for water is coming from all sources in the diet, not just plain water consumption. This means that food sources contribute and population data suggests that Americans typically meet 20% of this daily fluid needs from foods.   Non- water beverages count too, yes, even coffee! Coffee often comes into question due to the diuretic effect of caffeine however in the IOM report they concluded that for individuals with habitual intakes of significant caffeine, the caffeinated beverages appeared to contribute to total water intake similar to non-caffeinated beverages.  That’s good news for us coffee drinkers and one more reason to continue eating plenty of fruits and vegetables.

Fluid content of common foods:

Food Percent Water
cucumbers 96%
celery 95%
lettuce 95%
apples 86%
bananas 75%
eggs 75%
pasta 60%
Ground beef 49%
Olis 0%

Bottom Line:

For the majority of healthy adults drinking when you are thirsty will likely ensure you are meeting your daily hydration needs, but remember to be mindful of increasing you intake when outputs might be greater, for example, warmer temperatures and increased physical activity.  And pay attention to some of the common signs of dehydration including:

  • dark colored urine
  • thirst
  • dry mouth
  • fatigue
  • a headache
  • dizziness
  • confusion