Eight Cups of Water a Day is Not Ideal for Everyone

We all know the importance of hydration. You may have heard that drinking eight glasses of water a day is ideal for good health. However, a new study proves that this daily recommended amount is not universal.

According to Dale Schoeller, a nutritional scientist from the University of Wisconsin-Madison, science has never been able to back up the “eight glasses a day for optimal health” rule enough to make it a guideline, adding that much of the water we consume comes from our food and that can confuse the numbers.

While previous studies used mostly volunteered, self-reported stats from select groups of people, this new study was more expansive and objective. It looked at 5,600 individuals from all over the world, as young as eight days old, ranging to 96 years old. Researchers factored in specific environmental elements of the participants, including their age, sex, BMI, education, economic status, and even the weather and altitude where they lived.

Researchers set out to see how much water they drank throughout their life and their water output. They found that, on average, people consumed between one and six liters daily. (Four cups are in one liter, which adds up to between four and twenty-four cups a day). The authors of the study note that this is just an average.

Through “trackable” hydrogen and oxygen isotopes that were in the participants’ water, researchers could see how long it took for the water to work its way through the body, in through the mouth, and out through urine. The hydrogen isotope in the urine showed how much water the participants were replacing, whereas the oxygen isotope revealed how many calories were being burned.

Some interesting findings were that men’s water turnover volume tended to peak in their twenties, whereas women had a long plateau from their early twenties until their mid-fifties. The winner for the most daily water turnover was newborn babies (they replaced roughly 28% of their water daily).

The main environmental factor that played a role in water turnover was physical activity level, with sex coming in a close second. Other factors contributing to more water loss are living in humid climates and being more athletic.

“All things equal, men and women differ by about half a liter of water turnover. As a baseline of sorts, the study’s findings expect a male non-athlete (but of otherwise moderate physical activity) who is 20 years old, weighs 70kg (154 pounds), lives at sea level in a well-developed country in a mean air temperature of 10 degrees C (50 Fahrenheit) and a relative humidity of 50%, would take in and lose about 3.2 liters of water every day. A woman of the same age and activity level, weighing 60 kg (132 pounds) and living in the same spot, would go through 2.7 liters (91 ounces).”

These findings can help people better understand their unique water needs and not necessarily adhere to a generic recommendation without cause.

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