Water is essential and vital to life. Plain water is the healthiest choice for hydration, and you should drink enough water to replace the water you lose.
On average, a healthy, sedentary adult living in a temperate climate needs to drink around 1.5 liters of water per day to stay hydrated(*). That more or less breaks down to the old rule of thumb of 8 glasses**. Even though we get water from food and other beverages, it’s not really sufficient to maintain the balance of water that our bodies need (Jequier, 2010).
In fact, our bodies are mostly water—newborns are 75% water, an average adult is 60% water, and a senior is roughly 50% water.
Mild dehydration can occur when we lose even just one percent of our body weight due to insufficient water intake, and can result in symptoms ranging from increased heart rate, decreased blood volume, dry mouth, sleepiness, and at greater levels of dehydration, decreased cognitive function (Mayo Clinic, 2015 - Grandjean AC & Grandjean NR, 2007).
Juice and other sugar-sweetened beverages can be drunk on occasion, but they should not be your main source for staying hydrated all day long, because they are a major source of calories and over-consumption of sugar can have a negative impact on your health. Because water has zero calories and zero sugar, it’s the healthiest, safest way to meet your body’s basic needs.