Water is a basic life requirement. Our bodies are composed of (approx) 70% water-practically every system uses water. Blood, brain, breathing, organs, joints, muscles, skin...all of these things and more are dependent upon water. Dehydration is the #1 trigger for daytime fatigue-not lack of sleep!
What may be surprising to some is the link between weight loss (or optimal weight maintenance) and adequate hydration. I read on weightlossforall.com that water makes your metabolism burn 3% faster. In addition, being sufficiently hydrated allows your body to be working at peak performance. That's a desirable outcome.
