SECOND

What's your beverage of choice? If you said water, then congratulate yourself for recognising its importance in maintaining your body's fluid balance. It is said that water is the body's most important nutrient. The body needs more water each day than any other nutrient. Without water you can survive only few days.
Water has been recognised for a number of functions in the body involving the transportation of nutrients, the participation in metabolic reactions, the maintenance of body function, and the regulation of body temperature.  Research shows that water may affect mood and concentration. Results in a recent study indicated that mild dehydration in females increased perception of task difficulty, lowered concentration, and increased headache symptoms. In this study, 25 healthy females (age 23 ± 0.6 y) participated in 3 experiments separated by 28 days. In 2 of these dehydration was induced via walking on a treadmill with or without a diuretic pill. The women were given a number of tests in order to measure their concentration, memory, and mood when they were dehydrated and when they were not. Most aspects of cognitive performance were not affected by dehydration. However, they did have an increase in perception of task difficulty and lower concentration. In addition females were more fatigued. 

The results of the study highlight the importance of drinking water. We should drink at least 8-10 glasses of water daily. You will feel dehydrated by the time you become thirsty. And remember to consume moderate quantities of water both prior and after exercise in order to avoid mild dehydration...

Sources:British Journal of Nutrition 2012