Most people find it hard to believe that feeling thirsty for water is a sign that the body is slightly dehydrated.  A dry mouth usually occurs with thirst and is another sign that the body needs hydration.
7. Headaches
If you don’t drink enough water, your body will absorb water from all of your tissues to compensate for the lack of fluids.  Because of that, your brain tissue loses some of its moisture and shrinks, pulling away from the skull. This triggers the pain receptors and results in a headache. At the same time, when you don’t consume enough water, the blood volume drops, reducing the amount of oxygen being carried to the brain. In response to this, the blood vessels in the brain will dilate and the headache will intensify.
8. Fatigue
According to a study conducted in 2011, mild dehydration can cause fatigue, tiredness and lowers the energy.  When your body lacks water, the heart has to work harder to push oxygen and nutrients through the body. So, when you start to feel tired, skip the coffee (which is dehydrating!) and drink water instead.  Coffee, tea, soda and sports drinks are not substitutes for water.
9. Joint Pain
Water is a lubricant to the joints because it allows two bones to slide over each other as the joint moves.  If you don’t drink enough water, the joint will become nutrient deficient, which leads to damage of the cartilage cells.  If your organism is lacking water for an extended period of time, your cartilage will degenerate beyond repair.
10. Weak Immune System
Dehydration increases the concentration of toxins in the blood, which weakens the immune system.  Drinking enough water is very important for the elimination of toxins and helps the immune system to defend from infections.