See What Happens When You Drink Water On an Empty Stomach

Weight gain. When dehydrated, cells are depleted of energy. They then have to rely on energy generation from food rather than water. People thus tend to eat more when, in reality, the body is thirsty. In addition, the body will not metabolize fat unless the body is adequately hydrated to safely remove dangerous toxins that are often stored in fat cells.
Skin disorders. The skin is the largest elimination organ in the human body. In a dehydrated body, the first site for water conservation is the skin. Dehydration impairs the elimination of toxins through the skin and makes it more vulnerable to all types of skin disorders, including dermatitis and psoriasis, as well as premature wrinkling and discoloration.
Joint pain or stiffness. All joints have cartilage padding, which covers the bone structures in the joint, providing necessary lubrication. The cartilage itself is composed mainly of water. When the body is dehydrated, cartilage is weakened and joint repair is slow. According to Dr. Batmanghelidj, the joints in which the pain is felt depends on where the localized drought has set in.
Bladder or kidney problems. As with all of the organs of elimination, when the body is dehydrated, toxins are not eliminated as well. The accumulation of toxins and acid waste create an environment where bacteria thrive. Thus, when dehydrated the bladder and kidney become more prone to infection, inflammation and pain.