Dizziness. That unsteady feeling occurs most often during or right after strenuous physical activity, when your body is directing a lot of blood to muscles."Suddenly, your body doesn't have the capacity to get enough blood flow to the brain. At the same time, you're exerting yourself and that increases your body temperature and breathing rate, both of which cause the blood vessels in your brain to dilate," says Bergeron, leading to a dizzy spell.
Headaches. "Any change that alters the body's natural balance, such as dehydration, creates stress and that can trigger a headache. In fact, a headache is your body's alarm system. It's telling you your physiological equilibrium is off," says Juline Bryson, M.D., a neurologist with the Headache Institute at St. Luke's-Roosevelt Hospital Center in Manhattan. Also, when you're liquid-deprived, say headache experts, the blood becomes more concentrated, which causes inflammatory proteins circulating in the blood to irritate nerves surrounding the brain.
Impaired memory and concentration. While the reasons for these symptoms aren't yet clear, researchers at the University of Connecticut Human Performance Laboratory note that dehydration causes changes in electrolyte balances in the blood, which directly affect parts of the mind responsible for reasoning. Changes in electrolyte levels also can alter brain levels of serotonin, which influences mood.
Constipation. When we're dehydrated, the colon redirects fluid into the bloodstream. This leaves us with a harder stool that's harder to pass, says Nadya Swedan, M.D., a Manhattan-based rehabilitation specialist in private practice. And ironically, if you're dehydrated, you only worsen the situation if you try to counter constipation by eating high fiber foods like fruits, vegetables and whole grains. Dietary fiber stimulates the wave-like contractions that move food through the digestive tract, but only in the presence of the right amount of fluid. "If you're not drinking enough, these high fiber foods just sit in your gut," Swedan says.
Holly St. Lifer writes about health and fitness. She also teaches journalism at New York University.