Dehydration is an ailment with which all runners are familiar. Not drinking enough water, especially on hot or humid days, can lead to a variety of health problems. On the other hand, it is also possible to drink too much water, leading to an affliction called hyponatremia. Basically, hyponatremia occurs when the sodium content in your body becomes diluted, causing your body's water levels to rise and its cells to begin to swell. The symptoms of this illness include confusion, nausea, headache, etc. In incredibly rare cases, hyponatremia can even cause death.
But how rare is hyponatremia? A paper by Angus Whitfield in The British Journal of General Practice analyzes several studies and notes that it is more common than one would expect. One study of the Boston Marathon found that hyponatremia was prevalent in 13% of finishers. Most of those cases were mild, but there were some at critical levels.
Whitfield goes on to note that marathoners over 4 hours are significantly more likely to experience hyponatremia than those who ran faster. He recommends that runners be cautious about not drinking an excessive amount of water during exercise. Those runners that take a bit longer to complete marathons need to be especially vigilant about not over-indulging in water. When you are out running for 4-5 hours, it is easy to be concerned that you might become dehydrated. Keep in mind that it is ok if you are not replacing 100% of the fluids that you lose. Your body will function perfectly well unless you lose massive amounts of water.
With all that said, I have never met anyone who has ever fallen ill due to drinking too much water. Dehydration should be a concern for all runners and a fear of overhydrating should NOT stop you from drinking all together. Especially when it is hot or humid, make sure that you are drinking water, but at no point should you need to chug water. Also, be sure to take in drinks containing electrolytes in addition to just water to ensure that your sodium levels do not fall too low.
There is no need to overthink hydration. If you are thirsty, then drink some water. If you are not thirsty, do not force excess fluids down just because you are concerned about dehydration. Know that both dehydration and overhydration can lead to health issues, but as long as you maintain a balanced approach to hydration you will never have a problem.