There is not any optimal temperature for drinking water. It is different from man to man and place to place, dependng on the habitat or climate one lives/works in.....So the ideal temperature for water to consume is the temperature that your body lives at ease or soothingly ie. not more and less temperature that it feels uneasiness or your body is affected health wise!
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Depending on what you want to do. If you are looking to lose weight, drink it cold. Cold water helps to speed up the process of motabolisum because it works the digestive muscles harder. This is thought to help in weight loss. OR room temp is ok