We need about 1 liter or quart of water for every 1000 kcalories that we eat in order to hydrate the food molecules as they are separated apart i.e. digested in our intestine. Our bloodstream will not absorb nutrients that are more dense than our bloodstream, so water is required to dilute these food particles: digestion is a dehydrating process. You might think that drinking water only with meals would therefore be sufficient, but any excess that you drink during a meal will go straight to your bladder, and then as you digest food over the next couple hours you will dehydrate. So while drinking at meals is fine, to be hydrated you must drink in between meals. Drinking large amounts of water during meals can slow digestion for a few minutes while the water empties from the stomach, but there are no negatives associated with that (the next time someone suggests to you that drinking water with meals is bad, ask them for any evidence supporting this: they will be unable to provide you with any besides anecdotal hearsay). In fact, if you are dehydrated, it is highly recommended that you drink water when you eat. Keep in mind that drinking calories during meals is adding to the calories you are eating, forcing a larger insulin response and driving more of the calories in your meal into fat cells instead of active tissues, lowering your metabolism and energy levels.
Because of the calories in water, soda and fruit juice (about 100 kcal/8 oz cup), these fluids are only half as hydrating as water. Therefore, you would need to drink nearly 2000 kcals of these fluids in order to hydrate 2000 kcals of food you are consuming: Not a good idea since this would result in a gain of 1 lb of body fat every 2 days. Low-calorie sweeteners (such as in diet soda) have a greater than zero long-term cancer risk. Coffee and tea are high in xanthines (caffeine in coffee, other xanthines structurally similar to caffeine in tea), which are weak diuretics: More than 2 regular cups of coffee (~200 mg xanthines, such as caffeine) will dehydrate you. Any fluid containing more than 2% alcohol is dehydrating as well. For these reasons, milk, soda, fruit juice, fluids containing low-cal sweeteners, coffee, tea and alcohol should be consumed in moderation i.e. no more than 1-2 regular servings per day. If you are thirsty, drink water first, and then consume whatever beverage you enjoy drinking. If you want to drink something for its taste, or its caffeine content, do the same: Always water first. Drink at least an equal amount of water compared to the other fluids in your day. And carry a water bottle: if you only drink at meals you will spend roughly 20 of your 24 hour day dehydrated.
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