When someone suffers from diarrhea, especially a small child, it can also be tempting to serve them a glass of juice to keep them hydrated. But the sugars in fruit juice (and in pop) have an osmotic effect, requiring water (H2O) for their utilization in the body. These sugars (carbohydrates) are not all absorbed (because juice contains sooo much sugar/carbs), thus the excess sugars/carbs enter the large intestine. Once in the large intestine, they are excreted through feces, but for every sugar molecule that needs to leave the body, so does a water molecule, which leads to diarrhea or excessively soft/watery stool. Read more about this phenomenon, and why recommending juice to kids can be troublesome, here.
As for beer (or any type of alcohol), it reduces the body's level of anti-diuretic hormone (ADH), i.e. the hormone that in normal quantities, prevents us from peeing too much. But since beer decreases this hormone, we have to pee more than usual, resulting in more fluid loss and thus decreasing hydration.
|I promise this beer was preceded and proceeded by glasses of water!|
As for for coffee, it is a diuretic, meaning it increases your urination. But when you pee more, you become more dehydrated! So coffee is also a no-go when it comes to increasing hydration.
So, when you are thirsty and dehydrated, drink a glass of plain old water. If you still want some juice, pop, or alcohol, then at least make it a one-for-one deal (one glass of water for everyone one of these beverages).