 Water is by far the number one nutrient in our diet. Studies have suggested that proper hydration may lower a risk of heart disease and cancer, and may even make you a better kisser. Brushing artificial skin against the lips of young women, researchers found that hydrated lips showed greater sensitivity to light touch. Although it is well known that water is essential for human survival, only recently have it begun to understand its role in the maintenance of brain function. Make sense? Our brain is 75% water, and so when we get dehydrated, our brain actually shrinks. Even mild dehydration caused by exercising on a hot day has been shown to change brain function. I've talked about the role of hydration for cognitive function, and our current findings suggest that our mood states may also be positively influenced by water consumption. The effects of dehydration in real life have not been well documented. It wasn't until 2013 when the first study to investigate the effects of mild dehydration on a variety of feelings was published. What did they find? The most important effects of fluid deprivation were increased sleepiness and fatigue, lower levels of vigor and alertness, and increased confusion. But as soon as they gave them some water, the deleterious effects on alertness, happiness, and confusion were immediately reversed. Water absorption actually happens very rapidly, within 5 minutes from mouth to bloodstream, peaking around minute 20. And as an aside, the temperature of the water appears to matter. Which do you think is absorbed more rapidly? Cold water or warm body temperature water? Cold water gets sucked into the body about 20% faster. How can you tell if you're dehydrated or not? Why don't you ask your body? If you chugged down some water and then turned around and just peed it all right out, presumably that would have been your body's way of saying, I'm good, I'll top it off. But if you drank a bunch of water and your body kept most of it, then presumably your tank was low. So these researchers formalized the technique. So empty your bladder, and then chug down 11 milliliters per kilogram body weight, or 5 milliliters per pound, or about 240 milliliters in a cup. So that's about three cups of water. You drink three cups of water, and then an hour later, see how much you pee. The empty circles on this graph up in the corner are the dehydrated folks. So basically this says if you drink three cups and pee out less than one, then it's a good chance you were dehydrated.