 Drink a few cups of water, and within three minutes the level of the adrenal gland hormone noradrenaline in your bloodstream can shoot up 60%. Have people drink two cups of water with electrodes stuck in their legs, and within 20 minutes you can document about a 40% increase in bursts of fight-or-flight nerve activity. Chug two or three cups of water and blood flow squeezes down in your calves and arms, clamping down nearly in half as arteries to your limbs and skin constrict to divert blood to your core. That's why, for example, drinking water can be such a safe, simple, effective way to prevent yourself from fainting, known medically as syncope. Fainting is the sudden, brief loss of consciousness caused by diminished blood flow to your brain. About 1 in 5 people experience this at least once, about 1 in 10 may have repeated episodes, causing millions of emergency room visits and hospitalizations every year. Though fainting can be caused by heart problems, it is most often triggered by prolonged standing because blood pools in your legs or strong emotions, which can cause your blood pressure to bottom out. About 1 in 25 people have what's called blood injury or injection phobia. We're getting a needle stick, for example, that can cause you to faint. More than 150,000 people experience fainting or near-fainting spells each year when they donate blood. All you have to do to help prevent yourself from getting woozy, though, is just chug 2 cups of water 5 minutes before getting stuck. The secret isn't bolstering your overall blood volume. I mean, drinking 2 cups of water, even a whole quart, and your blood volume doesn't change more than like 1 or 2%. It's due rather to the shift in the distribution of blood towards your center, caused by the noradrenaline-induced peripheral artery constriction. Water drinking stimulates as much noradrenaline release as drinking a couple cups of coffee or smoking a couple unfiltered cigarettes. If the simple act of drinking water causes such a profound fight-or-flight reaction, why doesn't it cause our heart to pound and shoot our blood pressure through the roof? It's like the diving reflex I talked about in the last video. When we drink water, our body simultaneously sends signals to our heart to slow it down to still your beating heart. You can try it at home. Measure your heart rate before and after drinking 2 cups of water within 10 minutes. Your heart rate should slow by about 4 beats per minute, and by 15 minutes you should be down 6 or 7 beats. One of the ways scientists figure this out is by studying heart transplant patients. When you move a heart from one person to another, you have to sever all the attached nerves. Amazingly, some of the nerves grow back, but still give healed heart transplant patients two glasses of water and their blood pressure goes up as much as 29 points. The body is unable to sufficiently quell the effect of that burst of noradrenaline. Some people have a condition known as autonomic failure in which blood pressure regulation nerves don't work properly and their pressures can dangerously skyrocket over 100 points after chugging 2 cups of water. That's how powerful an effect the simple act of drinking a glass of water can be, and the only reason it doesn't happen to all of us is that we have an even more powerful counter-response to keep our heart in check. It reminds me of the poor woman who had a stroke after taking the ice bucket challenge due to an insufficient diving reflex to tamp down all that extra noradrenaline release. The remarkable water effect can be useful for people suffering from milder forms of autonomic failure such as orthostatic hypotension, which is when people get dizzy standing up suddenly. Drinking some water before getting out of bed in the morning can be a big help. But what about that metabolic boost with so much noradrenaline being released, with your adrenal gland hormones and overdrive, might drinking a few glasses of water cause you to burn more body fat? Could tap water be like a safe form of a fedrum? Although weight loss with a nice slowing of your heart rate instead. Researchers decided to put it to the test, which we'll explore next.