Below is an approximation of this video’s audio content. To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video.

Drink two cups of water and you can get a surge of the adrenal hormone noradrenaline in your bloodstream, as if you just smoked a few cigarettes or downed a few cups of coffee, which boosts your metabolic rate up to 30 percent within an hour––which, when put to the test in randomized controlled trials, appeared to accelerate weight loss by 44 percent, certainly making it the safest, simplest, and cheapest way to boost your metabolism.

Now, if you’re on a beta blocker drug, this entire strategy may fail. (Beta blockers are typically prescribed for heart conditions or high blood pressure, and tend to end with the letters “lol,” such as atenolol, nadolol, or propranolol, sold as Tenormin, Corgard, or Inderal, respectively.)  So, for example, if you give people the beta blocker drug metoprolol (sold as Lopressor) before they chug their two cups of water, the metabolic boost is effectively prevented. This makes sense, since the “beta” that’s being blocked in beta blockers are the beta receptors triggered by noradrenaline. Otherwise, though, the water should work. But what’s the best dose, type, temperature, and timing?

Just a single cup may be sufficient to rev up the noradrenaline nerves, but additional benefit is seen at two or more cups. Caution: one should never drink more than three cups in an hour, though, since that starts to exceed the amount of fluid your kidneys can handle. If you have heart or kidney failure, your physician may not want you drinking extra water at all, but even with healthy kidneys, any more than three cups of water an hour can start to critically dilute the electrolytes in your brain with potentially critical consequences. (In How Not to Diet, I talk about the first patient I ever killed in the hospital as an intern. It was a guy who drunk himself to death—with water. He suffered from a neurological condition that causes pathological thirst. I knew enough to order his liquids to be restricted and have his sink shut off, but didn’t think to turn off his toilet.).

Anyway, does it have to be plain, straight water? It shouldn’t seem to matter, right? Water is water, whether flavored or sweetened in some diet drink. But it does matter. When trying to prevent fainting before blood donation, something like juice doesn’t work as well as plain water. When trying to keep people from getting dizzy when they stand up, water works. But, the same amount of water with salt added doesn’t. What’s going on?

We used to think the trigger was stomach distention. When we eat, our body shifts blood flow to our digestive tract, in part by releasing noradrenaline to pull in blood from our limbs. This has been called the gastrovascular reflex. So, drinking water was thought to be just a zero-calorie way of stretching our stomach. But instead, drink two cups of saline (basically salt water), and the metabolic boost vanishes; so, stomach expansion can’t explain the water effect.

We now realize our body appears to detect osmolarity, the concentration of stuff within a liquid. Covertly slip liquids of different concentrations into people’s stomachs with a feeding tube, and you can demonstrate this by monitoring sweat production (which is a proxy for noradrenaline release). This may be a spinal reflex, as it’s preserved in quadriplegics, or picked up by the liver, as we see less noradrenaline release in liver transplant patients (who’ve had their liver nerves severed). Whichever the pathway, our body can tell. Thought we only had five senses? The current count is upwards of 33 (so, maybe the Bruce Willis movie should have been called The Thirty-FOURTH Sense).

In my Daily Dozen recommendation, I rank certain teas as among the healthiest beverages. After all, they have all the water of water with an antioxidant bonus. But from a weight-loss perspective, plain water may have an edge. That may explain the studies showing overweight and obese individuals randomized to replace diet beverages with water lost significantly more weight. This was chalked up to getting rid of all those artificial sweeteners, but maybe instead the diet drinks were too concentrated to offer the same water-induced metabolic boost. Diet soda, like tea, has about ten times the concentration of dissolved substances compared to tap water. So, plain water on an empty stomach may be the best.

Does the temperature of the water matter? In a journal published by the American Society of Mechanical Engineers, an engineering professor proposed that the “secret” of a raw food diet for weight loss was the temperature at which the food was served. To bring two cups of even just room temperature water up to body temperature, he calculated the body would have to dip into its fat stores and use up 6,000 calories. Just do the math, he says: a calorie is defined as the amount of energy required raise one gram of water one degree Celsius. So, since two cups of water is about 500 grams, and the difference between room temp and body temp is about a dozen degrees Celsius. 500 x 12 = 6,000-plus calories needed.

Anyone see the mistake? In nutrition, a “calorie” is actually a kilocalorie, a thousand times bigger than the same word used in the rest of the sciences. Confusing, right? Still, I’m shocked the paper was even published.

So, drinking two cups of room temperature water actually only takes six calories to warm up, not 6,000. Now, if you were a hummingbird drinking four times your body weight in chilly nectar, you could burn up to 2 percent of your energy reserves warming it up, but it doesn’t make as much of a difference for us.

What about really cold water, though? A letter called “The Ice Diet” published in the Annals of Internal Medicine estimated that eating about a quart of ice—like a really, really big snow cone with no syrup—could rob our body of more than 150 calories, the “same amount of energy as the calorie expenditure in running one mile.” It’s not like you directly burn fat to warm up the water, though. What your body does is just corrals more of the waste heat you normally give off by constricting blood flow to your skin. But how does it do that? Noradrenaline!

If you compare drinking body-temperature water, to room-temperature water, to cold water, there’s only a significant constriction in blood flow to the skin after the room temp and cold water. And neither the warm nor tepid water could boost metabolic rate as much as cold (fridge temperature) water. So, your body does, after all, end up at least indirectly burning off more calories when you drink your water cold.

So, two cups of cold water on an empty stomach a few times a day. Does it matter when? Yes, watch my Evidence-Based Weight Loss lecture to see how you can add the benefit of negative-calorie preloading by drinking that water right before your meals.

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