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If you randomize overweight adolescents into one of two groups, in which they’re either advised to drink eight cups (1.90 L) of water a day or not, what happens after six months? The first question you always have to ask with interventional studies is: Did the study participants actually comply? Both groups started out drinking around two cups (0.50 L) a day, and so the study was designed to see if there was a weight loss benefit of consuming six extra cups (1.45 L) of water. Unfortunately, the difference in water intake between the groups came out to be less than a cup and a half (0.35 L), which wasn’t enough to show any benefit. Only one or two teens in the water group reportedly reached the target intake.
To improve compliance, another set of researchers asked kids to check their pee. The group of overweight 9- to 12-year-olds randomized to the water intervention were told to increase their water intake to the point their urine became straw-colored (a pale yellow). Still, not every kid complied, but those who did lost significantly more weight.
Inspired by these small pilot studies, and early successes with school-based interventions in Europe, researchers launched the most ambitious study yet, involving more than a million students in New York City public schools. They compared obesity rates and weight gain in schools that implemented cooled, fast, filtered water dispensers compared to control schools that hadn’t, and the increased water access appeared to translate into less weight gain, and lower rates of overweight kids. In the very least, we should stop prohibiting water from classrooms. The accompanying editorial in the AMA’s pediatrics journal was entitled: “The Power of a Simple Intervention to Improve Student Health: Just Add Water.”
Couldn’t that result just be explained by decreased consumption of soft drinks? The study was performed a decade after NYC schools had already removed soda from all the vending machines. They still sold low-fat milk, though, and that’s what the researchers suspect may account for their findings—the 12 percent drop in milk purchases after the water machines were installed. Milk intake, like soda, results in weight gain. This is true for skim milk, or even just straight dairy protein, pure whey, or casein beverages without the sugars naturally occurring in milk. The increased fat mass from drinking milk may in part be from the increased insulin secretion caused by milk protein. Even dairy industry-funded studies have found that drinking less than a cup of milk (0.25 L) with a low-glycemic index meal can exaggerate the insulin spike as much as if you had just eaten high-glycemic white bread.
While milk can impair fat burning, water may have the opposite effect. To get to the bottom of the water and weight loss question, tightly controlled metabolic experiments were performed in which whole-body protein and fat breakdown were measured under different degrees of hydration. Well-hydrated individuals experienced the best of both worlds––increased fat burning and decreased protein breakdown. The way the body responds to high water intake is similar to how it responds to acute fasting––a switching towards fat as a fuel source, while trying to spare the muscle. These were proof-of-principle experiments, however, with limited real-world relevance, as the high fluid states were induced not only by having the participants drink 10 cups (2.5 L) of water over a 12-hour period, but also by dripping extra free water straight into their veins, and even giving them an antidiuretic hormone to cause them to retain even more water. There are, however, mechanisms by which our day-to-day hydration status can affect our metabolism.
When we get dehydrated, our blood volume actually shrinks. This drop is detected by our kidneys that release an enzyme into our bloodstream that triggers the cascade that results in the formation of a hormone called angiotensin. Angiotensin causes us to become thirsty, and constricts our blood vessels to compensate for the diminished blood volume. (That’s actually how a popular class of blood pressure lowering medications work. The ACE in “ACE inhibitors” like captopril stands for angiotensin-converting enzyme.) That’s not all that angiotensin does, though. Drip it on human fat cells in a petri dish, and they start piling in more fat. This may help explain why those with higher angiotensin levels in their bloodstream tend to be heavier. The thought is that those who don’t drink enough end up with chronically elevated angiotensin levels, which can lead to weight gain. The most convincing evidence comes from genetic studies showing that those born predisposed to higher angiotensin levels are significantly more likely to become obese. We can keep our levels down in the normal range, though, by staying adequately hydrated.
So how much water should you drink every day? Unless you have a condition like heart or kidney failure or your physician otherwise advises you to restrict your fluid intake, here’s how much water I recommend drinking every day for weight loss, based on the Institute of Medicine’s adequate intakes, assuming moderate physical activity at moderate ambient temperatures. Though many may distrust the safety of tap water, bottled water may be no safer, cleaner, or higher in quality than water straight out of the faucet.
There are risks to drinking too much water, though. Even healthy kidneys may only be able to handle about three cups (0.70 L) of water an hour. Beyond that, we risk washing the electrolytes out of our brain, with potentially lethal consequences. Can you just drink a sports drink instead? No. In fact, there was a high-profile case of a high school athlete who died after drinking two gallons of Gatorade. Drinking too much of anything can be dangerous.
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