Hydration and Weight Loss

Did you know that those who stay hydrated tend to maintain a healthier body weight. But is it cause and effect? Here’s our first story.

In my video series starting with “How to Get the Weight Loss Benefits of Ephedra Without the Risks,” I noted how drinking water affects the release of the adrenal hormone noradrenaline, making it the safest metabolic booster for weight loss. And I recommended drinking two cups (0.50 L) of cold water on an empty stomach a few times a day, ideally before meals, as a form of that negative calorie preloading, as I discuss in my video “Evidence-Based Weight Loss.” But what about just staying hydrated in general?

In a national survey, one of the weight control practices most associated with successful weight loss was “Drink plenty of water.” But it was also associated with unsuccessful attempts at weight loss. It’s just one of the most popular weight loss tips across the board, both in the mainstream media and a common recommendation given to patients by their physicians. But does it work? Can we just add water?

About a dozen studies have been published on the matter, and overall, there does appear to be a weight-reducing benefit to increased water consumption. What’s the obvious confounder, though? Confounding factors, also known as “lurking variables,” are some third element that ends up being the true explanation for a supposed link between two things. Here’s a textbook example: there may be a tight correlation between ice cream sales and drowning deaths, but that doesn’t mean ice cream causes drowning. A more likely explanation is that there is a lurking third variable—like hot weather, summertime—that explains why drowning deaths are highest when ice cream consumption is at its peak. So, what might be a confounding factor that offers an alternate explanation of why those who drink more tend to lose more weight? Maybe it’s because those who drink more water tend to drink less soda. So, maybe the reason more water is associated to more weight loss is because there’s a third variable, a confounding variable, like less soda consumption connected to both, and that’s the real cause.

The primary reason that the CDC, USDA, American Medical Association, American Diabetes Association, American Heart Association, and American Academy of Pediatrics all recommend drinking water for weight management is as a replacement of beverages with calories. Even replacing one sugary beverage or beer with water per day is associated with a lower incidence of obesity over time. American children and adolescents drink so much soda that replacing all sugary beverages with water could result in an average reduction of 235 calories a day.

And what about exercise? That’s another obvious confounder candidate. After all, who drinks lots of water? Those who spend hours working out. So, it’s no wonder water drinkers might be slimmer. A study of dieting overweight women took both soda and exercise into account, and still, however, found a benefit associated with increased water consumption. Over a year, those who drank at least a liter of water a day lost about five more pounds (2.30 kg) on average than those who didn’t. The researchers were able to control for physical activity and intake of other beverages. But what about other foods? It turns out that those who drink more water also tend to eat more fruits and vegetables, greens and beans, and whole grains, and less total sugar, and less fast food. No wonder they’re a healthier weight.

To control for dietary factors, the scientific world had to bring out the big guns, Harvard’s big cohort studies that followed the diets and health of more than 100,000 doctors and nurses for decades. They were able to control not just for other beverages, and lifestyle factors such as exercise, smoking, sleeping, and TV watching, but a wide range of healthy and unhealthy food intakes from fruits and vegetables to meat and candy consumption. They were the first to show that “increasing water intake per se was independently and significantly associated with less weight gain” over the long term.

Consumption patterns in these studies were by self-report, though. Participants were just asked to fill out detailed questionnaires about their diet. For more of an objective measure, researchers directly assessed people’s hydration status by assessing their blood and urine concentrations. In both adults and children, the more hydrated people were, the less likely they were to be obese. Spot-checking urine from nearly 10,000 men and women, researchers found that nearly half of obese individuals were walking around under-hydrated, compared to fewer than one in three individuals who were normal weight or lighter.

The problem with snapshot-in-time studies is that you don’t know which came first. Did under-hydration lead to obesity, or did obesity lead to under-hydration? At a heavier weight, you actually need more water. The daily water requirements of a man of average height weighing 210 pounds (95 kg) may actually be four cups (0.95 L) more than the same man at 160 (73 kg) pounds. And who’s more hydrated? Those who eat more water-rich foods, like fruits and vegetables. There’s that specter of confounding again. The only way to prove cause-and-effect is to put it to the test in an interventional trial, which I’ll cover next.

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.

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.