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.
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.
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