You’ve probably heard before that people overeat because they confuse hunger and thirst.
“You’re not really hungry, you’re just thirsty.”
At first, that sounds logical.
But if you stop and think about it…it actually doesn’t make a whole lot of sense.
Thirst and hunger are not similar sensations. Recall times when you’ve been really thirsty, like after running in the heat, versus when you’ve gone a long time without eating. Feels pretty different, right? There’s a good reason for that — your body delivers distinctive signals for hunger and thirst because dehydration, from a survival standpoint, is a much bigger deal.
More importantly, there’s literally no evidence out there to back this up.
At least, none that I can find.
Given how scientifically feeble that idea is, it would be pretty easy to dismiss water consumption as playing a role in weight regulation.
However, if you dig a little further into the literature on this subject, there does seem to be a link between drinking water and body weight, albeit through different mechanisms.
Let’s take a look at this relationship and how you can take advantage of it yourself.
The Link Between Hydration and Weight
One of the first clues that drinking water might play a role in body weight regulation comes from the National Health and Nutrition Examination Survey (NHANES), which captures the dietary habits of a representative sample of Americans through rigorous interviews and physical examinations.
In one analysis of NHANES data, researchers assessed hydration status in more than 9500 participants by looking at urine osmolality (meaning urine concentration). After adjusting for possible confounding variables like age and income, they found that people who were inadequately hydrated had higher BMIs on average, and almost 60% higher odds of being obese, compared to those who were well hydrated.
Another analysis of NHANES data went a little further, and instead looked at how changes in water consumption over time were related to energy intake. They found that a single percentage point increase in daily plain water intake was associated with a reduction in mean daily calorie intake of about 8.5 calories. Obviously a miniscule number, but modeling suggested that this could make a surprisingly big difference. When the researchers crunched the numbers, they calculated that an increase in daily plain water consumption of 1 cup would result in a reduction in daily total energy intake of 68 calories, and an increase by 3 cups would lead to a drop of up to 205 calories.
So there certainly is a correlation here, although this doesn't mean that inadequate hydration causes weight gain, or that drinking more water will help you keep weight off.
Experimental evidence, on the other hand, tells a somewhat more convincing story.
Lowering Caloric Intake
It is possible that drinking more water may help with weight regulation, in part, by dampening your appetite.
When your stomach is empty, your body releases the hunger hormone ghrelin, which sends a signal to your brain that it’s time to eat. Then, when you eat, the walls of your stomach are stretched with the contents of the meal, which in turn shoots a message to the brain causing your hunger to drop back down. This is one reason why bulkier foods, like fruits and vegetables, tend to be more satisfying - they take up more space in your stomach and do a better job of eliciting that stretch response.
Water, much like voluminous foods, expands the stomach, and can thus trigger a similar response. Could we exploit this and effectively “trick” our brains into thinking that we aren’t hungry?
Well, maybe. In one trial that put it to the test, healthy young men visited the lab on two separate occasions. On one visit, they were given 568 mL of water before being presented with an all-you-can-eat breakfast. For the other, they were offered the same ad libitum meal, but without water. When the guys drank water first, they reported feeling less hungry and wound up eating 22% less, compared to eating without drinking beforehand.
Now, this effect is temporary, of course, only lasting until the stomach empties again. And plain water clears the stomach pretty efficiently, which is probably why this seems to work better in older people, who tend to experience slower gastric emptying.
But if you were to keep it up long term, could this impact on appetite and food intake add up over time?
It appears so.
In another study, overweight adults were randomly assigned to two different groups. In one group, they were instructed to adhere to a lower-calorie diet designed to induce weight loss. In the other group, they were also assigned to such an eating plan, but they also were instructed to drink 500 mL of water before every meal.
Over the 12 weeks of the study, the water drinking group saw a 44% greater decline in weight — and this was attributed to lower energy intake at the meals.
Enhancing Caloric Expenditure
Drinking more water might also temporarily boost calorie burning, although the evidence behind this mechanism is a little less persuasive.
It has been known for some time that water consumption activates the sympathetic nervous system, which could in theory lead to transiently higher energy expenditure.
Researchers conducted a study a while back to see whether drinking water could influence metabolic rate in real life. They had 21 healthy young subjects drink 500 mL of water and sit in a metabolic chamber — a small sealed room equipped with devices that can calculate how many calories you're burning by measuring concentrations of gasses that you're breathing in and out.
They found that drinking the water increased metabolic rate by 30% in both men and women. This increase reached a maximum after about 30-40 minutes, with a total thermogenic response of around 25 calories. Thus, drinking two liters of water per day could be expected to increase energy expenditure by almost 100 calories.
Again, this is a relatively small and limited change, but you can imagine how it all might add up over time.
Putting it into Practice
Overall, the research suggests that drinking more water — especially if you’re not already particularly well hydrated — could be helpful for maintaining a healthy weight or even modestly lowering body fat. If nothing else, it’s certainly not likely to harm your efforts (and hey, water is cheap, right?).
Here are some tips, based on the above research, if you want to try this at home:
- Drink before (or during) your meals. These trials often use water as a “preload” with meals to help boost the stomach expansion that occurs when you eat. Replicating what studies did usually seems like a smart move. Plus it makes it easy to remember to drink enough water. (FYI: You might hear some people claim that taking in fluids while eating will mess up your digestion by “diluting” stomach acid. That’s…not how the stomach works, don’t listen to them.)
- A total of 1-2 liters of added water per day is a solid target. Most of the experiments discussed above gave people around 500 mL for each drinking bout. Consuming that amount with each meal, plus some extra if you sweat, makes good sense to me. I personally keep a water bottle that holds about 500 mL with me to ensure that I stick to my water habit.
- Cold fluids might work better. At least, with respect to increasing energy expenditure. In a subset of volunteers, researchers found that heating up the water to 98.6 degrees Fahrenheit before giving it to them slashed the metabolic effect by around 40%. This honestly wouldn’t make a huge difference though, so don’t sweat it if you’re an Earl Grey fan.
- Flavored (non-caloric) beverages are probably fine. If you’re not crazy about the taste of plain water, drinking something that has some kind of flavoring or calorie-free sweetener added is likely to work just as well for this purpose. One study that followed 303 individuals in a weight loss program found that people who were instructed to drink non-caloric sweetened drinks actually lost a little bit more weight than people who drank the same amount of plain water.
*You should always seek the advice of your physician or other qualified health care providers if you have questions regarding a medical condition or treatment or before starting or stopping any healthcare or health related regimen.