People who want to lose weight are frequently told to drink more water. With that being the case, you would think that there would be plenty of strong scientific evidence to support this recommendation. You would think that, and you would be completely wrong. I’ve only been able to identify four randomised trials that have increased overweight people’s water intake and then followed them for at least 12 weeks to see whether they lose weight (12 weeks is really the minimum at which it is reasonable to expect to see meaningful results).
Let’s take a look at each of the four trials in turn, and see what conclusions we can draw.
The first was published in the journal Obesity in 2010. 48 overweight and obese individuals aged 55-75 were randomised to either a recommendation to follow a low calorie diet and drink 500 ml of water half an hour before every main meal, or just a recommendation to follow a low calorie diet. They were followed for 12 weeks.
The low calorie diet recommendation was just a way to try to blind the participants to the purpose of the study and to whether they were in the intervention group or control group – if one group had been told to drink more water, and the other had been told to just keep doing what they were doing, then it would have been pretty obvious who was in the control group, and what the purpose of the study was.
It’s worth noting here though that the researchers weren’t blinded to who was in which group. In other words, the study was single-blind, not double-blind. This is always a weakness in a study, because it opens up for the researchers to manipulate the results in various ways, which can happen both consciously and unconsciously. The researchers who do these types of studies generally want to prove that “their” intervention works, and even if they’re not engaging in conscious fraud, they will often unconsciously tip the scales in various ways that make the intervention look better than it otherwise would. This is a problem we’re going to talk more about as we go along, because this first study wasn’t the only one that was only single-blind.
Before we get in to the results I will just note that the study was funded by the Institute for Public Health and Water Research. This organisation doesn’t have a functioning web site so it’s not possible to learn much about it, but if I were to guess, I would say the odds are about 99% that it’s a front for Coca-Cola and other companies that sell bottled water. Just so you know.
Ok, so what were the results?
Oddly, the researchers don’t tell us anywhere in the study report what the participants weighed at the end of the study. All we are told is that the participants in the water group lost 2 kg more weight than the control group, which amounts to “44% greater weight loss”. It always feels a bit fishy when one of the most important numbers the study produces isn’t listed anywhere. And it means we have to do some math, which really shouldn’t be necessary.
If 2 kg is 44% more than what the control group lost, then that means the control group lost roughly 4.5 kg, while the water group lost roughly 6.5 kg.
Is this a meaningful difference? Well, that’s impossible to know with just these numbers, because it depends on how much the two groups weighed to start. What we really want to know is how much weight the participants lost as a proportion of their starting weight. Why? Because heavier people will, all else being equal, lose more weight over a certain time period than less heavy people. So unless the two groups weighed the same amount (which they didn’t) we can’t draw any conclusions from the number of kilograms lost. We have to look at how much each group lost as a percentage of body weight.
Luckily, the researchers deemed fit to provide us with this information. The water group lost 7.8% of their starting weight, while the control group lost 6.5%. This amounts to an absolute difference of of 1.3%. Which sounds a lot less impressive than 44%!
A 1.3% difference in weight reduction over twelve weeks isn’t very impressive, but it is something, so it isn’t worth discounting completely. For a person weighing 100 kg, it would amount to an extra 1.3 kg (which is about 2.8 pounds) of weight loss over twelve weeks, which wouldn’t be bad at all if the weight loss followed a linear trend (i.e. you lost 1.3 kg extra weight every three months just by drinking more water).
Is the difference real though?
Well, the first thing to note is that the difference isn’t statistically significant. This could mean that the difference is just due to chance. It could also mean that the study is too small to tell if it’s a real difference or not – this was only a 48 person study after all. With such a small number of people, you generally need to see a big difference in results for them to reach statistical significance, regardless of whether the result is real or not.
We have another problem though. As mentioned, the researchers weren’t blinded. They could easily have created a small difference between the groups simply by rounding up the weight of the participants in the control group while rounding down the weight of the participants in the water group when weighing them, or by making sure the participants in the water group emptied their pockets before being weighed, while forgetting to do so with the participants in the control group. These types of small manipulations can easily happen even without the researchers really being consciously aware of what they’re doing. Which is why it’s important for studies to be double-blind whenever possible.
One thing that’s worth mentioning is that urine output was measured over a 24 hour period at the beginning and end of the study, as a way to try to determine how much water consumption had actually changed, and the participants in the water group had only increased their urine output by about 400 ml more than the participants in the control group, which is far less than you would expect based on the fact that they were supposed to consume an extra 1,500 ml of water per day.
Someone who really believes in increased water consumption as an effective weight loss method could therefore argue that the small difference in actual water consumption explains the small difference in results between the water group and the control group. The problem with that argument is that it is entirely hypothetical – there are at present no trials that have actually gotten people to increase their water consumption by 1500 ml, so no-one knows what would happen in that scenario.
Let’s move on to the next trial, which was published in The Journal of the Academy of Nutrition and Dietetics in 2013, and which is really a continuation of the previous trial. The participants who took part in the previous trial were asked if they wanted to take part in a longer term study upon completion of the first trial. They were then asked to continue with whichever recommendation they had been on in the previous study for a full year. That makes this the longest trial in existence of increased water consumption for weight loss. If the reason the previous trial wasn’t able to show a meaningful difference between the groups was that it was just too short, then this one, with a full year of follow-up, should be able to show a real difference.
Like the previous study, this one also has some shady funding behind it, having been paid for by the International Life Sciences Institute, which is a front for the food industry, and which has close ties to among others Coca-Cola and Pepsi, that make money selling bottled water, as well as companies like Nestlé and Kelloggs, that sell unhealthy processed foods, and that have an incentive to try to convince people that they can lose weight by drinking more water rather than by decreasing their intake of KitKats and Frosties.
So, what were the results?
At the end of twelve months, the participants in the water group had lost 2.3% of their weight (going from 83.7 kg to 81.8 kg), while the participants in the control group had lost 1.4% of their weight (going from 82.7 kg to 81.6 kg). In other words, the difference in weight loss between the groups was a measly 0.9%.
What I think is interesting here is that in the previous study, which ran for twelve weeks, the difference in weight loss was about 1% of starting weight, and in this study, which ran for a full year, the difference in weight loss was again about 1% of starting weight.
If it was actually the water having an effect, and it had an ongoing weight loss effect, then you would expect a widening gap over time between the two groups. You wouldn’t expect to see pretty much the exact same difference between the groups at a year out that you saw at twelve weeks out. Unless water has an initial small effect on weight loss that rapidly diminishes to nothing, in which case the benefits of increasing water consumption are so marginal as to be not worth bothering with.
As I see it, the most likely explanation for the fact that the difference in weight loss between the groups was the same after the twelve month trial as after the twelve week trial is that the difference is caused by the lack of blinding of the researchers, and the bias that automatically creeps in when that is the case. That bias could easily create a 1% difference between the groups in favour of the water group.
Another possible reason for the difference has to do with differences in protein consumption. As I’ve discussed previously on this blog, protein is the key lever controlling calorie intake in humans. The protein intake in the water group was slightly higher than in the control group, about 18.5% vs 16.5%. This is a difference that is big enough that it could in itself explain the small difference in weight loss that was seen.
The difference in urine production between the two groups was on average 300-400 ml over the course of the study, so just as with the previous study, an argument could be made that the difference in water consumption was too small to show a meaningful difference in weight loss, and that “if only” the participants in the water group had drunk a full liter and a half like they were supposed to, then they would have seen a much bigger weight loss.
As with the previous study, the difference in weight loss was not statistically significant. Since that study was small, that doesn’t necessarily mean the difference isn’t real, but it’s not encouraging.
Since the first trial and the second trial involve the same group of people who were followed continously, one thing we can do is to look at the total weight loss over the course of both studies. This means that we can increase the follow-up period to a year and three months. The comparison isn’t perfect, because there were some drop-outs between the first and second study, but it’s close enough that it should allow pretty decent conclusions to be drawn. If we do that, then we get a total weight loss in the water group over a year and three months of 10.1% and a total weight loss in the control group of 7.9%.
If we imagine that the difference is really due to the extra water, and imagine, for the sake of simplicity, that both groups had a starting weight of 100 kg, that would mean that consuming an extra 400 ml (about two glasses) a day over the course of a year and three months would allow for an extra 2 kg of weight loss. That isn’t much, especially when you consider that more weight was lost during the first twelve weeks than during the following year, which would mean that the effect declines rapidly over time, so the best that could ever be hoped for would probably only be 2-3 kg, even after a decade of extra water consumption. However, drinking a few extra glasses of water isn’t dangerous or expensive, so I guess a case could be made for trying it as an adjunct to more effective weight loss strategies (like following a high protein high fiber diet). If, that is, it could be shown that the difference was real and not just due to chance or bias or confounding variables (which, of course, isn’t the case).
Let’s take a look at the next trial, and see if it adds anything to our understanding. This one was published in Obesity in 2015. Before getting in to the study I will just note that it was part-funded by the European Hydration Institute, which is a known front for Coca-Cola and other companies that sell bottled water (are you starting to see a pattern here?).
This was a single-blind randomised controlled trial in which 84 obese adults (average age 57 years) were randomised to either a recommendation to drink 500 ml of water 30 minutes before of each of the day’s three main meals, or to imagine that their stomachs were full 30 minutes ahead of each meal. The purpose of getting the control group to image their stomachs being full was to give them something to do, so they wouldn’t realise that they were the control group. The participants were followed for twelve weeks.
What were the results?
The water group lost 2.6% of their starting weight (going from 92.2 kg to 89.8 kg), while the control group lost 1.3% of their starting weight (going from 93.5 kg to 92.3 kg). This gives a 1.3% difference in weight loss, amounting to roughly a 1 kg difference between the groups. The difference appeared to be statistically significant at first, but after the researchers adjusted for confounding variables, this was no longer the case.
So, all in all, a very similar result to the first study. Urine volume was 500 to 600 ml greater in the water group than the control group, so as with the previous studies, the researchers weren’t able to get the participants anywhere close to increasing intake by 1500 ml per day. Which I can understand – it’s hard to force yourself to drink if you’re not thirsty, and if you drink more now, you’ll probably just compensate by drinking less later. The body has pretty sophisticated mechanisms in place for controlling our fluid intake, and what these studies are basically trying to do is override those mechanisms and get people to drink a bunch of extra water that their bodies don’t think they need. It might, in fact, be impossible to get people to increase their water intake by 1500 ml per day beyond what they’re already taking in, unless you put a tube down their throat and force feed them, or hook them up to a drip.
Let’s move on to the final trial, which was published in JAMA Pediatrics in 2017. 38 overweight and obese 12-17 year olds were randomised to either “standard diet recommendations” plus a recommendation to drink 8 cups of water per day (1800 ml), or just “standard diet recommendations”. They were followed for six months.
For once, the study wasn’t funded by the food industry. As with the previous studies, the participants were blinded but the researchers weren’t (although apparently the people taking weight measurements were blinded, which is good, because it gets rid of some of the bias-inducing behaviours discussed above).
Ok, let’s get to the results.
Over the course of the six months, the participants in the water group lost 0.5% of their weight, while the participants in the control group lost 0.8%. In other words, the control group lost more weight than the water group, so the study is negative regardless of which way you try to slice the results.
No 24-hour urine measurements were carried out that could give us an objective measure of how much more water the participants in the water group were consuming, but the participants were asked about how much water they drank, and by their own accounts the participants in the water group increased consumption of water by about 400 ml per day more than the participants in the control group. This is of course nowhere near as reliable as actually measuring urine production – it’s easy to imagine the teenagers in the water group overestimating their water consumption when asked by the nice researchers how much water they drank (especially after being regularly bombarded with messages to drink more water by those same researchers for months on end).
Ok, I think we’re ready to wrap this up. So, we have four studies, three of which are funded by the food industry and one of which isn’t. The three that are funded by the food industry show marginally more weight loss in the water group than in the control group, however, in all cases the difference fails to reach the level of statistical significance. Additionally, the difference in weight loss is so small that it could easily be explained by the bias introduced by the fact that the studies were not double-blind.
The one study that isn’t funded by the food industry actually shows more weight loss in the control group than in the water group. However, it didn’t measure urine production, so we don’t actually know how much more water, if any, the participants in the water group actually drank.
Overall, I think it’s fair to conclude that the evidence that exists at present does not support recommending increased water intake to people who want to lose weight. If someone thinks water does have a role in weight loss, they will have to do a bigger and better designed trial to prove it.