Food and diet

Can water or diet drinks aid weight loss?

“Drinking water can help dieters lose at least five per cent of their bodyweight,” the Metro has reported.

The story is based on research that hoped to find evidence to support the common sense theory that replacing high calorie drinks with water or diet drinks helps weight loss. While patients in all groups lost weight on average, the study failed to prove this intuitive method of weight loss was better than simply advising overweight people on how to lose weight.

The news was based on a small study of overweight or obese adults, which looked at whether encouraging them to drink water or diet drinks rather than sugar-sweetened drinks could help with weight loss, without other dietary changes. It found that those who were encouraged to drink water lost on average 2.0% of their body weight, those on diet drinks lost 2.5% of their body weight and those in the control group lost 1.8%. The difference in weight loss between the groups was not statistically significant.

A separate analysis did find that people switching to water or diet drinks were more likely to lose 5% of their body weight, but the overall, more important findings suggest that, on average, the groups did not differ in weight loss.

It’s worth pointing out that the study was funded by a leading bottled water company in the US. It’s also noteworthy that people in the water/diet drinks groups were given the bottled water or dietary beverages. In real life, people who had to buy their own drinks might not be as good at sticking to drinking low calorie alternatives.

Where did the story come from?

The study was carried out by researchers from the University of North Carolina and was funded by Nestlé Waters USA. It was published in the peer-reviewed American Journal of Clinical Nutrition.

It was reported uncritically in the Metro, the Daily Express and the Daily Mail. The claim that people who switch to water or diet drinks are twice as likely to lose 5% of their body weight sounds dramatic, but overall there was no significant difference in average weight loss between the groups.

What kind of research was this?

This was a randomised controlled trial, which is the best way to find out about the effectiveness of an intervention.

The researchers investigated whether encouraging people to switch from sugar-sweetened beverages to water or diet drinks, without any other changes to diet or lifestyle, was an effective weight loss method. They did this by comparing drink replacement to just giving people information about a healthy diet.

The researchers point out that increased consumption of sugar-sweetened drinks is linked to several health problems including obesity. They argue that replacing high calorie drinks with water or low calorie drinks could be a simple strategy for promoting modest weight loss.

What did the research involve?

Between 2008 and 2010, the researchers recruited 318 overweight and obese people. The average age of the study’s participants was 42, most (84%) were women and a significant proportion were black (54%). To be eligible for the study people had to report consuming 280 calories or more of calorific drinks (including juice and juice drinks, sweetened coffee and tea, sports drinks and alcohol) daily.

Participants were allocated at random to one of three groups: the water group, the diet beverage group and the control (advice only) group. All three groups had the same contact time with researchers, monthly weigh-ins, group sessions and weekly monitoring.

Two of the groups were encouraged to replace two or more servings a day of drinks containing calories with either water or diet beverages. Four single servings of these drinks were provided daily, with two additional servings given in case family members drank them. Participants in both of these groups were given a choice of different beverages at their monthly treatment meetings. They were also given monthly group behavioural counselling to encourage them to adhere to their substituted beverages.

Members of the control group were given general weight loss information – for example, they were told to increase their physical activity and vegetable consumption, and to read product labels. They were not given specific weight loss plans, nor goals for physical activity. They were not encouraged to change beverage intake, nor were they provided with beverages.

All groups had access to a study website where they could report their weekly weight, receive feedback and view advice. The water group and the diet beverage group could also use the website to record how much of which drinks they consumed.

Participants’ body weight and height were measured at the start of the study, and at three and six months. Waist circumference and blood pressure were also measured. Researchers also collected information on dietary and calorie intake.

At six months, the researchers compared the weight of participants in all three groups, using standard statistical methods. In a further analysis, they examined whether more people in either the water or diet beverage groups had achieved a 5% weight loss target than in the control group.

What were the basic results?

At six months, all three groups had achieved small amounts of weight loss, but the difference between the groups was not statistically significant.

The average percentage weight loss in each group at six months was:

  • 2.03% (about 1.9kg) in the water group
  • 2.45% (about 2.6kg) in the diet beverage group
  • 1.76% (about 1.9kg) in the control group

In a separate analysis, researchers found that the chance of achieving 5% weight loss at six months was greater in the diet beverage group than in the control group (odds ratio 2.29, 95% confidence interval 1.05 to 5.01; p=0.04). In a combined analysis putting together both drink replacement groups, people in both the water and diet beverage groups were twice as likely to have achieved a 5% weight loss as those in the control group (odds ratio 2.07, 95% confidence interval 1.02 to 4.22). However, the study did not report exactly how many people in each group achieved this degree of weight loss.

How did the researchers interpret the results?

The researchers conclude that replacing high calorie drinks with water or diet drinks resulted in average weight loss of 2-2.5% of body weight. They say that replacing high calorie drinks with low calorie ones could be a simple strategy to reduce obesity and an important public health message.


This was an appropriately designed study for looking at the effect on weight of switching to non-calorific drinks. However, it does have some limitations, including that:

  • The study included only people who consumed more than 280 calories in beverages each day. Almost 40% of the people assessed did not consume this many calories in beverages, and could not be included in the study. The results of this study may not apply to those who consume fewer calories as beverages.
  • The researchers had calculated that their study should be large enough to detect a difference of 1.8kg in weight loss between the groups, but that it might not be statistically able to detect smaller differences in weight between groups. 
  • The study was relatively short and may not represent what would happen over a longer period of follow-up.
  • The people in the drink replacement groups received behavioural counselling to help them stick to the drink replacement programme – and they were more likely than the control group to attend group sessions.
  • Participants were provided with water or diet beverages as part of this study. People who had to buy their own drinks might not be as good at sticking to drinking low calorie alternatives.
  • The people taking part in the study were typically black, middle-aged women in the US. Its findings may not directly translate to all people in the UK.

Most importantly, the results did not show any significant difference in average weight loss between those who were encouraged to switch to water or diet drinks and the control group. A secondary analysis found that those in the water and diet beverage groups were about twice as likely to achieve a 5% weight loss as those in the control group. However, the proportion achieving this level of weight loss was not reported.

Sugar-sweetened drinks, juices, alcohol and similar beverages are a hidden source of calories and most dietitians would advise limiting intake of these in order to maintain a healthy weight. However, it is also important to maintain a healthy diet overall and to do regular exercise. There is no short cut to sustainable weight loss.

NHS Attribution