Despite an overload of diets to choose from, downing water is now the favoured weight loss strategy among older overweight people.
Regular sips throughout the day help slimmers shed the pounds by boosting metabolism, say scientists.
As well as burning up calories, it also cleanses the body of waste and dampens appetite.
Corresponding author Professor Lu Qi, a nutritionist at Tulane University in the United States, said: "Our analysis revealed an increasing trend in the use of increased water consumption as a weight loss strategy.
"This increase can be attributed to convincing evidence regarding the potentially important role of water in reducing energy intake, which thus contributes to the long term maintenance of weight loss."
The findings, published in JAMA Network Open, are based on data collected from about 48,000 men and women aged 40 to 64.
They were participants in the continuing National Health and Nutrition Examination Survey (NHANES) who were followed for up to 17 years.
Among those with a BMI (body mass index) of at least 25 who considered themselves overweight, almost four in ten (38%) tried to slim by "drinking a lot of water," said Prof Qi.
On the other hand, fewer than a fifth (18 per cent) chose exercise and one in nine (11 per cent) eating less.
Experts say if you are constantly dehydrated, your body tends to retain more water in an attempt to prevent levels getting too low.
It is important for liver and kidney health, too. Guidelines suggest about two litres per day.
Urine colour is a good indicator. If it is light yellow or fairly clear this is a sign of being well hydrated. Dark yellow or amber means you need more water.
Over the study period from 1999 to 2016 current individual BMI increased by an average of 20 per cent and weight by 6lbs (2.77 kilos), underlining the growing obesity epidemic.
Prof Qi said: "These increases were observed despite increases in the proportion of participants who attempted to lose weight and used weight loss strategies, such as reducing food consumption, exercising and consuming a large volume of water."
The proportion who had attempted to lose weight increased from around a third (34%) at the start of the study to more than four in ten (42%) by the end.
Prof Qi said: "Social pressure associated with an acceptable body weight and size might contribute to the increased reporting of weight loss attempts."
He added: "Taken together, these findings suggest that although 34 per cent to 42 per cent of adults in the United States in our study reported weight loss efforts, many of them might not have actually implemented weight loss strategies or applied a minimal level of effort, which yielded unsatisfactory results."
The proportion of participants who reported consuming more fruits, vegetables and salads, changing their eating habits, consuming less sugar, confectionery and junk food increased sharply.
Prof Qi said: "Evidence from other studies has shown these changes to be associated with less weight gain and, therefore, these strategies would be encouraged.
"Existing guidelines and compelling evidence also suggest longitudinal weight management relies on a combination of reductions in energy and fat intake, an increase in dietary fibre intake, regular physical activity, self monitoring, and other behavioural techniques.
"Reduction of either carbohydrates or fat has been similarly related to weight loss, especially in the context of low-calorie diets."
The prevalence of obesity among US adults increased from 34 to 40 percent between 2007 and 2016. Around a third of adults in the UK are obese.
Prof Qi added: "Obesity is associated with a variety of major chronic diseases, including cardiovascular disease, type 2 diabetes and cancer, as well as premature mortality.
"Compelling evidence suggests that even moderate weight loss significantly reduces the risk of obesity-related diseases and mortality.
"However, losing weight and maintaining a healthy weight remain significant challenges."
Earlier this year Hollywood star Chris Pratt revealed he lost almost a stone by "drinking water and weeing three times before 10am."
The 40-year-old actor once tipped the scales at a gut-busting 21 stone. He shed over five stone for the role of hunky Peter Quill in Guardians Of The Galaxy.