A little, and mainly alongside a diet. In the best-known trial, adults on a reduced-calorie diet who had 500 ml of water before each meal lost weight faster than those on the diet alone3, but when eight trials are pooled, extra water on its own made no clear difference5.
Water has no calories, so any effect has to come through eating a little less or using a little more energy. Here is what the nutrition trials measured for each.
What was studied
The main trial randomised 48 adults aged 55 to 75 with overweight or obesity to one of two groups for 12 weeks3. Both groups followed the same reduced-calorie diet taught by a dietitian, with weekly weigh-ins. Participants were told the study was about diet and weight loss, not water.
| What | Detail |
|---|---|
| People | 48 randomised, 41 completed; aged 55 to 75, BMI 25 to 40 |
| Diet (both groups) | 1,200 kcal a day for women, 1,500 for men |
| Water group | 500 ml of water before each of 3 daily meals (1.5 l a day) |
| Comparison group | offered water, no instruction |
| Length | 12 weeks |
| Body composition | measured before and after |
A "preload" is the drink before a meal. The idea is that it takes up room in the stomach, so the meal that follows is a little smaller.
In plain terms: two groups on the same diet; one added two glasses of water before each meal.
What the studies found
The rate of loss was faster with water. Both groups lost weight. In the authors' model, the water group's weight fell at a steeper rate: a slope of −0.87 against −0.60, which they describe as "a 44% greater decline (i.e., greater rate of weight loss)"3. In their discussion they put it at about 2 kg more lost over 12 weeks, as a group average.
Fat mass fell more. Fat mass dropped by 5.4 kg in the water group and 3.3 kg in the comparison group, group averages (P = 0.01)3.
But not every measure agreed. The share of starting weight lost was 7.8% against 6.5%, a difference that was not statistically significant (P = 0.17)3. Change in body fat percentage was not significantly different either. So "44% more weight lost" is a misreading: the 44% is a difference in the slope of a model, not in total kilograms.
The meal really was smaller, at first. In a lab breakfast test, people ate 498 kcal after the water against 541 kcal without it at the start of the study (P = 0.009)3. After 12 weeks, the gap was smaller and not significant: 480 against 506 kcal. People rated themselves fuller after the water; hunger ratings did not differ.
Breakfast eaten with and without 500 ml of water first · kcal
After water No water
A second trial, a smaller effect. A 2015 UK trial randomised 84 adults with obesity from general practices to 500 ml of water 30 minutes before main meals, or to an attention-control task (imagining a full stomach before meals)4. After 12 weeks, the water group lost 1.3 kg more (95% confidence interval −2.4 to −0.1). Adjusted for ethnicity, deprivation, age and gender, the difference was 1.2 kg and no longer significant (−2.4 to 0.07). A confidence interval is the range the true difference most likely sits in; when it crosses zero, "no difference" cannot be ruled out.
In plain terms: two trials point the same way, a kilogram or two over three months, and both sit near the edge of statistical noise.
When the trials are pooled
A 2024 meta-analysis pooled eight randomised trials in people with overweight or obesity that either added water or swapped it in for other drinks, lasting three to six months5. The pooled change in body weight was −0.33 kg (95% CI −1.75 to 1.08), not significant. Body mass index and waist size did not change significantly either.
Difference in weight change with extra water vs control · kg
What was replaced mattered. Among trials that swapped water in for sugar-sweetened drinks, weight fell by a borderline 0.81 kg (95% CI −1.66 to 0.03)5. That fits the simplest explanation: water that replaces calories removes calories. The other swap went the opposite way: in trials that replaced artificially sweetened drinks with water, body weight was 1.82 kg higher with water (95% CI 0.97 to 2.67)5. The authors concluded that water intake "may not significantly impact adiposity" in these groups, while replacing sugar-sweetened drinks with water "might offer a modest benefit".
Does water raise the calories you use?
A 2003 study reported that 500 ml of water raised metabolic rate by 30% in 14 healthy adults, a total of about 100 kJ (about 24 kcal)1. About 40% of that came from warming the water to body temperature.
A 2006 study tried to repeat it and could not. After roughly 500 ml of room-temperature water, energy expenditure did not rise at all; water chilled to 3 °C produced a small 4.5% rise over an hour, well below the energy cost of warming it2. The authors concluded that water is doubtful as a way to raise energy use for weight management.
In plain terms: if water helps, it is by filling you up or replacing drinks with calories, not by speeding up the body. The rest of the energy side is in calories in, calories out and the thermic effect of food.
Common misreadings
- "Water flushes out fat." Fat is not flushed. In the trials, any extra loss followed eating a little less.
- "Drinking water is a diet on its own." The main trial's effect appeared on top of a reduced-calorie diet3; adding water without other change made no clear difference in the pooled trials5.
- "More water is always better." No trial here tested amounts above about 1.5 l a day of added water. How much water people need is covered in how much water per day.
Any reduced-calorie diet has limits. The calculator on this site caps a deficit at 35% of TDEE and never sets a target below its calorie floor (1,500 kcal for men, 1,200 kcal for women on the standard setting); how big a calorie deficit should be explains both.
The studies at a glance
| Study | Who was studied | Key result | Year |
|---|---|---|---|
| Boschmann et al., *J Clin Endocrinol Metab*1 | 14 healthy normal-weight adults, one lab session | 500 ml of water raised metabolic rate 30%, about 100 kJ in total | 2003 |
| Brown et al., *J Clin Endocrinol Metab*2 | Healthy young adults, randomised crossover | Room-temperature water: no rise in energy use; very cold water: 4.5% | 2006 |
| Dennis et al., *Obesity*3 | 48 adults aged 55 to 75 on a reduced-calorie diet, 12 weeks | Water before meals: 44% steeper rate of loss, about 2 kg more; % of weight lost not different | 2010 |
| Parretti et al., *Obesity*4 | 84 adults with obesity, UK primary care, 12 weeks | 1.3 kg more loss with water before main meals; not significant after adjustment | 2015 |
| Chen et al., *Nutrients*5 | 8 RCTs, people with overweight or obesity, 3 to 6 months | Extra water: −0.33 kg, not significant; replacing sugary drinks: −0.81 kg, borderline | 2024 |
Limits worth knowing
- Small trials. 48 and 84 people, 12 weeks each. No long-term result exists.
- Who. The main trial studied mostly white adults aged 55 to 75. The authors note that young adults in earlier studies did not eat less after water3.
- Uneven groups. By chance, the water group had more men (12 of 23) than the comparison group (6 of 25).
- Compliance was self-reported, though urine tests suggested the water group did drink more.
- Funding. The main trial was funded by a research grant from the Institute for Public Health and Water Research3.
- Abstract only for the 2003, 2006 and 2015 papers.
Latest research
The newest study we opened is the 2024 meta-analysis of eight trials5. It is the most direct pooled answer to the question, and it is cautious: extra water alone did not change weight, BMI or waist size significantly, and only swapping water in for sugar-sweetened drinks leaned toward a small loss. The authors call for more and longer trials. Whether water beats low-calorie sweetened drinks is a separate question, taken up in our article on diet soda versus water.
The short version
- Water has no calories; any effect comes from eating a little less or replacing drinks that have calories.
- With a reduced-calorie diet, water before meals added about 2 kg of loss over 12 weeks in one trial, group average.
- A second trial found about 1.3 kg, which lost significance after adjustment.
- Pooled across eight trials, extra water alone made no significant difference.
- The idea that water raises energy use did not hold up when it was tested again.
Educational overview only — not medical advice.
