Drinking water could help you lose weight — but not because it's filling you up, a dietitian says

• A dietitian said evidence that drinking water aids weight loss because it fills you up is lacking.
• But Duane Mellor said that drinking water could help you reach a calorie deficit.

Drinking water could help you lose weight, but probably not because it makes you feel full or burns calories like you may have been led to believe, according to a dietitian.

There is some evidence to suggest that drinking water makes you feel more full and boosts your metabolism, helping your body to burn fat. But Duane Mellor, a registered dietitian and lead for evidence-based medicine and nutrition at Aston Medical School, UK, wrote in The Conversation that there is little quality evidence to support that this aids weight loss long term.

One way that drinking water can aid weight loss, however, is by helping you achieve a calorie deficit, as it contains no calories and is the healthiest drink you can choose, he said.

To lose weight you need to be in a calorie deficit

Insider previously reported that being in a calorie deficit, which means eating fewer calories than your body burns, is an effective way to lose weight.

While factors such as age, sex, current weight, and weight loss goal will affect the number of calories an individual needs, personal trainer and registered dietitian, Jesse Feder, previously told Insider that the general principle applies to everyone regardless of these differences.

For example, "If you burn 2,000 calories in a day, a caloric-deficit diet would have you eating less than 2,000 calories. The exact calorie intake will typically be around 1,500 to 1,700 calories per day, so the deficit would be 500 to 300 calories," he said.

The deficit can be reached by reducing portion sizes, consuming lower-calorie food and drink, and by increasing physical activity — although exercise plays a smaller role in weight loss than you might think.

"Swapping high-calorie drinks such as soda and alcohol for water may be an easy way of reducing the calories you consume daily, which may help with weight loss," Mellor said.

If you have existing health conditions such as type 2 diabetes, kidney problems, or high or low blood pressure, you should speak to your healthcare provider before starting a calorie-deficit diet, according to the Cleveland Clinic.

Water might curb appetite but that doesn't guarantee weight loss

Mellor said that there is some evidence suggesting that drinking water before a meal can lead to weight loss because it makes you feel fuller more quickly, but only in middle-aged and older people.

One study found that middle-aged and older adults who drank water before meals lost 2 kilograms over a 12-week period, while those who drank no water did not, according to Mellor. However, younger people, aged between 21 and 35, lost no weight regardless of water intake.

Mellor pointed out that the participants in the study were aware of the reason they were drinking water before they ate, which could have influenced how much they chose to eat. And the study only looked at whether they ate less during one daily meal after drinking water, in a clinical setting.

Overall, water might have some appetite-dulling effects, but that might not result in long-term weight change, he said.

Water helps burn (a few) more calories

Mellor highlighted two small studies that looked at how drinking water affects resting energy expenditure: the amount of calories our body burns before exercise.

The findings suggested that drinking water did increase the number of calories burned by the participants, but the effects only lasted for an hour and the increase was very small.

"For an average 70kg adult, they would only use an additional 20 calories — a quarter of a biscuit — for every 500ml of water they drank," Mellor said.