Plain-language note: what water before food gives and what it does not
A simple trick
Drink a glass of water half an hour before a main meal.
- trials used about half a litre
- you can start with less
What was seen
Older people on a diet who drank water before meals lost a little more.
- the difference built up over a few months
- more fat was lost
Older adults ate less
Water before lunch helped older adults eat less.
- young adults showed no such effect
- the effect faded over time
Water alone is not enough
Simply drinking more water showed no effect on weight in the review.
- water does not slim you down alone
- it works together with food
Instead of sugary drinks
If water replaces sugary drinks, the result is borderline.
- effect not proven
- swapping sweetener-containing drinks did not lower weight
Making it stick
Tie it to your routine.
- a glass half an hour before lunch and dinner
- water next to the plate
How much
If your doctor has not limited fluids.
- with heart or kidney disease, follow your doctor's plan
Important. Important: if your doctor has limited fluids, agree the amount of water with them.
When to see a doctor
If strong thirst does not go away, weight falls fast without a reason, or swelling appears.
- prescribed treatment is not changed
Key points
Water before a meal is the simplest trick, and it costs nothing. In a human trial of people aged 55 to 75 on a low-calorie diet, half a litre of water before each main meal gave about 2 kg of extra weight loss over 12 weeks, and more fat was lost. In older adults water half an hour before lunch reduced what they ate. But by itself water did not change weight in the review of trials: no effect shown.
| What | How much | How it helps | What studies showed |
|---|---|---|---|
| Water before meals on a diet | 500 mL before breakfast, lunch and dinner | Weight falls a little more | 48 people aged 55 to 75, 12 weeks: about 2 kg more; fat 5.4 kg versus 3.3 kg |
| Water before lunch | 500 mL (375 mL for women) 30 minutes before | Less food eaten | 21 older adults: 624 instead of 682 kcal at lunch; no effect in 29 young adults |
| More water or swapping drinks for it | In a review | No effect on weight shown | 7 trials: −0.33 kg (95% CI −1.75 to +1.08) (Chen Q Nutrients 2024. PMID 38612997[1]) |
| Water instead of sugar-sweetened drinks | A swap in the diet | Effect not proven | 3 trials: −0.81 kg (95% CI −1.66 to +0.03); swapping sweetener-containing drinks: +1.82 kg (Chen Q Nutrients 2024. PMID 38612997[1]) |
How to use it
Drink a glass of water half an hour before a main meal. The trials used 500 mL, but you can start with less. It helps alongside ordinary food and a diet, not instead of them.
When water replaced sugar-sweetened drinks in 3 trials, weight was 0.81 kg lower (95% CI −1.66 to +0.03): borderline, effect not proven. When water replaced drinks with sweeteners, weight was instead 1.82 kg higher (95% CI 0.97 to 2.67) (Chen Q Nutrients 2024. PMID 38612997[1]).
A real-life example
A man of 60 with excess weight decides to lose some. He does not give up favourite foods but drinks water half an hour before lunch and dinner and eats from his usual plate. At lunch and dinner he eats a little less. This is no miracle: in a trial of such people the effect was modest but noticeable together with a diet.
What the studies showed
The trial involved 48 people aged 55 to 75 with excess weight (48 started, 41 finished), all on a low-calorie diet. Some drank 500 mL of water before each meal, others only kept to the diet. Over 12 weeks the water group lost about 2 kg more. They lost 5.4 kg of fat versus 3.3 kg, but as a share of body weight the difference could be chance (p = 0.17) (Dennis E Obesity 2010. PMID 19661958[2]).
In the same trial, water 30 minutes before a test breakfast initially reduced food eaten (498 versus 541 kcal), while at 12 weeks the difference no longer reached significance: 480 versus 506 kcal (p = 0.069) (Dennis E Obesity 2010. PMID 19661958[2]).
In 21 older adults, water (375 mL for women, 500 mL for men) 30 minutes before lunch cut food eaten from 682 to 624 kcal (p = 0.02). In 29 young adults there was no difference (892 versus 913 kcal) (Van Walleghen E Obesity 2007. PMID 17228036[3]).
Water on its own gives a more modest picture. In a review of 7 trials that advised people with excess weight to drink more water or swap drinks for it, weight changed by −0.33 kg (95% CI −1.75 to +1.08), no effect shown (Chen Q Nutrients 2024. PMID 38612997[1]).
Who should see a doctor first
If your doctor has limited your fluids, for example in heart or kidney failure, agree the amount of water with them. See a doctor also for constant severe thirst, rapid weight loss without a reason, or swelling. Prescribed treatment is not changed on your own.
Bottom line
A glass of water half an hour before a meal is a simple habit that on a diet helps a little more and in older adults reduces food eaten. Water alone does not do the weight loss for you.
References
- Chen Q Nutrients 2024. PMID 38612997
- Dennis E Obesity 2010. PMID 19661958
- Van Walleghen E Obesity 2007. PMID 17228036
Key facts
- 500 mL of water before breakfast, lunch and dinner on a low-calorie diet for 12 weeks: in 48 people aged 55 to 75 (48 started, 41 finished), weight fell by about 2 kg more than on the diet without water (Dennis E Obesity 2010. PMID 19661958).
- In the same trial 5.4 kg of fat was lost versus 3.3 kg without water; as a share of body weight the difference could be chance (p = 0.17) (Dennis E Obesity 2010. PMID 19661958).
- In 21 older adults water 30 minutes before lunch cut food eaten at the meal from 682 to 624 kcal (p = 0.02); in 29 young adults there was no effect (Van Walleghen E Obesity 2007. PMID 17228036).
- In 7 review trials that advised drinking more water or swapping drinks for it, weight changed by −0.33 kg (95% CI −1.75 to +1.08): no effect shown (Chen Q Nutrients 2024. PMID 38612997).
- At 12 weeks in the trial the effect of water on food eaten at the test breakfast was no longer significant: 480 versus 506 kcal, p = 0.069 (Dennis E Obesity 2010. PMID 19661958).
