Most of the studies listed below looked at the effect of drinking one, 0.5 liter (17 oz) serving of water.

Drinking water increases the amount of calories you burn, which is known as resting energy expenditure (4).

In adults, resting energy expenditure has been shown to increase by 24–30% within 10 minutes of drinking water. This lasts at least 60 minutes (5, 6).

Supporting this, one study of overweight and obese children found a 25% increase in resting energy expenditure after drinking cold water (7).

A study of overweight women examined the effects of increasing water intake to over 1 liter (34 oz) per day. They found that over a 12-month period, this resulted in an extra 2 kg (4.4 lbs) of weight loss (8).

Since these women didn't make any lifestyle changes except to drink more water, these results are very impressive.

Additionally, both of these studies indicate that drinking 0.5 liters (17 oz) of water results in an extra 23 calories burned. On a yearly basis, that sums up to roughly 17,000 calories — or over 2 kg (4.4 lbs) of fat.

Several other studies have monitored overweight people who drank 1-1.5 liters (34–50 oz) of water daily for a few weeks. They found a significant reduction in weight, body mass index (BMI), waist circumference and body fat (8, 9, 10).

These results may be even more impressive when the water is cold. When you drink cold water, your body uses extra calories to warm the water up to body temperature.

BOTTOM LINE: Drinking 0.5 liters (17 oz) of water may increase the amount of calories burned for at least an hour. Some studies show that this can lead to modest weight loss.

Drinking Water Before Meals Can Reduce Appetite

Some people claim that drinking water before a meal reduces appetite.

There actually seems to be some truth behind this, but almost exclusively in middle-aged and older adults (11).

Studies of older adults have shown that drinking water before each meal may increase weight loss by 2 kg (4.4 lbs) over a 12-week period (4, 11).

In one study, middle-aged overweight and obese participants who drank water before each meal lost 44% more weight, compared to a group that did not drink more water (4).

Another study also showed that drinking water before breakfast reduced the amount of calories consumed during the meal by 13% (12).

Although this may be very beneficial for middle-aged and older people, studies of younger individuals have not shown the same impressive reduction in calorie intake.

BOTTOM LINE: Drinking water before meals may reduce appetite in middle-aged and older individuals. This decreases calorie intake, leading to weight loss.

Drinking More Water is Linked to Reduced Calorie Intake and a Lower Risk of Weight Gain

Since water is naturally calorie-free, it is generally linked with reduced calorie intake.

This is mainly because you then drink water instead of other beverages, which are often high in calories and sugar (13, 14, 15).

Observational studies have shown that people who drink mostly water have up to a 9% (or 200 calories) lower calorie intake, on average (16, 17).

Drinking water may also help prevent long-term weight gain. In general, the average person gains about 1.45 kg (3.2 lbs) every 4 years (18).

This amount may be reduced by:

• Adding 1 cup of water: Increasing your daily water consumption by 1 cup may reduce this weight gain by 0.13 kg (0.23 lbs).
• Replacing other drinks with water: Substituting a serving of a sugar-sweetened beverage with 1 cup of water may reduce the 4-year weight gain by 0.5 kg (1.1 lbs).

It is especially important to encourage children to drink water, as it can help prevent them from becoming overweight or obese (17, 3).

A recent, school-based study aimed to reduce obesity rates by encouraging children to drink water. They installed water fountains in 17 schools and provided classroom lessons about water consumption for 2nd and 3rd graders.

After one school year, the risk of obesity had been reduced by a whopping 31% in the schools where water intake was increased (19).

BOTTOM LINE: Drinking more water may lead to decreased calorie intake and reduce the risk of long-term weight gain and obesity, especially in children.

How Much Water Should You Drink?

Many health authorities recommend drinking eight, 8-oz glasses of water (about 2 liters) per day.

However, this number is completely random. As with so many things, water requirements depend entirely on the individual (20).

For example, people who sweat a lot or exercise regularly may need more water than those who are not very active.

Older people and breast-feeding mothers also need to monitor their water intake more closely (21).

Keep in mind that you also get water from many foods and beverages, such as coffee, tea, meat, fish, milk, and especially fruits and vegetables.

As a good rule of thumb, you should always drink water when you're thirsty, and drink enough to quench your thirst.

If you find you have a headache, are in a bad mood, are constantly hungry or have trouble concentrating, then you may suffer from mild dehydration. Drinking more water may help fix this (22, 23, 24).

Based on the studies, drinking 1-2 liters of water per day should be sufficient to help with weight loss.

Here's how much water you should drink, in different measurements:

• Liters: 1–2.
• Ounces: 34–67.
• Glasses (8-oz): 4–8.

However, this is just a general guideline. Some people may need less, while others may need a lot more.

Also, it is not recommended to drink too much water either, as it may cause water toxicity. This has even caused death in extreme cases, such as during water drinking contests.

BOTTOM LINE: According to the studies, 1–2 liters of water per day is enough to assist with weight loss, especially when consumed before meals.

Take Home Message

Water can be really helpful for weight loss.

It is 100% calorie-free, helps you burn more calories and may even suppress your appetite if consumed before meals.

The benefits are even greater when you replace sugary beverages with water. It is a very easy way to cut back on sugar and calories.

However, keep in mind that you're going to have to do a lot more than just drink water if you need to lose a significant amount of weight.

Water is just one, very small piece of the puzzle.