Nutrition Myths
How much water should you drink to lose weight?

SUMMARY

  • The effects on weight loss may occur if you drink 500ml of water before, not during, each meal.
  • A daily 2L of water drunk before meals (based on an average of four meals per day), may result in the most optimal weight loss.
  • Although this result is based on studies conducted on middle age and elderly people, there is no harm in experimenting if you are young.
  • Please note that drinking water during meals may be not beneficial for weight loss and could cause some negative health effects.
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How much water should you drink to lose weight?

There is some evidence showing that water consumption may help reduce weight, but only in those individuals who are dieting for weight loss or for weight maintenance.

In other words, drinking water will not have any effect if you are not making an effort to lose weight. The possible reasons of why this happens are shown below.

Please note, however, that the evidence explaining this mechanism is not strong.

NOTE: having water before meals may have a different effect than during meals. While having about 2 glasses of water before meals may suppress appetite, drinking water with meals may have a negative outcome described below in the below section.

How can drinking water help you lose weight?

Drinking water before meals may help you lose weight, although the exact causes are still unknown.

The following are some hypothesis from the results of studies that can help explain the possible reasons for the association between drinking water and weight loss.

  1. Temporary appetite reduction

    Some studies show that drinking half a liter of water (500ml) half an hour before a meal temporarily suppresses hunger.

    This results in eating less during the next meal (approximately 13% in energy terms) and in possible weight reduction over the long term.

    These studies, however, only apply to middle age to elderly overweight or obese individuals.It is not known what factors are responsible for this reduction of energy intake.

    There is also no information in these studies about whether the participants’ subsequent meals were higher in energy to compensate for the previous one. (1, 2)

    More studies are, therefore, needed to prove that the overall dietary energy intake is reduced throughout the day. More research is also necessary to show how drinking water before meals affects the dietary energy intake in young adults.

    This interesting documentary shows somewhat contradictory findings. In Michael Mosley’s documentary “10 Things You Need to Know about Losing Weight”, a group of doctors and scientists conducted an experiment using a group of soldiers in order to prove some facts about the speed of water passing through the digestive system.

    Magnetic Resonance Imaging (MRI) scans show that water, even when consumed during meals, passes through the stomach within only a few minutes and gives a temporary feeling of fullness (satiety). Once the sensation has passed, the hunger returns.

  2. Increased metabolic rate

    Two studies showed that increasing water consumption raises the metabolic rate and causes weight loss through the elevated energy expenditure.

    Drinking 500 ml of water before a meal can boost the metabolism up to 30% and burns 24 extra calories (100kJ).

    The extra energy burning includes the effect of drinking water at room temperature which requires extra calories to warm it up to the body temperature.

    In other words, drinking cooler water causes your body to burn slightly more calories.This could mean that drinking 500ml of water before each meal (e.g. 4 times per day) may result in up to 96 extra calories burned per day. (3)

    However, these associations are inconsistent with two other studies. These not only don’t confirm these results, they show that the thermogenic effects are minor. (4, 5)

    Therefore, more studies are needed.

Other hypotheses on drinking water and weight loss

Fluid retention

Dr. Donald S Robertson claims (REF: “Water, how 8 glasses a day keep fat away.” 1986. McCall’s Magazine.) that not drinking enough water may result in edema – fluid retention. 

Our bodies’ sense of survival causes it to accumulate water when the intake is insufficient. Water gets stored outside of the cells, resulting in swollen feet, legs and hands. 

The known causes of edema are: eating too much salt, sunburn, heart failure, kidney failure, liver cirrhosis, pregnancy, issues with the lymph nodes after mastectomy, some medicines and standing or walking in hot weather. (6)

Despite Dr. Robertson’s claims, I haven’t found any additional scientific studies that supports it.

Additional reading: “8 glasses of water per day” myth.

Does the lack of water cause more fat to be stored in the body cells?

Another claim by Dr. Robertson is that if we don’t drink enough water, our kidneys do not function well. As a result, they pass some of their functions to the liver.

The liver (responsible for fat metabolism), in turn, gets overloaded with extra work and becomes inefficient. This causes the fat to “escape” the breakdown process and accumulate in the fat cells.

I have found no scientific studies that confirm this claim.

Drinking water with meals

Drinking water while eating may cause a quicker absorption of glucose from food.

Dr. Rob Thompson in his book “The Glycemic-Load Diet: A powerful new program for losing weight and reversing insulin resistance.” (7) states that drinking water with food may actually make you eat more rather than less.

Water entering the stomach and quickly exiting, may take with it some carbohydrates. Since it rapidly passes through the stomach and gets absorbed through the walls of the small intestine, the sugars that left the stomach with water may cause an increase in the blood glucose and insulin levels, leading to insulin resistance. 

Insulin resistance is associated with sugar cravings and overeating carbohydrates. However, additional research is needed on this hypothesis.

Drinking water may dilute other nutrients that will pass through without being absorbed by the small intestine.

Some scientists and nutrition experts think that drinking while eating may dilute some enzymes, causing incomplete/inefficient digestion and releasing incompletely digested food to the small intestine together with a quick passing of water.

Undigested food cannot be absorbed by the intestine and gets excreted.

The effect on the amount of food eaten during the following meals, however, is unknown. Once again, more research is needed.

Drinking water with meals may worsen reflux symptoms

Drinking fluids while eating food may cause a temporary overfilling of the stomach.

The stomach has about one liter capacity. However, it can stretch when we stuff it with a lot of food and/or fluid. This can have negative consequences for people with dyspepsia or reflux/GERD related issues.

When digesting, the stomach muscles churn the food while stomach acids and enzymes break it down.

If water is added, the volume of food inside increases and in the case of people predisposed to reflux, the food together with the stomach acids are pushed up to the esophagus through a malfunctioning muscle called the sphincter.

This influx of acid burns not only the delicate tissue of the esophagus but also the sphincter, making it swell.

Once it is swollen, the sphincter cannot close properly and more acid is likely to pass through to the esophagus either at the next meal, when lying down, during exercise, bending over etc. (8, 9)

NOTE: You should be aware that some studies are sponsored by bottled water companies and, therefore, may be biased.

NUTRITION FACTS VS NUTRITION MYTHS

You will find a summary of the most common nutrition myths and evidence-based nutrition facts here.

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