Water and the Human Body Series: Chapter 3 – Methods of Gaining Water into the Human Body

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“Water does not stay in one place, it flows quickly and carves its own way, even through stone – and when something blocks its way, water makes a new path.”

— Memoirs of a Geisha

Three Methods of Gaining Water in the Body

There are 3 methods of gaining water in the human body:

  • Drinking water and other fluids (accounts for ~60%-70% of the total fluid gain)
  • Dietary intake of food  (accounts for ~30%-20% of the total fluid gain)
  • Metabolic water produced by human cells through cellular respiration  (accounts for ~10% of the total fluid gain)

Drinking water and other fluids

The most common way of adding water to the body is by drinking water or other fluids.  These other fluids will varying on the amount of water content, for example, pure water compared to vegetable juices. 

Some fluids may actually create a net fluid loss or at least not contribute to water gain in the body.  For example, ethanol or drinking water is approximately 60% water depending on the type of spirit, yet it can be a very dehydrating beverage in excess quantities and does not replenish water content in the human body.  

All water-containing drinks can contribute to the total required for hydration including fruit juice, soft drinks, tea, coffee, dilute alcoholic drinks such as beer, as well as pure water itself.

Dietary intake of food

Most foods, even those that look hard and dry, contain water. The body can get approximately 20 to 30 percent of its total water requirements from solid foods alone.

Living or raw foods have a higher content of water since they have not been cooked by heating.  Cooked foods usually have less water content, for example, cooked broccoli versus raw broccoli.  Processed foods may contain a very low water content and are not preferred in the diet for water gains.

The variety of the diet will dictate the amount of water consumed in the diet.  The higher the consumption of water-rich foods (e.g., fruits, vegetables or soup), the higher the intake of water from food. Fruits and vegetables are indeed the food group which contains the most water: from 96% in a cucumber to 72% in an avocado, most contain more than 85% water.

The Figures 1 and 2 below list the water content of various food:

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Metabolic water

Metabolic water refers to water created inside a living organism through metabolism, by oxidizing energy-containing substances in their food. Humans obtain only about 8-10% of their water needs through metabolic water production.  

Each macronutrient that is metabolized in the body produces different quantities of water as a by-product.  Lipid (fat) oxidation produces the most water per gram.

The Table below illustrates the metabolic water production for all three macronutrients:   1

Metabolic Water from Macronutrients

Macronutrient in 100 gramsMetabolic Water Produced
Fat110 grams
Protein41.3 grams
Carbohydrate55 grams

One liter equals 1,000 grams, so 100 grams is the equivalent to 0.10 liters.

The amount of metabolic water produced by an average body is approximately 250 ml to 350 ml per day, depending on the variety of the diet.  Metabolic water production is proportional to the energy intake. As a result a sednetary individual may produce 250 ml of metabolic water per day versus a physically active individual engaging in strenuous exercise may produce up to 600 ml per day of metabolic water.

Metabolism of Water in the Body

Once water is ingested, it is absorbed in the gastrointestinal tract starting with the stomach in which a small portion is absorbed.  After leaving the stomach, water is absorbed mostly in the early segments of the small intestine, the duodenum and the jejunum. The small intestine absorbs about 6.5 liters per day and the colon absorbs about 1.3 liters per day.

From there the water enters the cardiovascular system by passing from the intestinal lumen into plasma mainly by passive transport, regulated by osmotic gradients.  The absorption process is very rapid where within 5 minutes after ingestion of water it us present in the plasma and blood cells. 

The vascular system then transports it to the interstitial tissue spaces between cells.  It then crosses the cell membrane via aquaporins, which are specific integral membrane proteins for water transport into the cell.

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Figure 3:  Aquaporin illustration

Recommendations for daily water intake

Insufficient scientific research exists that addresses the issue of the amount of water required to prevent disease or improve health. Due to this no exact consumption thresholds clearly exist or are linked to a specific health benefit or risk.  However, there are a number of agreed upon guidelines as to the minimum amount of water to be consumed depending on a variety of factors, including age, weight, health status, gender, level of activity (exercise) and environment.

In 2010 the European Food Safety Authority (EFSA) published official guidelines for total water intakes.  These guidelines are based on physiological parameters based on age.  These recommendations assumes no physical activity which will account for extra fluid loss and will need to be restored.

The Table below lists the recommended adequate intake values for water:

Dietary Reference Values for Water

Age rangeDaily adequate water intake
Infants
0-6 months680  mL/day or 100-190  mL/kg/day. From human milk
6-12 months0.8-1.0  L/day. From human milk and complementary foods and beverages
1-2 years1.1-1.2  L/day
Children
2-3 years1.3  L/day
4-8 years1.6  L/day
Adolescents
9-13 years – Males2.1  L/day
9-13 years – Females1.9  L/day
14-18 years – Males2.5  L/day
14-18 years – Females2.0  L/day
Adults
19-70 years – Males2.5  L/day
19-70 years – Females2.0  L/day
Special cases
Pregnant women2.3  L/day
Lactating women2.7  L/day

Source:  EFSA Panel on Dietetic Products, Nutrition, and Allergies (NDA); Scientific Opinion on Dietary reference values for water. EFSA Journal 2010; 8(3):1459. [48 pp.]. doi:10.2903/j.efsa.2010.1459.

Previously published Chapters in the Series

Chapter 1 – Water Content in the Human Body

Chapter 2 – The Function of Water in the Human Body

Future Chapters in the Series:

Chapter 4 – Water Loss by the Human Body

Chapter 5 – Water Balance in the Human Body

Chapter 6 – Dehydration

Chapter 7 – Waters Effect on Neurological Health

Chapter 8 – Edema – Fluid Retention

Previously published Chapters in the Series

Chapter 1 – Water Content in the Human Body

Chapter 2 – The Function of Water in the Human Body

Future Chapters in the Series:

Chapter 4 – Water Loss by the Human Body

Chapter 5 – Water Balance in the Human Body

Chapter 6 – Dehydration

Chapter 7 – Waters Effect on Neurological Health

Chapter 8 – Edema – Fluid Retention