This week we are studying how nutrients affect fluid balance. We can’t live without fluid. The human body is about 70% water. That means every chemical reaction that takes place in our body takes place in the presence of water. Without adequate water, chemical reactions necessary for life is slowed down or stopped. The amount of water that our body contains will depend on our age, on our gender and the amount of lean muscle we have. Men tend to hold more water since they have more lean muscle than women. The body mass in elderly people will have less water due to muscle loss associated with aging. 

Water has a number of functions in the body including:

  1. Acting as a universal solvent to dissolve nutrients
  2. Transporting nutrients to cells in the body 
  3. Maintaining our blood pressure
  4. Keeping us cool
  5. Maintaining body temperature  
  6. Lubricating tissues and joints 

How do we maintain adequate water in our body?

We lose water continuously either by sensible (observable) or insensible (unobservable) water loss. Sensible water loss include loss through urine and feces while insensible water loss is water loss through sweating and breathing. The amount of fluid loss could be influenced by temperature and humidity of the day, illness resulting in vomiting and diarrhea, taking diuretics, exercising and being pregnant. Pregnant women will need to drink more water since some water will be diverted to the growing fetus. We maintain body fluid by the following two mechanisms: 

Mechanism 1: Maintenance of solute concentration 

When we lack water, this causes solutes (dissolved solids) in our body fluid to become more concentrated. This is detected by the hypothalamus which sends a message to the the pituitary gland to release anti-diuretic hormone (ADH). ADH then causes the kidney to reabsorb more water, reducing the amount that is lost through the urine and restoring solute concentration.

Mechanism 2: Maintenance of blood volume 

When we lack water this causes a drop in blood volume. This drop is detected by the kidneys which responds by secreting an enzyme called renin. Renin causes the liver to produce a protein called angiotensinogen which gets converted to angiotensinogen II. Angiotensinogen II is a powerful vaso-constrictor hormone that causes blood vessels to constrict (get smaller) and thereby increasing blood pressure back to normal levels. At the same time, angiotensinogen II causes the adrenal glands above the kidneys to produce a hormone called aldosterone. Aldosterone causes the the kidney to retain more salt. Remember that wherever salt goes, water follows. This results in more water being retained and hence blood volume is restored.  

Reference: Thompson,&  J., Manore, M., Vaughan, L. (2020). The science of nutrition (5th ed.). New York. Pearson

Courtney Simons
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Courtney Simons is a food science professor. He holds a BS degree in food science and a PhD in cereal science from North Dakota State University.
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