Why Does Diabetes Cause Excessive Urination and Thirst? A Lesson on Osmosis

A TABA Seminar on Diabetes

I have the pleasure of being an executive member of the Toronto Applied Biostatistics Association (TABA), a volunteer-run professional organization here in Toronto that organizes seminars on biostatistics.  During this past Tuesday, Dr. Loren Grossman from the LMC Diabetes and Endocrinology Centre generously donated his time to deliver an introductory seminar on diabetes for biostatisticians.  The Institute for Clinical and Evaluative Sciences (ICES) at Sunnybrook Hospital kindly hosted us and provided the venue for the seminar.  As a chemist and a former pre-medical student who studied physiology, I really enjoyed this intellectual treat, especially since Loren was clear, informative, and very knowledgeable about the subject.

blue circle

The blue circle is a global symbol for diabetes.

Source: Wikimedia Commons


Diabetes is a group of metabolic diseases that are characterized by a high concentration of glucose in the bloodstream.  Glucose is a common monomer of carbohydrates that exists in many foods, including bread, pasta, rice, fruits, vegetables, and refined sugar. It provides the fuel for the cells of our bodies to function.

glucose open-chain     glucose cyclic

Chemical Structures of Open-Chain and Cyclic Glucose

Source: Acdx via Wikimedia Commons

For a variety of reasons that distinguish the different types of diabetes, diabetics cannot absorb glucose normally, leaving an excess of glucose in the bloodstream.  Diabetes leads to many health problems, like kidney failure, blindness, heart attacks and strokes.

The Growing Prevalence of Diabetes

It was interesting but sad for me to learn about the increased prevalence of diabetes in North America and around the world.  Loren commented that diabetes was a specialized niche area in endocrinology when he began his research in this field over 25 years ago, but it is now a major area of study in medical research because of its epidemic proportions.  According to the Canadian Diabetes Association, there are roughly 285 million people in the world who are affected by diabetes. and this number is expected to hit 438 million by 2030.  Some direct quotations, with emphases added:

– “People with diabetes incur medical costs that are 2 to 3 times higher than those without diabetes.  A person with diabetes can face direct costs for medication and supplies ranging from $1,000 to $15,000 a year.”

– “Life expectancy for people with type 1 diabetes may be shortened by as much as 15 years. Life expectancy for people with type 2 diabetes may be shortened by 5 to 10 years.”

Why Does Diabetes Cause Excessive Urination and Thirst?

Two common symptoms for all diabetics are frequent urination (polyuria) and increased thirst (polydipsia).  A fundamental but interesting chemical question arises: Why does a high concentration of glucose in the bloodstream cause excessive urination and thirst?  It turns out that a basic understanding of osmosis helps to understand this connection.

Osmosis is the net movement of solvent molecules in a solution with a semi-permeable membrane from a region with a higher proportion of the solvent to the region with a lower proportion of solvent.  Essentially, the solvent molecules will diffuse so that the two sides of the semi-permeable membrane have the same proportion of solvent.  Equivalently, the solvent molecules will diffuse from the side with the lower concentration of solutes to the side with the higher concentration of solutes until the solute concentration is the same across both sides.  In chemistry and in many biological systems, the most common solvent is water.  In biology, osmosis occurs all the time across the walls of our cells, with our cell membranes being the semi-permeable membranes.


How Osmosis Works

Source: Hans Hillewaert via Wikimedia Commons

In the case of diabetes, the solvent is water, and the solute of interest is glucose.  Diabetics have high blood glucose, and, since it is not absorbed by the cells of the body, it eventually goes to the kidneys.  Some of it diffuses into the urine and is excreted, which causes the sweet smell/taste of urine from diabetics.  (In fact, Loren told us that tasting urine for sweetness is a former method of diagnosis of diabetes.)  Since the urine becomes more concentrated with glucose, water leaves the bloodstream and diffuses into the urine, and this leads to more frequent urination.  Of course, since the bloodstream is more concentrated with solutes, it now lacks water, and this leads to increased thirst.


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