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Diabetes

Low Blood Sugar, High Urine Sugar

09/14/2009

Question:

I know others have asked around this question, but I hope to give you more information so that you can try to help as much as possible. My background: Growing up, sugar was prevalent in my house, as was drinking plenty of water. I was overweight, but not nutritiously aware. I went to college and stopped drinking as much water, but started having UTIs like crazy. Every time, they would say it had to do with the sugar - my urine was incredibly high in sugar. This has been going on for about 5-6 years now. I`ve had several kidney infections, all related to sugar-related UTIs. For the past 9 months or so, I`ve worked very hard to cut sugar out of my diet, and it seems to work. However, anytime I have any sugar and don`t drink enough water (as in, a soda and a brownie in one day and only drank two glasses of water within the 4-hour block I had the sugar), I get a UTI. I was just tested for diabetes (which does not run in my family, nor am I overweight). They did a FPG that came back with 85 after I had been fasting for over 14 hours, and they took a urine sample to be cultured (test results not back yet, but I`m sure they`ll find the same thing they always find - sugar). With that level of blood glucose, it doesn`t seem like I`m diabetic (I have done the oral test yet). Its been mentioned in other advice responses that there are other things this could be. Any advice in what I should look into would be helpful. I`m getting my MS in Biology, so I do know a bit about the science behind this, so feel free to be as specific as you feel you can be. Thank you for your help.

Answer:

The one explanation I can think of off hand is something I have heard about but never seen somebody with:  it could be that there is a variant transporter of glucose in the kidney tubule cells that results in a failure to re-absorb glucose from the filtrate the kidneys make when filtering blood.  Ordinarily, the glomerulus (plural glomeruli) which is the structures in the individual kidney functional unit called a nephron would filter the plasma, creating a filtrate that contains all the molecules in plasma below a certain molecular size cut-off, perhaps with some selectivity for charge.  That filtrate flows down the nephron and components are then re-absorbed via molecular transporters across the nephron wall back into the blood.  This would ordinarily result in essentially all the glucose in the filtrate being saved or re-absorbed as long as the blood glucose is below about 180 mg/dl - because of the shape of the mathematical function describing this, it is referred to as the "tubular maximum" or Tm for glucose.  A person with an abnormal transporter might have a reduced Tm for glucose, resulting in a net "spilling" of sugar into the urine.  Sometimes this is selective for sugar and sometimes it affects not only sugars but also other molecules, e.g. some amino acids.

Indeed, drug companies have done research on compounds that inhibit the function of this transporter as a possible treatment for diabetes, causing urinary loss of sugar - and in fact I believe one of the major side effects they have encountered which may limit the ability to use this treatment strategy is a high incidence of urinary tract infections.  i believe there are a number of different transporters that can mediate this effect, and you can read about these.  If you want a medical opinion about this, you would probably want to see a university-based endocrinologist or kidney specialist.

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Response by:

Robert M Cohen, MD Robert M Cohen, MD
Professor of Clinical Medicine
College of Medicine
University of Cincinnati