What does the fasting blood sugar test really tell you?

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New Member

Date Joined Oct 2007
Total Posts : 11
   Posted 12/21/2007 5:17 PM (GMT -7)   
I was just wondering what does the glucose test (fasting blood sugar, in particular) tell you.

Does it tell you what your state is based on the food (sugar levels) of last night's meal or does it work in a cumulative way like cholesterol such that when you eat a lot of fatty food for like 6 months, then cholesterol will be higher in you take the lab test after compared to before.

I'm asking because it got me thinking, since it is in a fasting state, it may just be based on when you don't have food in your system, or if its basis is short-term then its basis will be the food you ate the day before or two days before only (for ex. you may have high fasting blood sugar but if you eat less or stop carbs for like 2 days before your lab test, then the fasting blood sugar results come out at normal levels even if ordinarily you should be quite high), or does it take its basis on a long-term like 2 or 3 months (for ex. if one takes the test november and eats a lot of sweets/foods in december and early january due to parties and does back to their normal diet for the rest of january, when they take the test at early february, the results will show the increase over november's result due to the excessive eating done in december & january even if the normal diet is average in carbs/sweets), thus, more cumulative in nature?

Sorry, if this is a silly question, just needed clarification in order to understand the test.


Lanie G
Forum Moderator

Date Joined Nov 2006
Total Posts : 6074
   Posted 12/21/2007 7:06 PM (GMT -7)   
Hi arzoo, I'm presuming you're not talking about when you test your blood sugar in the morning at home.  There are two lab tests (maybe more!  what do I know?) that I am familiar with: the glucose tolerance test or OGTT for which you have to fast for at least 8 hours beforehand.  That test is probably better for determining if you are really diabetic because it's so controlled.  When you drink a glucose solution, your blood is taken after 1 hour and then 2 hours and will show the doctor how your body metabolizes glucose.  It would be hard to fool the lab on that test.  The second test is the HbA1c or simply called the A1c in which blood is taken after fasting 8 hours and the results give the doctor an average of how glycated the red blood cells have been over the previous 3 months.  When glucose is in the blood stream, it pairs up with red blood cells and joins the hemoglobin within the cells for the lifespan of the red blood cell (which is about 120 days).  A poorly controlled or undiagnosed diabetic will have high blood sugar, of course, and this test will give a little history of how high it's been over the previous three months.  The A1c can only give an average though.  It cannot tell you that you've had highs, normals or lows.  It's possible to have a normal A1c but still be diabetic like me because I am controlling my blood sugar, but when I take the OGTT, then the results show I cannot metabolize glucose normally because my blood sugar will be very high on that test.  So, let's say you stay pretty well on a low carb plan and control your blood sugar on a daily basis but during the holidays you go ahead and party, eat some cake, pies, etc. a couple of times, but then you go back and stay on your good diet, then your A1c might still be ok as long as during the rest of the time, you were careful with what you ate.  Your A1c might be a little higher but not critical.  I think during the holiday season, it's very hard, even unreasonable, to say you will never eat a piece of pie or cake or a cookie.  I will but I'll choose wisely.  I'll eat a piece of cake but a smaller piece and leave the icing.  I'll eat 1 cookie but not 6.  In that way, I hope that I won't mess up too much.  Anyway, I don't think it's a good idea to binge.  I hope that helps answer your question? 

forum moderator - diabetes
"pre-diabetic" controlled so far by diet and exercise
following low/no carb diet, no meds

Post Edited (LanieG) : 12/21/2007 7:10:31 PM (GMT-7)

Veteran Member

Date Joined Apr 2004
Total Posts : 6056
   Posted 12/22/2007 2:28 PM (GMT -7)   
Yeah.... er.... what Lanie said!
~ Jeannie, Forum Moderator/Diabetes & Fibromyalgia
~Please remember that 50% of all doctors graduated in the bottom half of their class! Yours may be one of them...
"People are like stained glass windows: They sparkle and shine when the sun's out, but when the darkness sets in, their true beauty is revealed only if there is light within."- Elizabeth Kubler-Ross

Regular Member

Date Joined Mar 2007
Total Posts : 422
   Posted 1/9/2008 1:04 PM (GMT -7)   

I am bumping this post to put it up close to a post I plan of making.


Age: 63
07/05 - Biopsy: Suspicious looking cells. 
12/27/06 - Biopsy
1/24/07 - Dignosed cancer: Gleasn Grade: 3+3=6
02-14-07 Radical Open Nerve-sparing surgery
Pathology Stage of cancer: T2c
4-18-07: First Post-Op psa = 0.011
05/31/07: Cystoscope reveals "Stricture"
06/12/07: Procedure to remove stone & dilate uretha at stricture: Successful procedure.
8/16/07: Start bleeding again in urine.  Get to see a new urologist same day.
8/20/07: Go back to dr. for cystoscope.  "No sign of stricture"!  Wonderful news! Still leaking.
8/31/07: Back to new urologist for regular check up & received 2nd post-op Psa.  It was 0.05
8/31/07:  First Bio Feedback session.
9/19/07: Finished 4 Bio Feedback sessions.  No imporovement with leaking. 
9/26/07 - Started injections 
10/27-07 - Began using clamp at uro's encouragement.
1/6/08 - Still incontinent & impotent... and still "down."   

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