Endocrinology (including Diabetes)/fasting glucose test


QUESTION: Sheri, when just a fasting glucose blood test is done, does it at all reflect how someone ate for the past several days or weeks? The reason I ask is, as I wrote you recently, my reading was above normal and I plan to take the test again (the HbA1C test will follow if need be). But I was first going to cut way down on carbs and sugar for 2-3 months before taking it. Or would doing that not really affect the results of the test? Thanks

  The fasting blood glucose reflects mostly what you did the day before, such as whether you exercised, drank alcohol the night before, had a large dinner, etc.  In some people it also reflects that their liver simply overproduces glucose overnight (their livers are insulin resistant).  An A1C test gives the overall BG levels for the past 2 months or so, which includes the AM values as well as after-meal spikes.  It's also possible to do an oral glucose tolerance test, which really assesses your body's ability to handle large loads of rapidly-absorbed carbs.  So, all three test different things, but any one of them can be used to diagnose diabetes or prediabetes.  So, would cutting down your carb and sugar for 2-3 months affect your fasting BG?  Hard to know.  It would definitely lower an A1C test, though.  Sheri


---------- FOLLOW-UP ----------

QUESTION: Sheri, I do apologize bugging you again with yet another question, but you seem to have a good handle on Diabetes and testing and are clear in your answers. If someone eats very low to almost no carbs the day before a fasting blood glucose test, would that backfire? Meaning, would that cause the body to compensate and produce more glucose than usual, resulting in a higher level than if I ate a moderate amount of those? I also plan to take my usual brisk walk the day before, assuming that will help lower the reading. Thanks again!

  Although I'm not absolutely sure what the response would be in someone without diabetes, it is possible that putting yourself into ketosis from not eating carbs the day before could potentially increase insulin resistance.  Fasting for 24 hours certainly does increase levels of blood fats and insulin resistance, but I'm not certain you'd have exactly the same response if you still ate enough calories (albeit no carbs).  if I were you, I would eat most of my carbs earlier in the day (and before exercising) and then eat fewer in the evening before you do the test the next morning.  Insulin action stays higher when you haven't fully repleted the muscle glycogen (carb) stores.  Sheri

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Sheri Colberg, PhD


I am an exercise physiologist with a PhD who specializes in diabetes--as such I CANNOT answer general questions about other endocrine problems as I am neither an expert in all areas of endocrinology nor am I a medical doctor. My expertise lies in answering questions about diabetes (of any type) and physical activity, so please limit your questions to those areas. I can help you if you want to begin exercise or if you're already a diabetic athlete, and I am prepared to respond to questions about physical activity to which even your diabetologist may not know the answer. I can give suggestions about changes in your diabetic medications that differing types and intensities of exercise may necessitate, but I will have to refer you to your regular health care team to get final approval to make such changes. I can also answer questions about physical fitness, exercise metabolism, prediabetes reversal, and prevention of type 2 diabetes and diabetic complications.


I have both personal and professional experience in the areas of diabetes and exercise/physical activity. On a personal level, I have had type 1 diabetes since 1968, and I have been an avid exerciser since I was a child. Professionally, I have been conducting clinical studies on diabetes and exercise since 1992, largely with funding from the American Diabetes Association. I am also the author of 8 books related to diabetes, exercise, and more: The Diabetic Athlete (2001), Diabetes-Free Kids (2005), The 7 Step Diabetes Fitness Plan (2006), 50 Secrets of the Longest Living People with Diabetes (2007), The Science of Staying Young (2007), Matt Hoover's Guide to Life, Love, and Losing Weight (2008), Diabetic Athlete's Handbook (2009), and Diabetes? No Problema! (2009).

I am a Fellow of the American College of Sports Medicine, a professional member of the American Diabetes Association (and professional volunteer for the ADA), and a former member of the Board of Directors of the Diabetes Exercise & Sports Association.

I have published research and review articles in the following journals and magazines: Diabetes Care, Diabetes, Journal of Diabetes & Its Complications, Diabetes Self-Management, Medicine & Science in Sports & Exercise, Journal of Applied Physiology, The Physician and Sportsmedicine, Journal of Clinical Investigation, International Journal of Obesity, FASEB Journal, The Diabetes Educator, Journal of Physical Education, Recreation, and Dance, Clinical Exercise Physiology, Clinical Diabetes Reviews, Insulin, ACSM's Health & Fitness Journal, Biomechanics, On the Cutting Edge, Diabetes Technology & Therapeutics, Microvascular Research, Drug Benefit Trends, ACSM Certified News, Diabetes Health, SportEX Health, Diabetes Focus, Diabetes In Control, dLife-For Your Diabetes Life, Pediatrics for Parents, and My TCOYD (Taking Control of Your Diabetes) Newsletter. I have also been interviewed in myriad other magazines, such as Men's Health, Men's Fitness, Diabetes Forecast, Countdown Magazine, Joe Weider's Muscle & Fitness, Health, Tidewater Parent, Barron's News, Diabetes New Day, and Newsweek International.

I have an undergraduate degree (1985) from Stanford University, a Master's degree in exercise physiology (1987) from the University of California, Davis, and a Ph.D. (1992) from the University of California, Berkeley, in the same field. I also spent two years in an NIH-funded postdoctoral research position in endocrinology (studying obesity, diabetes, metabolism, and exercise) at the University of Pittsburgh School of Medicine (1993-1994).

Awards and Honors
Fellow, American College of Sports Medicine (FACSM) - 1996 Old Dominion University Darden College of Education Young Investigator Grant Award 2003 Honor Society of Phi Kappa Phi, Old Dominion University Chapter 2004-Present Great Women of the 21st Century by the American Biographical Institute 2005 Edition Old Dominion University Darden College of Education Largest Research Grant Award 2006 Old Dominion University Darden College of Education Publications Award (for greatest number) 2006 Saint Louis University, The Max K. Horwitt Memorial Lecture Distinguished Lectureship Award 2008 Old Dominion University Darden College of Education Publications Award 2009 Old Dominion University Darden College of Research Grants Award 2009

Past/Present Clients
I have consulted for numerous groups, including Can-Am Care, Numera|Social, California Commission on Peace Officer Standards and Training(about candidates with diabetes), Animas Corporation (an insulin pump company), Therasense, Inc. (makers of the Freestyle blood glucose meters), Council of Healthcare Advisors, and the City of Chesapeake (Virginia) Health Department.

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