The Dreaded Oral Glucose Tolerance Test

Maybe you ladies out there remember the oral glucose tolerance test (OGTT) from when you were pregnant.  It’s mostly used these days as a screening for gestational diabetes, but for some reason, it was prolific in this study.  I had the privilege of doing three over a 21-day period!

The first one was to start the study; the second was midway through when changing continuous glucose monitoring sensors; the final one was the last day of the study.  They were all very different, but yet had some similarities that I want to discuss here.  I am not convinced that the OGTT is the best measure of whether someone has a healthy blood sugar response, especially when they’re eating a low-carb diet, but there are work arounds for “passing” the test under those conditions.

I read a little about the OGTT and how it worked for low-carb diets before doing my first test.  Basically the consensus was to eat over 150g of carbohydrates for three days preceding the test.  Of course I didn’t do that prior to my first test; rather, I went in after eating decently for a few days.  And, as expected, I “failed” the test.

The way the test works is you drink a 75g glucose solution after fasting a minimum of 10 hours.  Your glucose and insulin measurements are taken periodically (in this case, every 15 minutes).  In order to pass, your two-hour glucose measurement needs to be less than 140mg/dL.  When your body is used to dealing with glucose, this may not seem like a huge deal, but if you’re either are insulin resistant or don’t make enough insulin, this could be problematic.  Imagine what would happen if you ingest that much sugar when your body is not accustomed to it… Definitely another concern.

For the first test, my fasting insulin and blood glucose were both low, 1.4mIU/mL and 85mg/dL (81mg/dL from the lab sample), respectively. My insulin level was actually below the normal range, so perhaps it was too low, exacerbating the effect of my body having no idea what to do with all of that glucose.  

During the test, my body did not respond well to the sugar at all.  I immediately felt cold, started burping, and then nausea set in.  A little over an hour in, I got a horrible headache and had light sensitivity.  Once my glucose levels started to decrease, I started to feel better. It was interesting that I was able to attribute those symptoms to sugar, as I’ve felt then before, and couldn’t isolate their cause. I suspected dairy, alcohol and sugar, but wasn’t able to find confirmation before.

The curve for my first OGTT is below:


My glucose concentration started out pretty modest, but then increased to a max of 214mg/dL staying above 200mg/dL for an hour!  That was my highest ever glucose level and ingesting 75g of sugar isn’t a ton by Standard American Diet (SAD) standards!  At the 2-hour mark, my glucose was at 185mg/dL, so I definitely failed the test.  I suspected this was because my insulin was slow to respond and it maxed out at 51.7mIU/mL.  I didn’t know if that was a good or bad insulin value, but I remembered reading in Dr. Hyman’s “The Blood Sugar Connection” that your insulin should never be above 30mIU/mL during the test.  That information is available in an article here. I was pretty confused by this because wouldn’t less insulin make my glucose decrease even slower?  My fasting insulin was super low, so I shouldn’t be insulin resistant, but why did my blood sugar go so high and stay so high when by Dr. Hyman’s standards, I still had too much insulin at the maximum? Neither my doctor, nor the study doctor felt my insulin spiked too high.

I’m not really sure what happened to my insulin at t=150 minutes.  It’s like it fell and then rose again, which makes no sense. Perhaps it was a bad lab analysis?  I’m also missing the final data point due to a lab issue. My glucose was pretty slow to respond to the insulin and the concentration decreased much more slowly than expected.  In fact, I didn’t even feel hypoglycemic (shaky, hot flashes, general feeling sick) until I was about to eat an hour later.  Eating a low carb meal helped my blood glucose stabilize and I felt fine after that.

Ten days later, I had the pleasure of my second OGTT.  This was also after decent eating, excluding a few carefully timed experiments with my continuous glucose monitor (plenty more to come about this). However, with these experiments and the first OGTT, I let my blood sugar stabilize before consuming additional carbohydrates.  My eating over the first ten days of the study was very controlled, although I probably consumed more carbs than I normally would have.  I had similar symptoms during the test, including nausea, headache, and light sensitivity.  However, my second response curve ended up picture perfect!


My fasting glucose and insulin were a little higher than the first OGTT, coming in at 90mg/dL (75mg/dL from the lab sample) and 2.5mIU/mL, respectively.  My glucose concentration increased rapidly to a maximum of 241mg/dL, which broke the record for my highest blood glucose value to date.  My insulin also increased comparatively and maxed at 99.9mIU/mL, almost double what I saw in the first OGTT! I feel like my body recognized the 75g of glucose this time and knew what to do about it. Unfortunately, it still wasn’t efficient at removing it, as at the 2-hour mark, my glucose concentration was 176mg/dL.  I failed again!

However, my glucose was clearly on the decline at a much faster pace this time and bottomed out shortly after the study.  I made it to my low-carb eating establishment when the crash happened and it was pretty severe.  The alarms on my continuous glucose monitors were going crazy! To make matters worse, eating my low-carb meal did not alleviate my hypoglycemia as it did during the first OGTT, so I had to eat sugar to raise my blood glucose to normal levels. That put me on a pendulum swing from low to high sugar, which was impossible to recover from. Once my blood sugar raised, I drove home and took a nap because I felt so horrible.

Unfortunately, the sugar roller coaster continued over the Memorial Day weekend. This encouraged me to change my experiments for the second ten days while wearing the continuous glucose monitor. It was at this point I decided to see if eating the SAD would help me pass the OGTT. Was their validity to the notion of containing 150g of carbohydrate for at least three days prior to the OGTT?

I’ll tell you more about that next time…. and eventually, I’ll get to the glucose experiments while wearing the continuous glucose monitor!

7 thoughts on “The Dreaded Oral Glucose Tolerance Test

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