Question from San Antonio, Texas, USA:

I am 24 years old and was diagnosed with gestational diabetes at 27 weeks. The results of my three-hour test were: 68 mg/dl [3.8 mmol/L], 178 mg/dl [9.9 mmol/L], 169 mg/dl [9.4 mmol/L], and 150 mg/dl [ 8.3 mmol/L]. So, the last two numbers were slightly higher than they should have been. However, I was not given very detailed instructions before the test, and I actually fasted for over 17 hours because of an early dinner and no snack. I understand that the recommendation is to fast 8-14 hours, with the most common time being 9-10 hours. I have read that fasting for over 14 hours can lead to falsely high numbers because the body may try to overcompensate. However, I don’t completely understand this.

I asked for another test at 30 weeks and only fasted for 9 hours this time. The results from this second three-hour test showed that I do not have gestational diabetes although I don’t have the actual numbers yet. However, my doctor says that the three-hour test takes into account that people eat a wide variety of things and that fasting for 17 hours before the test would not make a difference. He believes the first test is the most accurate. I understand that the hormones in pregnancy that cause the insulin resistance usually peak around 32 weeks. So if the first test was accurate, I would expect the second test to be a bit higher (or at least close to the same) because the hormones should be stronger at 30 weeks than at 27 weeks.

I am currently monitoring four times a day, and my results are well within the guidelines. My average fasting levels are around 80 mg/dl [4.4 mmol/L], average two-hour after breakfast is around 80 mg/dl [4.4 mmol/L], lunch around 85 mg/dl [4.7 mmol/L], and dinner around 95 mg/dl [5.3 mmol/L].

Is it true that the body could overcompensate after 17 hours of fasting and that this can result in falsely high numbers? If the time of the fast really does not matter, why would organizations such as the American Diabetes Association bother with specifically stating that the fast should be no more than 14 hours?