With the rise in popularity of low-carb and keto diets has come a curious unintended consequence: expecting mothers receiving a false positive on their Gestational Diabetes Mellitus (GDM) test.
Gestational Diabetes Mellitus (GDM)
GDM is diabetes diagnosed in the second or third trimester of pregnancy that is not obviously type 1 or 2.[1] Women with diabetes in the first trimester would be classified as having type 2 diabetes.[1] The essential contributing factor to GDM is pancreatic insufficiency.[2] Essentially, your pancreas is not making enough insulin for your body to overcome the increased insulin resistance due to the placental hormones of pregnancy and increased maternal fat tissue, which could potentially cause your blood glucose levels to remain high.
GDM = overworked pancreas. Not making enough insulin for you and the baby.
Testing
There are two test for GDM, the one-step and two-step procedure.
The one-step test is the first test for diagnosing GDM. The diagnosis of GDM is made when any of the following plasma glucose values are met or exceeded.[3]
Fasting: 92 mg/dL (5.1 mmol/L)
1 hour into the test: 180 mg/dL (10.0 mmol/L)
2 hours into the test: 153 mg/dL (8.5 mmol/L)
If for whatever reason you do not pass the one-step Oral Glucose Tolerance Test (OGTT), ask for the two-step test or ask them to confirm your results by testing your HbA1c, as it shows blood sugar levels over time as opposed to the brief snapshot these one and two-step tests provide. HbA1C can be measured with a simple blood test. The two-step test involves the following.[3]
Step 1: Perform a 50 g Glucose Loading Test (GLT) (non-fasting), with plasma glucose measurement at 1 hour. If the plasma glucose level is ≥140 mg/dL (7.8 mmol/L), proceed to a 100 g OGTT.
Step 2: The 100 g OGTT should be performed when the patient is fasting. The diagnosis of GDM is made if at least two of the following four plasma glucose levels (measured fasting and 1 h, 2 h, 3 h after the OGTT) are met or exceeded the levels in the chart below. There are two different sets of proposed cutoff points, so it my vary by your doctor.
| Carpenter/Coustan | NDDG (57) | |
|---|---|---|
| Fasting | 95 mg/dL (5.3 mmol/L) | 105 mg/dL (5.8 mmol/L) |
| 1 Hour | 180 mg/dL (10.0 mmol/L) | 190 mg/dL (10.6 mmol/L) |
| 2 Hours | 155 mg/dL (8.6 mmol/L) | 165 mg/dL (9.2 mmol/L) |
| 3 Hours | 140 mg/dL (7.8 mmol/L) | 145 mg/dL (8.0 mmol/L) |
Low Carb Diets and OGTT
So, if you are on a low-carb diet (less than ~130 g of carbs per day) you may have developed peripheral insulin resistance (aka physiological insulin resistance). During this state, any carbohydrate that you eat is preferably going to be diverted to your brain because the many of the tissues in the rest of your body will not be taking up glucose as readily.[4]
If you take either the one or two step test while on a low-carb diet the results are more than likely to indicate you have gestational diabetes. This is because the muscles will not be able to absorb the large dose of glucose provided in these tests, thus causing your blood glucose levels to remain high.
In order to prevent a false positive, about 5–7 days before the test is conducted (but no less than 3) make sure you are eating plenty of carbs — 200+ grams a day minimum. Go ham with those sweet and white potatoes, rice, oats, squash, grains, etc. If you are already eating sufficient carbs then this shouldn’t be a problem.
A Final Note
It is not advised that women try to lose weight while pregnant. If you are eating low-carb with the intention of weight loss, please speak with your doctor. Below, the folks at Precision Nutrition have put together an infographic on what to eat while pregnant. Click on the picture to jump to their guide.
Sources
1 - American Diabetes Association. Classification and diagnosis of diabetes. Sec. 2. In Standards of Medical Care in Diabetes—2015. Diabetes Care 2015;38(Suppl. 1):S8–S16. http://care.diabetesjournals.org/content/38/Supplement_1/S8.full#T5
2 - Retnakaran R, Qi Y, Sermer M, Connelly PW, Hanley AJG, Zinman B. β-Cell Function Declines Within the First Year Postpartum in Women With Recent Glucose Intolerance in Pregnancy. Diabetes Care 2010;33(8):1798-1804. doi:10.2337/dc10-0351. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2909065/
3 - American Diabetes Association. Diagnosis and Classification of Diabetes Mellitus. In Standards of Medical Care in Diabetes—2015. Diabetes Care January 2014 vol. 37 no. Supplement 1 S81-S90. http://care.diabetesjournals.org/content/37/Supplement_1/S81.full
4 - Rennan de Oliveira Caminhotto, Fabio Bessa Lima. Impaired glucose tolerance in low-carbohydrate diet: maybe only a physiological state. American Journal of Physiology - Endocrinology and Metabolism. Published 15 December 2013. Vol. 305 no. 12, E1521DOI: 10.1152/ajpendo.00580.2013. http://ajpendo.physiology.org/content/305/12/E1521