Carbohydrate Intake Prior to Oral Glucose Tolerance Testing

Abstract

With the emergence of glycated hemoglobin as a diagnostic test for diabetes, oral glucose tolerance tests (OGTTs) have become rare in endocrinology practice. As they have moved out of favor, the importance of patient instructions on preparation prior to OGTT has faded from memory. Decades old literature, well known to endocrinologists a generation ago, emphasize the importance of carbohydrate intake prior to OGTT. In this expert endocrine consult, we discuss an OGTT performed in a research setting without adequate carbohydrate intake at the evening meal prior to the OGTT. The resultant elevated plasma glucoses at 1-hour and 2-hours mimicked the loss of first-phase insulin release seen in early type 1 and type 2 diabetes. With clinical concern that the research participant had evolving type 1 or type 2 diabetes, the volunteer was subjected to additional testing and experienced anxiety. Repeat OGTT was normal after adequate carbohydrate intake (>150 grams/day and >50 grams the evening prior to overnight fast for the study). The physiology of this phenomenon is explored and is likely mediated through beta-cell adaptation and alteration in peripheral glucose uptake in response to nutrient exposure. The learnings of decades ago have clearly faded, and this literature should be revisited to ensure that OGTT results are not compromised when ordered for clinical or research purposes.

Prolonged Glycemic Adaptation Following Transition From a Low- to High-Carbohydrate Diet: A Randomized Controlled Feeding Trial

“…Prolonged Glycemic Adaptation Following Transition From a Low-to High-Carbohydrate Diet: A Randomized Controlled Feeding Trial protocols recommend an unrestricted diet containing $150 g/day carbohydrate for 3 days prior to an oral glucose tolerance test (OGTT) (7). However, the adequacy of this 3-day preparatory period specifically and the process of physiological adaptation to changes in macronutrients in general have not been well characterized.…”

Weight Trajectory Since Birth, Current Body Composition, Dietary Intake, and Glucose Tolerance in Young Underweight Japanese Women

“…Although lower insulin secretion, lower muscle function, insulin resistance in muscle, and adipose tissue may contribute to elevated postload glucose values, 35 , 36 a low carbohydrate intake before oral glucose testing is another forgotten factor, which is associated with elevated postload glucose values. 37 As young underweight women in the latter study were underweight probably due to desire to be thin, 36 young underweight women with IGT consumed, on average, 134 g of carbohydrate a day (46.7% of 1152 kcal), which is lower than the recommended daily carbohydrate intake (>150 g). 37 In contrast, four underweight women with IGT in this study consumed, on average, 251 g a day (204–273 g).…”

“… 37 As young underweight women in the latter study were underweight probably due to desire to be thin, 36 young underweight women with IGT consumed, on average, 134 g of carbohydrate a day (46.7% of 1152 kcal), which is lower than the recommended daily carbohydrate intake (>150 g). 37 In contrast, four underweight women with IGT in this study consumed, on average, 251 g a day (204–273 g).…”

The Effect of a Lifestyle Intervention on Type 2 Diabetes Pathophysiology and Remission: The Stevenshof Pilot Study

“…This was especially the case for the OGTT performed at baseline, since all participants stopped using insulin during the intervention period. Additionally, the OGTT was not performed after three days of unrestricted diet with at least 150 g of carbohydrates as recommended by the WHO [ 27 ], but guidelines from standard clinical practice were followed, as the DLI was implemented as part of regular primary care. These clinical practice guidelines for the OGTT only dictate fasting of at least 10 h prior to the test.…”

“…OGTT guidelines as used in standard clinical practice were used. No specific instructions on carbohydrate intake prior to the OGTT were provided, as suggested by Klein et al (2021) [ 27 ]. At three and six months, diet was in line with the dietary instructions provided to the participants.…”