Development of minimally invasive 13C-glucose breath test to examine different exogenous carbohydrate sources in patients with glycogen storage disease type Ia

Abstract

Background: Glycogen storage disease type Ia (GSD Ia) is an autosomal recessive disorder caused by deficiency of glucose-6-phosphatase (G6Pase), resulting in fasting hypoglycemia. Dietary treatment with provision of uncooked cornstarch (UCCS) or a novel modified cornstarch (Glycosade®) is available to treat hypoglycemia, yet choice of carbohydrate to achieve a desirable glycemic control is debated.¹³C-glucose breath test (¹³C-GBT) can be used to examine glucose metabolism from different carbohydrate sources via ¹³CO₂ in breath.

Objectives: Our objectives were: 1) establishing the use of a minimally invasive ¹³C-GBT to examine in vivo glucose metabolism in healthy adults, and 2) using ¹³C-GBT to measure utilization of the standard UCCS vs. Glycosade® in GSD Ia and healthy controls.

Design: Experiment 1- Ten healthy adults (6F: 4 M, 22-33y) underwent ¹³C-GBT protocol twice as a proof-of-principle, once with oral isotope dose (glucose 75 g + [U-¹³C₆] d-glucose 75 mg) and once without isotope (only glucose 75 g) to test sensitivity of natural ¹³C-enrichment. Breath samples were collected at baseline and every 20 min for 240 min. Rate of CO₂ production was measured at 120 min using indirect calorimetry. Finger-prick blood glucose was measured using a glucometer hourly to test hypoglycemia (glucose <4 mmol/L). Experiment 2- Three GSD Ia (12y, 13y, and 28y) and six healthy controls (2F: 4 M, 10-32y) underwent ¹³C-GBT protocol twice: with UCCS or Glycosade® (based on their current prescribed dose 42-100 g) after ~4 h fast based on our GSD Ia patients with fasting tolerance.

Results: Findings 1- Maximum ¹³C-enrichments occurred at 200 min without and with [U-¹³C₆] d-glucose in all healthy adults, suggesting natural enrichment is sensitive for the ¹³C-GBT. Findings 2- Glycosade® utilization was lower than UCCS utilization in 12y and 13y GSD Ia, but was similar in the 28y GSD Ia.

Conclusions: ¹³C-GBT is a novel minimally invasive functional test to examine glucose metabolism in GSD Ia, and test new products like Glycosade®, which has the potential to improve nutritional management and individualized carbohydrate supply in GSD.

Keywords: 13C-GBT, 13C-glucose breath test; 13C-glucose; APE, atom percent excess; AUC, area under the curve; BIA, bioelectrical impedance analysis; BMI, body mass index; Breath test; CGM, continuous glucose monitor; CREU, clinical research and evaluation unit; Cmax, maximum peak enrichment in 13CO2 oxidation; F13CO2, rate of glucose oxidation; FCO2, CO2 production rate using indirect calorimetry; FFM, fat free mass; FM, fat mass; G6P, glucose-6-phosphate; G6Pase, glucose-6-phosphatase; GSD I, glycogen storage disease type I; GSD Ia, glycogen storage disease type Ia; Glucose-6-phosphatase; Glycogen storage disease type Ia; Glycosade®; HSCT, hematopoietic stem cell transplantation; OGTT, oral glucose tolerance test; PKU, phenylketonuria; REE, resting energy expenditure; UCCS, uncooked cornstarch; Uncooked cornstarch; VCO2, rate of carbon dioxide production; tmax, time to reach maximum 13CO2 oxidation.

Fig. 1

CONSORT flow chart depicting enrollment and allocation of patients with glycogen storage disease type I (GSD I). GSD Ia, glycogen storage disease type Ia; GSD Ib, glycogen storage disease type Ib; HSCT, hematopoietic stem cell transplantation; UCCS, uncooked cornstarch.

Fig. 2

Study day protocol for the 2nd experiment in patients with GSD Ia and healthy controls using the ¹³C-glucose breath test (¹³C-GBT), based on the natural enrichment of ¹³C in UCCS and Glycosade®.

Fig. 3

¹³C-glucose breath test (¹³C-GBT) and finger-prick blood glucose concentrations in healthy adults. A. Rate of ¹³C-glucose oxidation (F¹³CO₂) in healthy adults without vs. with stable isotope [U-¹³C₆] d-glucose for 240 min study day protocol. B. Finger-prick blood glucose concentrations in healthy adults from glucose without vs. with stable isotope [U-¹³C₆] d-glucose. Values are means ± SDs.

Fig. 4

¹³C-glucose breath test (¹³C-GBT) in the 12y GSD Ia and healthy age-matched controls. A. Glucose oxidation in GSD Ia (12y, F) who received UCCS vs. Glycosade® (dose 42 g). B. Glucose oxidation in healthy children who received UCCS vs. Glycosade® (dose 42 g).

Fig. 5

¹³C-glucose breath test (¹³C-GBT) in the 13y GSD Ia and healthy age-matched control. A. Glucose oxidation in GSD Ia (13y, M) who received UCCS vs. Glycosade® (dose 43 g). B. Glucose oxidation in a healthy child who received UCCS vs. Glycosade® (dose 43 g).

Fig. 6

¹³C-glucose breath test (¹³C-GBT) in the 28y GSD Ia and healthy age-matched controls. A. Glucose oxidation in GSD Ia (28y, M) who received UCCS vs. Glycosade® (dose 100 g). B. Glucose oxidation in healthy adults who received UCCS vs. Glycosade® (dose 100 g).

Fig. 7

Finger-prick blood glucose concentrations from UCCS vs. Glycosade®. A. GSD Ia (12y, F), B. Healthy children's controls, C. GSD Ia (28y, M), D. Healthy adults, E. GSD Ia (13y, M), and F. Healthy child.