Glial acetate metabolism is increased following a 72-h fast in metabolically healthy men and correlates with susceptibility to hypoglycemia

Abstract

Aims: Prior exposure to insulin-induced hypoglycemia was shown to increase glial acetate metabolism (GAM) during subsequent exposure to hypoglycemia in diabetic individuals. However, it remained unclear whether this effect was dependent on the disease state or the antecedent cause of hypoglycemia. We aimed to establish whether exposure to fasting-induced hypoglycemia was sufficient to produce alterations in GAM in non-diabetic individuals.

Methods: GAM was measured via carbon-13 magnetic resonance spectroscopy during infusion of [1-13C] acetate before and after a 72-h fast in six metabolically healthy men. All participants were male, aged 18-40 years, with a Body Mass Index of 20.0-27.9 kg/m2, who consented to reside at Pennington Biomedical Research Center for 4 days. The main outcome measure was the percent enhancement of cerebral [1-13C] bicarbonate (the primary metabolic byproduct of glial oxidation of [1-13C] acetate). Continuous glucose monitoring was used to measure hypoglycemic episodes during the 72-h fast.

Results: As expected, 72 h of fasting significantly reduced blood glucose levels and resulted in a high frequency of hypoglycemic episodes. Steady-state GAM increased from 53.5 ± 3.7 to 61.9 ± 1.7% following the 72-h fast (p = 0.005). This increase correlated with greater duration of hypoglycemia experienced during the fast (r = 0.967). In addition, subjects with greater GAM at baseline experienced a greater increase in the duration of hypoglycemia experienced during the 72-h fast (r = 0.979).

Conclusions: GAM has potential as a biomarker for susceptibility to hypoglycemic episodes.

Trail registration: Clinicaltrials.gov ID: NCT02690168.

Keywords: Acetates; Fasting; Glucose; Humans; Hypoglycemia; Magnetic resonance spectroscopy; Neuroglia.

Conflict of interest statement

Conflict of interest

The authors have stated explicitly that there are no conflicts of interest in connection with this article.

Ethical standard statement

All study procedures and literature were approved by the Institutional Review Board at Pennington Biomedical Research Center (PBRC). All subjects provided informed consent prior to enrollment in the study (NCT02690168). All study procedures and measurements, except plasma acetate assays (conducted at the University of Tennessee, Knoxville, TN, USA), were performed at PBRC, Baton Rouge, LA, USA.

Informed consent

Informed consent was obtained from all individual participants in the study.

Figures

Fig. 1

Glial acetate metabolism (GAM) is increased following a 72-h fast. GAM was measured via carbon 13 magnetic resonance spectroscopy during [1-13C] acetate infusion. a Time course of the percent enhancement (PE) of 13C bicarbonate (HCO3; the primary metabolite of [1-13C] acetate oxidation) following the start of infusion in a representative subject after 12 h of fasting (day 0) and 72 h of fasting (day 3). The solid line represents the best fit of a mono-exponential function modeling GAM. b Comparison of the average best fit across subjects via a linear mixed effects model demonstrates a clear increase from day 0 to 3 (p < 0.0001). c Average steady-state PE of HCO3 was increased on day 3 relative to day 0 (p = 0.0053). Furthermore, the PE of HCO3 was increased on day 3 relative to day 0 in all subjects who completed both scans (symbols with connecting lines)

Fig. 2

Continuous glucose monitoring (CGM) demonstrates that 72 h of fasting leads to frequent bouts of hypoglycemia. CGM was used to monitor blood glucose levels at 5-min intervals throughout a 72-h fast. The mean blood glucose for all six subjects is plotted in the top panel. The gray shading represents the 95% confidence limit and the dotted line represents the clinical threshold for Level 1 hypoglycemia, 70 mg/dL. The lower panel represents the percent of subjects which experienced a hypoglycemic episode during each hour of the 72-h fast. A subject was determined to have had a hypoglycemic episode during a given hour of the fast if a CGM reading of ≤ 70 mg/dL was maintained for three consecutive CGM readings (15 min)

Fig. 3

Individual differences in glial acetate metabolism (GAM) are related to the duration of hypoglycemia experienced during a 72-h fast. GAM, as measured by the percent enhancement of 13C bicarbonate (PE of HCO3) following an acetate infusion, is both predictive of and responsive to subsequent fasting-induced hypoglycemia. a Relationship between glia acetate metabolism measured just prior to 72 h of fasting (day 0) and the subsequent exposure to hypoglycemia during the fast (percent time ≤ 70 mg/dL). A correlation analysis between these two variables demonstrated a robust positive relationship between GAM and the duration of hypoglycemia experienced during fasting (Pearson’s r2 = 0.96, p = 0.004). b GAM measured following a 72-h fast (day 3) increase in a dose-dependent manner based on the magnitude of exposure to hypoglycemia during the fast (Pearson’s r2 = 0.94, p = 0.007)