Hypoglycemia is the most frequent acute complication of insulin therapy in patients with type 1 diabetes (T1DM). Recurrent hypoglycemia may lead to impaired awareness of hypoglycemia (IAH), characterized by suppression of hypoglycemic symptoms. Although the precise mechanisms underlying the development of IAH still remain to be revealed, there may be a pivotal role for the brain’s handling of lactate as a non-glucose energy substrate.^1,2,3 The aim of this study was to quantify brain lactate by ¹H-MRS under euglycemic and hypoglycemic conditions to test the hypothesis that brain lactate concentrations decrease in response to hypoglycemia, possibly as a consequence of increased lactate oxidation, and that this effect is enhanced in patients with T1DM and IAH.

Subjects: After an overnight fast, seven patients with T1DM and IAH, seven patients with normal awareness of hypoglycemia (NAH) and seven healthy, non-diabetic subjects underwent a two-step hyperinsulinemic euglycemic (5.0 mmol/L, 30 min) hypoglycemic (2.8 mmol/L, 45 min) glucose clamp, inside a 3T MR system (TIM Magnetom Trio, Siemens). Arterial plasma glucose and lactate levels were determined every 5 minutes. Subjects completed a semi-quantitative symptom questionnaire just prior to initiating the euglycemic glucose clamp and at the end of the hypoglycemic phase.

MR protocol: Brain lactate concentrations were measured continuously with ¹H-MRS, using a J-editing semi-LASER sequence^4,5 (TE 144 ms, TR 3000 ms, 32 averages and TA 1.40 min). J-editing was performed with MEGA-pulses with a bandwidth of 75 Hz centered on the lactate quartet at 4.1 ppm (MEGA on) and subsequently at -3 ppm (MEGA off). Data were acquired from a 25 cm³ voxel placed in the periventricular brain region. Additionally, spectra without water suppression (TE 30 ms, TR 5000 ms, 8 averages) were acquired.

Post-processing: After zero-filling (from 1024 to 2048 points) and Fourier transformation, all J-edited spectra from each subject were phase and frequency aligned with the first spectrum recorded. MEGA-on and MEGA-off spectra were subtracted and the difference spectra were apodized with a 5 Hz Lorentzian. For a better signal-to-noise ratio per spectrum, moving averaging with a sliding window of three scans was performed. In the final difference spectra (figure 1), the lactate doublet was fitted with the AMARES algorithm in jMRUI⁶. Absolute quantification of cerebral lactate was performed using the unsuppressed water signal as a reference, taking voxel composition and differences in T2 relaxation into account. A paired t-test was performed to determine significant differences (p<0.05) between both glycemic conditions. Analysis of variance (ANOVA) with post-hoc testing (Bonferonni) was used to analyze group differences. All data are expressed as mean±SEM.

The groups were well-matched for age, gender, BMI, and (where applicable) for HbA_1C and duration of diabetes. One subject with T1DM and NAH was excluded from MR data analysis due to significant movement of the head during data acquisition. During the clamp, plasma glucose levels stabilized at comparable levels during both the euglycemic and hypoglycemic phase (figure 2A). While plasma lactate levels increased during hypoglycemia in healthy controls (+0.3±0.08 mmol/L, p<0.01), they dropped in both diabetic groups (by -0.3±0.05 mmol/L and -0.2±0.06 mmol/L in T1DM IAH and T1DM NAH respectively, both p<0.05) (figure 2B). Hypoglycemic symptom scores increased significantly in response to hypoglycemia in both healthy volunteers (+17.4±3.7) and in T1DM patients with NAH (+12.9±3.9), but not in T1DM patients with IAH (+2±0.9, p<0.05 versus the other groups).

Brain lactate concentrations fell by -0.11±0.02 µmol/g ww, in response to hypoglycemia in T1DM with IAH (p<0.001), but remained stable in both healthy controls and in T1DM with NAH (figure 3). There were no differences between groups in absolute brain lactate concentrations, neither during euglycemia nor during hypoglycemia (table 1).

We demonstrated a ~20% decrease in brain lactate concentration in T1DM patients with IAH in response to hypoglycemia, whereas no such change was observed in the other two groups. The reduction in brain lactate T1DM patients with IAH may reflect increased lactate oxidation. It seems plausible that increased lactate oxidation, as an adaptation to recurrent exposure to hypoglycemia, is able to preserve brain metabolism during hypoglycemia and that lactate can thus be used as a supplemental or alternative fuel when glucose supply is low. Together these findings suggest an important role for brain lactate in the pathophysiological mechanism of hypoglycemia unawareness.