Impact of fasting on human brain acid-base homeostasis using natural abundance ¹³C and ³¹P MRS

Purpose To use ¹³C magnetic resonance spectroscopy (MRS) and ³¹P MRS to develop a direct assay for regional [HCO3-] in the human brain and to define brain pH and physiological response of [HCO3-] to fasting. Materials and Methods Seven healthy subjects underwent MRS examinations on a 1.5T MRI scanner. Subjects were well fed with repeated examinations performed after 4 and 12 hours of fasting. Proton noise decoupling ¹³C MRS were acquired using pulse and acquired acquisition while ³¹P MRS were acquired using a 2D chemical shift imaging method with relaxation time (TR) of 2 seconds. Results Fasting brain bicarbonate concentrations (6.7 ± 2.5 mM for 12-hour fasting, P = 0.002 and 8.3 ± 2.1 mM for 4-hour fasting, P = 0.015) are significantly reduced compared to fed state (11.6 ± 1.3 mM). However, no significant difference in brain pH was observed, confirming the critical role of pCO₂ in intracerebral pH homeostasis. Conclusion We demonstrated that the intracellular HCO3- in human brain is readily modified by diet but appears to have no measurable effect on cerebral pH. Natural abundance ¹³C can provide useful information relevant to human brain pH homeostasis by providing information for HCO3-.