In mild cognitive impairment (MCI), posterior cingulate cortex glucose hypometabolism may results from posterior cingulum (PCg) alterations. We suggest that raising ketone availability to the brain may overcome the brain energy deficit. We developed a dual-tracer PET/dMRI tractometry method to assess whether a ketogenic supplement has impact on fuel uptake in the PCg of MCI participants. Mean fuel uptake in the PCg was unchanged post-supplementation, but tract-profiling enabled the identification of sections with lower glucose uptake. Energy supply in white matter fascicles is crucial to sustain adequate axonal function and may be linked to the pathogenesis of MCI.