Glucose is the brain’s principal energy substrate. In Alzheimer’s disease (AD), there appears to be a pathological decrease in the brain’s ability to use glucose. Neurobiological evidence suggests that ketone bodies are an effective alternative energy substrate for the brain. Elevation of plasma ketone body levels through an oral dose of medium chain triglycerides (MCTs) may improve cognitive functioning in older adults with memory disorders. On separate days, 20 subjects with AD or mild cognitive impairment consumed a drink containing emulsified MCTs or placebo. Significant increases in levels of the ketone body β-hydroxybutyrate (β-OHB) were observed 90 min after treatment (P=0.007) when cognitive tests were administered. β-OHB elevations were moderated by apolipoprotein E (APOE) genotype (P=0.036). For ε4+ subjects, β-OHB levels continued to rise between the 90 and 120 min blood draws in the treatment condition, while the β-OHB levels of ε4− subjects held constant (P