Objective To explore the characteristics of apolipoprotein E(APOE)gene polymorphism in patients with Alzheimer's disease(AD)and mild cognitive impairment(MCI)due to AD,as well as its corre-lation with baseline levels of ketone bodies.Methods A total of 110 AD patients from the outpatient and neu-rology wards of the hospital from January 2020 to October 2023 were selected as the AD group,105 patients(none of whom had used anti dementia drugs)were selected as the MCI group,and 110 healthy elderly exami-nees in the physical examination center were selected as the control group.APOE gene polymorphism,and the levels of serum β-hydroxybutyrate(HB)and urine ketone bodies were measured.The distribution of APOE genotype among the three groups was analyzed,and the differences of the levels of serum HB and urine ketone bodies were compared among those carried APOE ε4 allele and those did not.Results Among the three groups,the statistical significance was found in the differences of APOE genotype and ε2,ε3,ε4 allele(P<0.05).The proportion of APOE ε4 allele carriers in the AD group and the MCI group was higher than that in the control group(P<0.05).The levels of serum βHB in the AD group and the MCI group were lower than that in the control group(P<0.05).The levels of serum βHB in those carried APOE ε4 in the AD group were significantly lower than that in the control individuals(P<0.05).There was no statistically significant differ-ence in serum βHB levels between individuals carried and not carried APOE ε4 in the three groups(P>0.05).There was no statistically significant difference in the levels of urinary ketones among the three groups(P>0.05).There was no statistically significant difference in urine ketone bodies levels between individuals carried and not carried APOE ε4 in the three groups(P>0.05).Conclusion The reduced baseline levels of serum βHB in AD patients are associated with APOE ε4 allele.