TTR and Huntington disease: In both the HD and PD groups, the the serum levels of prealbumin showed a positive correlation with the Shannon index (r = 0.731 and r = 0.688) and a negative correlation with the Simpson index (r = − 0.59 and r = − 0.653) (p < 0.05), indicating that a higher bacterial diversity was accompanied by higher serum levels of prealbumin in both groups.