Sex differences in the association of photoperiod with hippocampal subfield volumes in older adults: A crosssectional study in the UK Biobank cohort

Introduction
Even though seasonal and sex‐dependent changes in hippocampal and subfield volumes are well known in animals, little is known about changes in humans. We hypothesized that changes in photoperiod would predict changes in hippocampal subfield volumes and that this association would be different between females and males.

Methods
A total of 10,033 participants ranging in age from 45 to 79 years were scanned by MRI in a single location as part of the UK Biobank project. Hippocampal subfield volumes were obtained using automated processing and segmentation algorithms using the developmental version of the FreeSurfer v 6.0. Photoperiod was defined as the number of hours between sunrise and sunset on the day of scan.

Results
Photoperiod correlated positively with total hippocampal volume and all subfield volumes across participants as well as in each sex individually, with females showing greater seasonal variation in a majority of left subfield volumes compared with males. ANCOVAs revealed significant differences in rate of change in only left subiculum, CA‐4, and GC‐ML‐DG between females and males. PLS showed highest loadings of hippocampal subfields in both females and males in GC‐ML‐DG, CA1, CA4, subiculum, and CA3 for left hemisphere and CA1, GC‐ML‐DG, CA4; subiculum and CA3 for right hemisphere in females; GC‐ML‐DG, CA1, subiculum, CA4 and CA3 for left hemisphere; CA1, GC‐ML‐DG, subiculum, CA4 and CA3 for right hemisphere in males.

Conclusion
The influence of day length on hippocampal volume has implications for modeling age‐related decline in memory in older adults, and sex differences suggest an important role for hormones in these effects.