Insulin resistance has been increasingly implicated as a metabolic contributor to Alzheimer's disease (AD). The hippocampus is a core region affected in AD, and structural changes in hippocampal subregions are closely tied to clinical progression. The authors aimed to clarify how insulin resistance relates to hippocampal gray matter volume across diagnostic stages, hypothesizing that associations may differ between mild cognitive impairment (MCI) and established AD.
The study used neuroimaging and clinical data from the Alzheimer's Disease Neuroimaging Initiative (ADNI), a large-scale repository of brain MRI and clinical measures. Insulin resistance was estimated with the Homeostatic Model Assessment of Insulin Resistance (HOMA-IR). The neuroimaging outcome of interest was anterior hippocampal gray matter volume, analyzed in relation to HOMA-IR and cognitive performance. Cognitive status was assessed with the Mini-Mental State Examination (MMSE).
The investigators evaluated relationships between HOMA-IR and hippocampal gray matter volume separately across diagnostic groups. They also tested associations between anterior hippocampal volume and MMSE scores. An exploratory mediation analysis was performed in the MCI group to assess whether anterior hippocampal volume might act as an intermediate variable linking HOMA-IR to cognitive performance. Details on specific covariates, model specifications, and sample sizes were not reported in the abstract.
In individuals with MCI, higher HOMA-IR was associated with larger anterior hippocampal gray matter volume. This positive relationship indicates that, at this diagnostic stage, greater peripheral insulin resistance corresponded to greater anterior hippocampal volume in this dataset.
By contrast, in the AD group the association between HOMA-IR and anterior hippocampal gray matter volume was reversed: higher HOMA-IR related to smaller anterior hippocampal volume. The authors highlight this opposite-direction pattern across the two diagnostic categories.
Within the MCI cohort, larger anterior hippocampal gray matter volume was positively associated with higher MMSE scores, indicating better global cognitive performance. The exploratory mediation analysis conducted in the MCI group suggested a significant indirect association in which anterior hippocampal volume linked HOMA-IR to cognitive performance. The abstract reports this mediation as exploratory; full mediation statistics and effect sizes were not provided in the abstract.
The study's principal finding is that the relationship between systemic metabolic status—indexed by insulin resistance—and hippocampal structure differs by disease stage. A positive association in MCI but an inverse association in AD suggests stage-dependent or nonlinear interactions between metabolic dysfunction and neurodegeneration. These results imply that metabolic alterations such as insulin resistance may have complex roles across the course of cognitive decline and that diagnostic status should be accounted for in studies linking metabolic measures to brain structure and function.
This article is a preprint and has not been peer reviewed. The abstract does not report key methodological details such as sample sizes, adjustment covariates, imaging processing methods, or formal statistics for the reported associations and mediation. Those details would be necessary to fully evaluate effect sizes, potential confounders, and the robustness of the observed stage-dependent relationships. Readers should consult the full preprint for supplemental methods and results.
The authors declared no competing interests. Funding sources listed include the Japan Agency for Medical Research and Development, the Japan Society for the Promotion of Science, the Nakatomi Foundation, Watanabe Foundation, and the Promotion and Mutual Aid Corporation for Private Schools of Japan. The work used data from ADNI.
Using ADNI data, the investigators report differential associations between HOMA-IR and anterior hippocampal gray matter volume: higher insulin resistance corresponded to larger anterior hippocampal volume in MCI, but to smaller volume in AD. In MCI, anterior hippocampal volume was positively associated with MMSE, and an exploratory mediation analysis suggested anterior hippocampal volume may mediate an indirect link between insulin resistance and cognition. These findings underscore the importance of considering disease stage when evaluating metabolic contributions to AD-related brain changes. The preprint format means these observations await validation through peer review and detailed methodological scrutiny.