PURPOSE: As global temperatures rise, health risks such as acute kidney injury (AKI) and chronic kidney disease are increasing, making it crucial to identify heat-related health risks in vulnerable populations. Older adults experience higher rates of AKI, kidney-related hospitalizations, and mortality during periods of high environmental heat. However, it is unclear whether older females living in hotter climates also exhibit kidney stress during summer compared to winter. Thus, the present study investigated the impact of seasonal temperature changes on AKI biomarkers. We hypothesized that postmenopausal females would exhibit higher concentrations of AKI biomarkers in summer compared to winter. METHODS: Thirteen Tallahassee, FL postmenopausal female residents (66 ± 7 years old) without overt disease completed laboratory visits in January (winter) and August (summer). Hydration was quantified based on urine specific gravity (USG) from 24-hour urine samples. Urinary AKI biomarkers, including insulin-like growth factor 7 (IGFBP7), tissue inhibitor of metalloproteinases 2 (TIMP2), neutrophil gelatinase-associated lipocalin (NGAL), and nephrin, were measured from 24-hour urine samples and normalized to urine flow rate (i.e., excretion rate). Step count and moderate to vigorous physical activity (MVPA) were recorded using wrist actigraphy, while ambient temperature and heat index were monitored with waist-worn Kestrel temperature and humidity loggers for a period of three days of free-living. We compared variables between seasons using pairwise two-tailed analyses. We used multiple linear regression to assess potential predictors of the change in the product of IGFBP7*TIMP2 (the only FDA-approved biomarker for AKI risk). Data are presented as mean ± SD with Cohen’s d (d) for normally distributed data, and as medianIQR with rank biserial correlation (r) for non-normally distributed data. RESULTS: Despite a significant increase in 24-hour individual-experienced heat index from winter (21 ± 2°C 70 ± 4°F) to summer (27 ± 2°C 80 ± 4°F, p< 0.0001, d=2.650), USG (winter: 1.010 ± 0.003 vs. summer: 1.010 ± 0.004, p=0.530, d=0.179) did not differ between seasons. While not statistically significant, IGFBP7*TIMP2 demonstrated a moderate effect size, indicating higher values in the summer (winter: 0.0060.036 vs. summer: 0.0080.077 (ng/mL)2/1000 per mL/min, p=0.068, r=0.582). Additionally, NGAL and nephrin did not exhibit seasonal changes (both p≥0.233, r≤0.410). Multiple regression identified higher average heat index (estimate=0.012, p=0.0002), minutes spent over 27°C (80°F) (estimate=8.96×10 - 5 , p=0.0002), and average step count (estimate=6.36×10 - 6 , p=0.013) as significant predictors of the winter-to-summer change in IGFBP7*TIMP2. MVPA was not a significant predictor (p=0.760) and was therefore removed from the model. The model explained 86.4% of the variance (R 2 =0.864). CONCLUSION: In postmenopausal females, hydration status remained consistent between seasons in this pilot study, suggesting that any fluid losses from higher summer ambient heat exposure may have been compensated for through greater fluid intake. Ambient environment indices and step count are independently associated with seasonal changes in AKI biomarker excretion. These pilot findings suggest environmental factors, rather than hydration status, may be important in predicting variability in kidney stress during periods of high ambient temperature. This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.
Domeier et al. (Fri,) studied this question.