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Objectives: To determine how age affects insulin resistance during the menstrual cycle and insulin resistance-associated indices: the Triglyceride-glucose and Triglyceride-glucose-BMI indexes., Methods: This prospective observational study used fasting plasma glucose, fasting insulin, triglycerides, body mass index (BMI), and days since the start of the menstrual period collected from the NHANES dataset (1999-2006). Insulin resistance was determined using the Homeostasis Model Assessment of Insulin Resistance (HOMA-IR). The participants were categorized as young (16-34 years) or older (>35 years). Rhythmicity during the menstrual cycle was analyzed using the Cosinor and Cosinor2 packages for R., Main Outcome Measures: Cosine fit curves for insulin resistance during the menstrual cycle and age-associated effects on rhythmicity., Results: Using 1256 participants, rhythmicity was observed for fasting insulin and HOMA-IR (p < 0.05) but not for fasting plasma glucose, the Triglyceride-glucose index, or the Triglyceride-glucose-BMI index. Significant amplitudes for fasting insulin and HOMA-IR were observed when age was considered. Acrophases for fasting insulin and HOMA-IR were significant only for the younger group, and the differences between these groups were significant, suggesting that the changes in scores for insulin resistance for the younger and older groups occur at different times of their menstrual cycle., Conclusions: Insulin resistance does fluctuate during the menstrual cycle, and it is at a maximum at different times for younger and older women. Since these results are unadjusted, this study is preliminary and further investigation is required., Competing Interests: Declaration of competing interest The authors declare that they have no competing interest., (Copyright © 2024 The Authors. Published by Elsevier B.V. All rights reserved.)