Age-related differences in fluctuations in insulin resistance evaluated with HOMA-IR and triglyceride and glucose-based indices during the menstrual cycle, as determined using the NHANES cross-sectional dataset

•In this study, insulin resistance fluctuated during the menstrual cycle.•Older females showed a different rhythmicity than younger females.•The Triglyceride-glucose-BMI index is better associated with the Homeostasis Model Assessment of Insulin Resistance (HOMA-IR) than other indices.•The Triglyceride-glucose-BMI index did not fluctuate during the menstrual cycle.•To assess insulin resistance, the day of the menstrual cycle must be considered. To determine how age affects insulin resistance during the menstrual cycle and insulin resistance-associated indices: the Triglyceride-glucose and Triglyceride-glucose-BMI indexes. 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. Cosine fit curves for insulin resistance during the menstrual cycle and age-associated effects on rhythmicity. Using 1256 participants, rhythmicity was observed for fasting insulin and HOMA-IR (p