Very low fasting insulin needs to be investigated. Include a comprehensive assessment of glucose regulation including C-peptide. Also, assess extreme dietary restrictions of carbohydrates. Diabetes mellitus and hypopituitarism should be ruled out.
A fasting insulin significantly below optimal and standard range should be repeated following an 8-12 hour fast and investigated further if persistently low. Of course, it is imperative that you assess all blood glucose regulation biomarkers in order to develop a clearer clinical picture. If fasting glucose is high and fasting insulin is low, then diabetes must be ruled out.
Some research has observed an inverse association between fasting insulin and the risk of atrial fibrillation. Researchers note that insulin has a vasodilatory effect and increases blood flow. Low insulin levels are also associated with increased risk of hypertension in those at increased genetic risk for diabetes (Johnson 2014). Low fasting insulin may also be associated with hypopituitarism (Pagana 2019).
References
Johnson, Linda S B et al. “Low fasting plasma insulin is associated atrial fibrillation in men from a cohort study--the Malmö preventive project.” BMC cardiovascular disorders vol. 14 107. 24 Aug. 2014, doi:10.1186/1471-2261-14-107 [R]
Pagana, Kathleen Deska; Pagana, Timothy J.; Pagana, Theresa N. Mosby's Diagnostic and Laboratory Test Reference. Elsevier Health Sciences. 2019.