What is Fasting Insulin?
Insulin is a hormone produced by pancreatic beta cells that enables glucose to enter cells for energy production. Fasting insulin measures how much insulin the body requires in a fasted state to maintain normal blood glucose levels. In a healthy metabolic state, fasting insulin should be low , the body requires minimal insulin to keep blood sugar stable when not eating. As insulin resistance develops, the pancreas must produce progressively more insulin to achieve the same blood sugar control, making fasting insulin one of the earliest detectable markers of metabolic dysfunction , often years before fasting glucose or HbA1c become abnormal.
Fasting insulin is critical safety context before GH secretagogue and GLP-1 research. GH elevation from secretagogue protocols transiently reduces insulin sensitivity , starting a GH protocol in someone with already elevated fasting insulin adds metabolic stress on top of existing dysfunction. GLP-1 agonist research is specifically studied to improve insulin sensitivity , baseline fasting insulin establishes the starting point for objectively monitoring this effect. It is also the key marker for calculating HOMA-IR (Homeostatic Model Assessment of Insulin Resistance) when combined with fasting glucose.
What do the numbers mean?
Under 8 µIU/mL (acceptable)
Lab reference ranges vary by laboratory, age, sex, and testing method. Always interpret your results with your healthcare provider , do not self-diagnose based on these ranges.
Why this marker matters before peptide research.
In GH secretagogue research, pre-existing insulin resistance is a significant confounding variable , it blunts the body composition benefits of GH elevation and increases the risk of glucose dysregulation. GH is a counter-regulatory hormone to insulin: it promotes lipolysis and reduces peripheral glucose uptake. In an insulin-sensitive individual, this is well-tolerated and produces the favorable body composition effects seen in research. In someone with significant insulin resistance, adding GH axis stimulation creates competing metabolic pressures that may worsen glycemic control.
In GLP-1 agonist research, fasting insulin is a primary efficacy marker. Studies with semaglutide and tirzepatide consistently show meaningful reductions in fasting insulin alongside weight loss and HbA1c improvements , often before fasting glucose normalizes, making it the more sensitive early indicator of metabolic improvement. Monitoring fasting insulin at baseline and at 8–12 week intervals provides objective evidence of whether a metabolic protocol is producing its intended insulin-sensitizing effects.
Fasting insulin is also valuable in combination with fasting glucose as the HOMA-IR formula: (fasting insulin × fasting glucose) ÷ 405 (using mg/dL for glucose). A HOMA-IR above 2.0 suggests insulin resistance; above 2.9 indicates moderate to significant resistance. This calculated index adds diagnostic resolution beyond either marker alone and is a standard research outcome measure in metabolic intervention studies.
How to get this test.
Where to order
Standard blood draw at any major lab , LabCorp, Quest Diagnostics, or through your physician. Often ordered alongside fasting glucose as part of a metabolic panel. Available through direct-to-consumer testing services. Order with fasting glucose to calculate HOMA-IR.
How to prepare
True fasting required , minimum 8–12 hours with no food or caloric beverages. Water is fine. Draw in the morning before any food. Avoid intense exercise the day before , it transiently improves insulin sensitivity and may produce an artificially low result that doesn't reflect chronic metabolic status.
What to ask for
"Fasting Insulin" , a simple standalone test that is not included in most standard metabolic panels and must be requested specifically. Add "Fasting Glucose" if not already on your panel to enable HOMA-IR calculation. Both tests require the same fasted blood draw.