Why is my fasting insulin low?

Because insulin rises to compensate for insulin resistance [4], low fasting insulin with normal glucose generally reflects good sensitivity. In one US clinic, people in the lowest insulin quartile averaged 4.9 µIU/mL and had the lowest odds of prediabetes [5].

Common reasons for a low reading with normal glucose:

• Lean body composition and regular exercise [6].
• A low-carbohydrate or ketogenic diet, which lowers insulin [7] [8].
• A longer fast than usual before the test.
• Lab factors: labs reject hemolyzed samples for insulin [9], and assays differ [10].

Is lower always better? Not necessarily. In lean Japanese adults with a BMI below 22, the lowest HOMA-IR groups had higher mortality than the middle group, a J-shaped curve [11]. This is one cohort and does not prove cause, but it argues against chasing ever-lower numbers.

The key is the pairing with glucose. Type 1 diabetes, including latent autoimmune diabetes in adults (LADA), is caused by autoimmune destruction of beta cells and usually leads to absolute insulin deficiency [1]. Diseases of the exocrine pancreas are another cause of diabetes the ADA classifies separately [1].

In these conditions fasting insulin can be low or low-normal while glucose rises. In people who already inject insulin, a fasting insulin test cannot tell you how much insulin your own pancreas makes, because the assay measures injected insulin too and antibodies interfere [9].

When to seek care. See a clinician promptly if a low insulin comes with a fasting glucose of 126 mg/dL (7.0 mmol/L)7.0 mmol/L (126 mg/dL) or more, an HbA1c of 6.5% or more [1], or symptoms such as frequent urination, excessive thirst or unexplained weight loss [1]. Seek urgent care for vomiting, abdominal pain, rapid breathing or confusion with high glucose, which can signal diabetic ketoacidosis.

An international consensus statement estimates that LADA accounts for 2% to 12% of adult-onset diabetes. At diagnosis people resemble type 2 patients because they do not need insulin, but they carry the immune markers of type 1 diabetes. The panel advised screening for LADA in newly diagnosed diabetes that does not yet need insulin [2].

Clues that diabetes may be autoimmune rather than type 2:

• Diabetes in an adult with a normal or low BMI. The ADA lists a BMI of 25 or more and no weight loss among the features of type 2 diabetes [1].
• Unexplained weight loss, or glucose that keeps rising on standard type 2 treatment.
• A personal or family history of autoimmune conditions, such as thyroid disease (see TSH).

Testing for islet autoantibodies and C-peptide is how clinicians sort this out [1] [2].

C-peptide is cut from proinsulin at the same time as insulin, so it tracks insulin secretion, and injected insulin does not contain it [3].

Thresholds clinicians use:

The ADA says a C-peptide test is indicated only in people receiving insulin treatment, and that a random sample with concurrent glucose within 5 hours of eating can replace a formal stimulation test for classification [1]. C-peptide assays are not standardized either, so thresholds depend on the method [12].

The Endocrine Society recommends evaluating low blood sugar only when Whipple's triad is documented: symptoms, a low measured glucose and resolution of symptoms when glucose is raised. When the body may be making too much insulin, it advises measuring glucose, insulin, C-peptide, proinsulin and beta-hydroxybutyrate during an episode, plus insulin antibodies [13].

In people with diabetes, insulin or insulin secretagogue treatment is the most common cause of low blood sugar [13]. A routine fasting insulin test is not the right tool for this question.