Fasting Insulin: The Blood Test That Should Be Routine but Isn't
Fasting Insulin: The Blood Test That Should Be Routine but Isn’t
If you have ever had blood work done through your family physician, you have likely had your fasting glucose measured. You may have also had an HbA1c. These are the standard screening tools for diabetes in Canadian medicine. They are also remarkably late indicators.
By the time fasting glucose crosses the diagnostic threshold for type 2 diabetes, the metabolic dysfunction that caused it has been building for 10 to 15 years. HbA1c catches the problem somewhat earlier, but still well downstream of the initial disturbance.
Fasting insulin tells the story that glucose and HbA1c cannot.
The Physiology in Plain Terms
When you eat, blood glucose rises, and the pancreas releases insulin to shuttle that glucose into cells. In a metabolically healthy person, a modest amount of insulin does the job efficiently. Glucose enters the cells, blood sugar normalizes, and insulin drops back to baseline.
In insulin resistance, cells become less responsive to insulin’s signal. The pancreas compensates by producing more. Blood glucose stays normal because the pancreas is working overtime to keep it there. This compensation can persist for years, even decades.
During this entire period, fasting glucose looks fine. HbA1c looks fine. The patient is told they are metabolically healthy. Meanwhile, fasting insulin is elevated, the pancreas is under strain, and the downstream consequences are accumulating.
What Elevated Fasting Insulin Actually Means
A fasting insulin above 8 to 10 µIU/mL begins to signal that the metabolic machinery is working harder than it should. Optimal is generally considered to be below 5 to 6 µIU/mL, though context matters.
Elevated fasting insulin, even with normal glucose, is associated with increased visceral adiposity, rising triglycerides, declining HDL, elevated blood pressure, increased inflammatory markers, and accelerated vascular aging. These are not theoretical associations. They are well established in the epidemiological and clinical literature.
The condition described above has a name. It is metabolic syndrome, or its precursor, and it falls squarely within the domain of metabolic health. It is invisible on standard screening blood work until the glucose itself begins to fail.
The Connection to Hormonal Health
This is where fasting insulin becomes particularly relevant to hormonal optimization.
In men, insulin resistance suppresses gonadotropin releasing hormone (GnRH) at the hypothalamic level, which reduces LH secretion, which reduces testicular testosterone production. Simultaneously, elevated insulin increases aromatase activity in adipose tissue, accelerating the conversion of testosterone to estradiol. Visceral fat accumulates, which drives further aromatase activity. The result is a self-amplifying cycle of declining testosterone and worsening metabolic function.
A man presenting with low testosterone and undiagnosed insulin resistance will respond poorly to testosterone therapy alone. The metabolic environment is actively working against hormonal optimization. Addressing insulin resistance alongside testosterone replacement produces a qualitatively different result.
In women, insulin resistance disrupts ovarian function, contributes to irregular cycles, drives androgen excess (as seen in polycystic ovarian syndrome), and worsens the metabolic consequences of declining estrogen during perimenopause. A woman entering the menopausal transition with unrecognized insulin resistance faces compounding metabolic headwinds.
Why Most Physicians Do Not Order It
Fasting insulin is not part of standard screening guidelines in Ontario or in most Canadian provinces. It is not included in routine annual blood work panels. OHIP covers it only when there is a documented clinical indication, which typically means the patient already has an established diagnosis.
The standard of care follows glucose. If glucose is normal, the system considers the patient metabolically cleared. This approach works well for diagnosing diabetes. It does not work well for preventing it. The assumption is that abnormal glucose is a necessary prerequisite for intervention. But by the time glucose rises, the metabolic infrastructure has already been compromised for years. The earlier detection window—years of elevated insulin with normal glucose—is where preventive intervention is most effective and most straightforward.
Functional medicine physicians order fasting insulin as a baseline because the clinical utility is too significant to ignore. Identifying insulin resistance 10 years before it becomes diabetes changes the treatment window from reactive to preventive.
HOMA-IR: A More Complete Picture
Fasting insulin alone is informative. Combined with fasting glucose in a calculation called HOMA-IR (Homeostatic Model Assessment of Insulin Resistance), it becomes more precise.
The formula is straightforward: fasting insulin (µIU/mL) multiplied by fasting glucose (mmol/L), divided by 22.5. A HOMA-IR below 1.0 is considered optimal. Between 1.0 and 1.9 suggests early insulin resistance. Above 2.0 is consistent with significant insulin resistance.
This single calculation, derived from two inexpensive blood tests, provides more actionable metabolic information than most standard panels.
What to Do with the Information
Identifying insulin resistance early opens a treatment window where lifestyle intervention is effective and pharmacological intervention is rarely needed.
The most potent intervention is reducing refined carbohydrate intake, particularly sugars and processed starches, which directly reduce the insulin demand on the pancreas. The mechanism is direct: fewer carbohydrates means less glucose enters the bloodstream, which means the pancreas does not need to secrete as much insulin to manage it. Over weeks and months, as fasting insulin drops, the cell’s responsiveness to insulin begins to recover—a durable improvement in the condition itself.
Time-restricted eating patterns, where the daily eating window is compressed to 8 to 10 hours, have demonstrated consistent improvements in fasting insulin in clinical trials. The mechanism appears to involve both reduced total caloric intake and an extended fasting window that allows insulin levels and insulin-sensitive enzymes (like AMPK) to reset.
Exercise, particularly resistance training and sustained aerobic work, improves insulin sensitivity through both acute and chronic mechanisms. A single bout of resistance training can improve insulin sensitivity for 24 to 48 hours—the glucose transporters in muscle cells become transiently more responsive, which is why a post-training meal is preferentially taken up by working muscle rather than being stored as fat. Consistent training over weeks and months produces durable, structural improvements in glucose disposal: the muscle develops greater mitochondrial capacity, more glucose transporters, and greater enzymatic capacity to handle glucose.
Sleep deprivation independently worsens insulin sensitivity. One study demonstrated that four nights of sleep restricted to 4.5 hours produced insulin resistance comparable to that seen in pre-diabetic subjects. Restoring sleep quality is a metabolic intervention.
When lifestyle measures are insufficient, metformin remains a well-studied pharmacological option with a favourable safety profile. GLP-1 receptor agonists represent a newer class with potent effects on insulin sensitivity, weight, and cardiovascular risk.
How This Fits into the Vis Viva Framework
At Manus Solis, fasting insulin is part of every baseline Pulsus assessment. It is tracked longitudinally alongside HbA1c, lipids, and inflammatory markers. Changes in fasting insulin are correlated with Sensus measures (energy, cognitive clarity, sleep quality) and Virtus measures (body composition, exercise capacity, wearable data).
This three-domain approach ensures that metabolic improvements are not just biochemical but are felt and performed. A dropping fasting insulin that coincides with better sleep, sharper cognition, and improved VO2 max tells a coherent clinical story.
Ask for the Test
If you have never had your fasting insulin measured, ask your physician for it. If the answer is that your glucose is normal and therefore it is unnecessary, understand that this reflects a different clinical philosophy, one oriented toward disease diagnosis rather than disease prevention.
You deserve to know the full metabolic picture. Fasting insulin is one of the most cost-effective, actionable blood tests in medicine. It should not be reserved for patients who are already sick.
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Dr. Handsun Xiao is a McGill trained physician (MD, CCFP) practicing functional medicine and bioidentical hormone therapy in Toronto, with virtual consultations available to patients across Ontario. He holds advanced BHRT certification through WorldLink Medical and IFM AFMCP training. Manus Solis offers physician led BHRT consultations with custom compounding through a dedicated Ontario pharmacy partner. To learn more or book a virtual consultation, visit manussolis.ca.
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