Blood Sugar Markers Explained: Fasting Glucose, HbA1c, Fasting Insulin

Why Three Markers Are Better Than One

A single fasting glucose measurement captures blood sugar at one point in time—useful, but incomplete. HbA1c reflects average glucose over the preceding 2–3 months.[3] Fasting insulin reveals how hard the pancreas is working to keep glucose in check. Together, these three markers expose a spectrum of metabolic dysfunction that begins years before a diabetes diagnosis.

Fasting Glucose

Fasting blood glucose measures the concentration of glucose in plasma after at least 8–10 hours without caloric intake. Normal fasting glucose is 70–99 mg/dL (3.9–5.5 mmol/L). A reading of 100–125 mg/dL (5.6–6.9 mmol/L) on two separate occasions meets the diagnostic criterion for prediabetes (impaired fasting glucose). A value of 126 mg/dL (7.0 mmol/L) or above confirms a diabetes diagnosis when repeated or confirmed by symptoms.[1]

Fasting glucose can be transiently elevated by stress, illness, poor sleep, or early-morning cortisol release (the "dawn phenomenon"). For this reason, a single elevated reading is always confirmed before a clinical diagnosis is made. You can track your fasting glucose values over time using the Health3 blood glucose biomarker page.

HbA1c: The Long-View Marker

Haemoglobin A1c (HbA1c) measures the percentage of haemoglobin molecules that have glucose attached to them—a process called glycation. Because red blood cells circulate for roughly 90–120 days, HbA1c reflects mean blood glucose over that period.[3] An HbA1c below 5.7% is normal; 5.7–6.4% indicates prediabetes; and 6.5% or above on two separate tests confirms type 2 diabetes.[1] Optimal metabolic health is often associated with HbA1c in the 4.8–5.4% range.

HbA1c has important limitations. It can be falsely lowered by conditions that shorten red blood cell lifespan—haemolytic anaemia, iron deficiency, recent blood transfusion—and falsely elevated by iron deficiency anaemia or certain haemoglobin variants. In such cases, fructosamine or continuous glucose monitoring provides a more accurate picture of glycaemic control.

Fasting Insulin: The Earliest Warning Signal

Fasting insulin is rarely included in routine panels but is arguably the most sensitive early indicator of insulin resistance. When cells become resistant to insulin signalling, the pancreas compensates by secreting more insulin to maintain normal glucose levels.[4] Fasting glucose can appear perfectly normal for years while fasting insulin climbs—a state called compensated insulin resistance.

Optimal fasting insulin is generally cited as below 5–8 µIU/mL, with many functional medicine practitioners targeting below 5 µIU/mL. Values above 10–15 µIU/mL with a normal fasting glucose suggest insulin resistance is already underway. You can learn more about the clinical context of this marker on the Health3 fasting insulin biomarker page.

HOMA-IR: Quantifying Insulin Resistance

The Homeostatic Model Assessment of Insulin Resistance (HOMA-IR) combines fasting glucose and fasting insulin into a single number that quantifies insulin resistance. The formula is: HOMA-IR = (fasting insulin in µIU/mL × fasting glucose in mg/dL) ÷ 405.[2] A HOMA-IR below 1.0 indicates excellent insulin sensitivity; 1.0–1.9 is normal; 2.0–2.9 is early insulin resistance; and values above 3.0 indicate significant insulin resistance.

HOMA-IR is validated in large population studies and correlates well with the gold-standard hyperinsulinaemic-euglycaemic clamp.[2] It is particularly useful for tracking the effect of lifestyle interventions over time because it is sensitive enough to detect improvements before HbA1c or fasting glucose shift significantly.

The Progression from Normal to Diabetes

Insulin resistance typically develops first, driven by excess caloric intake, physical inactivity, visceral adiposity, and chronic sleep deprivation.[4] Fasting insulin rises while glucose remains normal. Over years, the pancreatic beta cells exhaust their compensatory capacity. Fasting glucose then enters the prediabetic range, followed by rising HbA1c, and eventually frank type 2 diabetes.[5] Catching dysfunction at the fasting insulin or HOMA-IR stage—rather than waiting for elevated glucose—provides the longest runway for reversal.

Practical Steps to Improve All Three Markers

The lifestyle interventions with the strongest evidence for improving glycaemic markers are: reducing total refined carbohydrate and added sugar intake; replacing ultra-processed foods with whole foods; achieving and maintaining a healthy body weight; performing at least 150 minutes of moderate aerobic exercise per week, with resistance training two or more days per week; prioritising 7–9 hours of quality sleep; and managing chronic psychological stress. A 10% reduction in body weight can lower fasting glucose by 5–10 mg/dL, reduce HbA1c by 0.5–1.0 percentage points, and cut fasting insulin substantially.

When to Discuss Medication

Metformin is the most widely prescribed first-line agent for type 2 diabetes and is also used off-label for prediabetes in high-risk individuals. Newer drug classes—GLP-1 receptor agonists and SGLT-2 inhibitors—offer additional cardiovascular and renal benefits beyond glucose lowering and are increasingly recommended earlier in the treatment algorithm. The decision to initiate pharmacotherapy is always made alongside a thorough assessment of cardiovascular risk, kidney function, and patient preferences.

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