Fasting Glucose
Blood glucose after ≥ 8-hour fast – baseline marker for carbohydrate metabolism
Also known as
Definition
Fasting plasma glucose (after ≥ 8–12 hours without food) is the standard marker for evaluating carbohydrate metabolism. It reflects hepatic glucose production (gluconeogenesis, glycogenolysis) and baseline insulin action. It is globally standardized as a diagnostic criterion for diabetes mellitus and prediabetes. Together with fasting insulin, it allows calculation of the HOMA-IR as a measure of insulin resistance.
| Parameter | Value |
|---|---|
| Unit | mmol/l (mg/dl) |
| Reference Range | < 5.6 mmol/l (< 100 mg/dl): Normal | 5.6–6.9 mmol/l (100–125 mg/dl): Prediabetes (IFG) | ≥ 7.0 mmol/l (≥ 126 mg/dl): Diabetes mellitus (two measurements) |
| Optimal Range (lab2go) | 4.4–5.0 mmol/l (79–90 mg/dl) (lab2go Optimum Zone — metabolically healthy range; ≥ 5.2 mmol/l continuously increases cardiovascular risk) |
Conversion: 1 mmol/l = 18.02 mg/dl. US labs use mg/dl (normal < 100 mg/dl, prediabetes 100–125 mg/dl, diabetes ≥ 126 mg/dl).
↓ What a low value means
Hypoglycemia (< 3.9 mmol/l / < 70 mg/dl) can indicate insulinoma, hypopituitarism, adrenal insufficiency, reactive hypoglycemia, or overdosage of insulin-stimulating medications. Symptoms: tremor, sweating, palpitations, confusion, loss of consciousness below 2.8 mmol/l.
↑ What a high value means
Elevated fasting glucose signals impaired carbohydrate metabolism. In the prediabetes range: reversible through lifestyle change. At ≥ 7.0 mmol/l: diabetes mellitus, high risk for micro- (retinopathy, nephropathy, neuropathy) and macroangiopathy (coronary heart disease, stroke).
✓ How to optimize this marker
Lifestyle: exercise (aerobic + strength training lowers fasting glucose by 0.3–0.8 mmol/l), weight loss, Mediterranean or low-carb diet, sleep optimization, stress reduction. Dietary strategy: fiber before carbohydrates (sequence lowers postprandial peak). Metformin for prediabetes with high-risk features.
When to test
Annually from age 45; earlier with overweight (BMI > 25), family history of diabetes, gestational diabetes history, PCOS, or hypertension. Morning measurement after ≥ 8 hours fasting; no activity before the blood draw.
Frequently asked questions
Is fasting glucose enough, or do I also need HbA1c? +
Both are complementary: fasting glucose shows the current basal level (a daily snapshot), HbA1c reflects the 3-month average. HbA1c can be falsified by hemolytic conditions or anemia. The combination of both significantly increases sensitivity for early detection of prediabetes.
Why is my morning blood glucose sometimes higher than in the evening? +
This is the 'dawn phenomenon': cortisol, glucagon, and GH rise in the early morning hours (~4–8 AM) and stimulate hepatic glucose production. This physiological rise serves energy provision upon waking. With insulin resistance or diabetes, the effect is more pronounced.
What is prediabetes and how do I treat it? +
Prediabetes (fasting glucose 5.6–6.9 mmol/l or HbA1c 5.7–6.4%) is a reversible metabolic disorder. Studies show that 5–10% weight loss combined with 150 min/week of moderate activity reduces diabetes risk by 58% (DPP trial). Without intervention, ~25–70% of those with prediabetes develop type 2 diabetes within 3–5 years.
Sources
Last Reviewed: May 28, 2026 · sina
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.