Summary
Fasting glucose measures the concentration of glucose in the blood after at least 8 hours without food. It is the most direct measure of your body's ability to maintain normal blood sugar in the resting, fasted state. Elevated fasting glucose — alongside HbA1c — is used to diagnose type 2 diabetes and identify impaired fasting glucose (IFG), the pre-diabetic state.
In healthy individuals, fasting glucose is tightly maintained between 3.9 and 5.6 mmol/L by a balance of insulin (which promotes glucose uptake into cells) and glucagon (which releases glucose from liver stores). As insulin resistance develops, higher insulin levels are required to maintain normal glucose — and eventually, fasting glucose begins to rise.
Elevated fasting glucose is one of the earliest detectable signs of metabolic dysfunction — often rising years before type 2 diabetes develops. Impaired fasting glucose (5.6–6.9 mmol/L) identifies people at high risk who can benefit most from early lifestyle intervention.
Fasting glucose must be combined with HbA1c and, ideally, fasting insulin and HOMA-IR for a complete metabolic health assessment — each marker provides different and complementary information.
What It Is
Glucose is the primary circulating fuel for all human cells. In the fasted state, blood glucose is maintained by: (1) hepatic glucose output from glycogenolysis (breakdown of stored glycogen) and gluconeogenesis (synthesis from amino acids and glycerol); (2) insulin from the pancreatic beta cells, which suppresses hepatic glucose production and promotes peripheral glucose uptake.
Fasting plasma glucose is measured after a minimum 8-hour fast. The WHO diagnostic threshold for diabetes is ≥ 7.0 mmol/L; for IFG (impaired fasting glucose, or prediabetes) it is 5.6–6.9 mmol/L. The UK uses HbA1c as the primary diagnostic criterion, but fasting glucose remains important when HbA1c is unreliable or when assessing metabolic syndrome.
Diagnostic thresholds: Normal < 5.6 mmol/L; IFG 5.6–6.9 mmol/L; Diabetes ≥ 7.0 mmol/L.
Functions
Insulin secretion and resistance probe
Elevated fasting glucose directly reflects impaired insulin secretion or resistance — the core defect in type 2 diabetes.
Diabetes and prediabetes diagnosis
Fasting glucose ≥ 7.0 mmol/L on two occasions diagnoses diabetes; 5.6–6.9 mmol/L identifies impaired fasting glucose.
Metabolic syndrome component
Fasting glucose is one of five criteria for metabolic syndrome — elevated levels significantly increase cardiovascular risk.
HOMA-IR calculation input
Combined with fasting insulin, fasting glucose enables calculation of HOMA-IR — the insulin resistance index.
Reference Ranges
Fasting Plasma Glucose
Measured in mmol/L| Status | Range (mmol/L) | Range (mg/dL) | What it means |
|---|---|---|---|
| Normal | < 5.6 | < 100 | Normal fasting glucose — adequate insulin sensitivity in the fasted state. |
| Prediabetes | 5.6–6.9 | 100–125 | Impaired fasting glucose — significant risk of progression to type 2 diabetes. |
| Diabetes | ≥ 7.0 | ≥ 126 | Diabetes range — confirm with repeat test or HbA1c. Medical review required. |
A single fasting glucose ≥ 7.0 mmol/L in a symptomatic person diagnoses diabetes; in asymptomatic people, two separate tests are required. Fasting must be ≥ 8 hours — even coffee with milk affects the result.
Symptoms of Imbalance
Mildly elevated fasting glucose (prediabetes) is usually completely asymptomatic — this is why screening is important.
- Hypoglycaemia (< 3.9 mmol/L): shakiness, sweating, dizziness, confusion
- Palpitations
- Pallor and anxiety
- Hunger — intense craving for food
- Often asymptomatic in prediabetes
- Increased thirst and frequent urination
- Unexplained fatigue
- Blurred vision
- Slow-healing wounds
- Tingling or numbness in the feet (early neuropathy in chronic high glucose)
Causes of Imbalance
- Insulin overdose or medication-induced hypoglycaemia
- Prolonged fasting or excessive exercise without food
- Insulinoma (rare insulin-secreting tumour)
- Severe liver failure
- Addison's disease (adrenal insufficiency)
- Type 2 diabetes — insulin resistance and beta-cell dysfunction
- Type 1 diabetes — absolute insulin deficiency
- Prediabetes / impaired fasting glucose
- Corticosteroid therapy
- Cushing's syndrome (excess cortisol)
- Pancreatitis or pancreatectomy
- Stress hyperglycaemia (acute illness — transient)
FAQs
A true fasting glucose requires at least 8–10 hours without eating or drinking anything other than plain water. Even a small amount of food, fruit juice, milk in coffee, or a glucose-containing drink will invalidate the test. Timing matters too — fasting glucose should be taken in the morning, as cortisol and growth hormone cause a natural rise in blood glucose in the late afternoon (the dawn phenomenon).
A fasting glucose of 5.9 mmol/L falls in the impaired fasting glucose (IFG) range — also called prediabetes. It does not mean you have diabetes, but it does indicate that your blood sugar regulation is under strain. Combined with HbA1c, this gives a clearer picture. Crucially, IFG is highly reversible with sustained lifestyle changes — losing 5–10% of body weight and increasing physical activity significantly reduces the risk of progression.
Yes. Psychological stress triggers cortisol and adrenaline release, which stimulate hepatic glucose production and impair insulin signalling — causing stress hyperglycaemia. Acute illness, surgery, and injury can also cause transient glucose elevation. A single high fasting glucose in the context of acute stress should be repeated once stable before clinical decisions are made.
The OGTT involves measuring glucose at fasting and 2 hours after drinking a 75g glucose solution. It detects impaired glucose tolerance (IGT) — elevated glucose at 2 hours despite a normal fasting level. IGT indicates poor post-meal glucose clearance and significant diabetes risk, but is missed by fasting glucose alone. OGTT is recommended in pregnancy (gestational diabetes screening) and when clinical suspicion is high despite normal fasting glucose and HbA1c.
Yes. Fasting glucose is measured after ≥ 8 hours without food and reflects baseline glucose regulation. Random glucose is taken at any time and is used when diabetes symptoms are severe — a random glucose ≥ 11.1 mmol/L with symptoms diagnoses diabetes without fasting. Random glucose is less useful for screening because post-meal levels vary widely depending on what was eaten and when.
References
- WHO/IDF. Definition and diagnosis of diabetes mellitus and intermediate hyperglycaemia. 2006. View source
- NICE. Type 2 diabetes in adults: management. NG28. Updated 2022. View source
- Tabák AG, et al. Prediabetes: a high-risk state for diabetes development. Lancet. 2012;379(9833):2279–2290. View source