Blood glucose is the concentration of glucose — the simple sugar your cells use as immediate fuel — circulating in your blood at a given moment, reported in milligrams per deciliter (mg/dL) in the US and millimoles per liter (mmol/L) in most other countries. The body defends this number within a narrow band, mainly with insulin to push glucose into cells and glucagon to release it from the liver. It rises after every meal and falls again over the following hours, so the number only means something once you know when it was taken. Most clinical decisions rest on a fasting value, measured after roughly 8 to 12 hours without food.
What it measures
A standard lab result comes from a venous blood draw, and the glucose is measured in plasma rather than whole blood. A home meter reads a fingertip capillary drop and can differ from a lab plasma value by a noticeable margin, and a continuous glucose monitor measures glucose in the fluid between cells, which trails blood levels by several minutes. Three different tests answer three different questions: a fasting sample is a snapshot of your baseline, an oral glucose tolerance test shows how quickly you clear a standard 75 g sugar load, and Glycated hemoglobin (HbA1c) estimates your average glucose over roughly the previous two to three months. The deeper reference on Blood glucose (fasting) walks through the testing conditions that distort a single result, from an incomplete fast to an acute infection.
Typical values
The bands below are the fasting plasma glucose criteria used by the American Diabetes Association. They are diagnostic thresholds, not laws of biology: other bodies draw the lower prediabetes boundary higher, at 110 mg/dL (6.1 mmol/L), and a diagnosis is never made on one reading alone.
| Fasting plasma glucose | mg/dL | mmol/L |
|---|---|---|
| Normal | Below 100 | Below 5.6 |
| Prediabetes (impaired fasting glucose) | 100-125 | 5.6-6.9 |
| Diabetes range | 126 or above | 7.0 or above |
For the two-hour value in an oral glucose tolerance test, below 140 mg/dL (7.8 mmol/L) is normal, 140-199 mg/dL (7.8-11.0 mmol/L) is impaired glucose tolerance, and 200 mg/dL (11.1 mmol/L) or above is in the diabetes range. Your lab report may print slightly different reference limits depending on the assay it uses, so read your own result against the range printed beside it.
Why it matters for longevity
Higher fasting glucose is consistently associated with cardiovascular disease, kidney and eye damage, dementia and all-cause mortality in large observational studies, and the association does not switch on abruptly at a diagnostic cut-off — risk tends to grade upward across the upper part of the normal range as well. Chronically high glucose also drives glycation, the non-enzymatic binding of sugar to proteins that stiffens collagen in skin and arteries. Fasting glucose is a late signal rather than an early one: Insulin resistance usually develops years before the fasting number moves, because rising insulin keeps glucose looking normal for a long time. The encouraging part is interventional: randomized lifestyle programs in people with prediabetes have cut progression to type 2 diabetes by roughly half compared with usual care.
What changes it
Losing excess weight, particularly visceral fat around the abdomen, is the single most reliable lever. Beyond that: the amount and quality of carbohydrate you eat, a 10 to 15 minute walk after meals, resistance training that builds muscle to store glucose in, aerobic exercise, and adequate sleep — short or broken sleep raises next-morning glucose in controlled studies. Illness, acute stress, and drugs such as corticosteroids push readings up temporarily, which is why one high value during a bad week is not a diagnosis. Metformin and GLP-1 receptor agonists lower glucose pharmacologically when lifestyle is not enough. What you cannot change: your genetics, a gradual upward drift with age, and pregnancy, which alters glucose handling on its own.
Related reading
- Which Blood Tests Reflect Aging and How to Read Them
- How to Improve Metabolic Health: A Practical Guide
- Glycation and AGEs: How Sugar Ages Your Tissues
- Heart Health: A Prevention Guide for Longevity
Disclaimer. This article is for information only and does not replace medical advice. Talk to a qualified clinician before changing anything about your health.