Fasting glucose

What is fasting glucose?

Glucose is the sugar your cells run on. Almost every carbohydrate you eat, from bread to fruit, ends up as glucose in your blood, and your body keeps that level within a narrow band: too low and your brain runs short of fuel, too high and the sugar slowly damages your vessels. Insulin from your pancreas is the regulator. It moves glucose into your muscles, liver and fat tissue as soon as more comes in than you are burning at that moment. Fasting glucose measures that level at the quietest moment of the day: after a night without eating, when your last meal has long been processed. What you see then is not your breakfast but the level your body falls back to. Your liver makes glucose of its own overnight to keep your brain running, and insulin keeps that production in check. The fasting value shows how well those two are in balance. Your report shows one number in mmol/L. Normal is 4.0 to 6.0 mmol/L, measured in blood from a vein, and almost every Dutch lab uses that range, at most one step wider at either end. Unlike cholesterol this is a measured range and not a target: it is what you find in healthy people. Where you sit within that range says more than whether you sit inside it, and that is what the rest of this page is about.

Why is fasting glucose relevant?

A blood sugar that hangs on the high side for years does silent damage. Glucose attaches itself to proteins in the wall of your blood vessels and in your nerves, and over time that makes the small vessels of eyes and kidneys leak and the large vessels stiffen. You feel none of it. Type 2 diabetes is the end of that road, but the damage begins well before it, in the years when blood sugar creeps up slowly. The awkward part is that fasting glucose is the number that moves last. Insulin resistance, the metabolism in which your muscles and liver listen less and less to insulin, begins years earlier. Your pancreas compensates by making more insulin, and as long as that works your fasting glucose stays neatly normal. Only when the pancreas can no longer keep up does the value shoot up. A value pressing against 6.0 mmol/L is therefore not the same as a value around 4.0 mmol/L, even though both are normal. That is why, for anyone who wants to stay healthy, this is a value to get to know young and to follow. The direction over the years, from the low to the high side of normal, is the earliest signal you can get out of this number, and it is exactly the signal lifestyle does the most with. Knowing where you stand at thirty gives you decades to set the direction. The limits above it are sharp. Between 6.1 and 6.9 mmol/L it is called impaired fasting glucose, the category between normal and diabetes. From 7.0 mmol/L fasting it counts as diabetes, provided a second measurement on another day shows the same; in the Netherlands the diagnosis is made on this test, not on HbA1c. A fasting value alone does miss about 30 percent of the people who already have diabetes, because in them it is mainly the blood sugar after meals that is too high. HbA1c beside it catches that part.

Fasting glucose reference ranges

The first row is a measured range: what you find in healthy people, in blood from a vein and in mmol/L. The rows below it are thresholds, not normal values; they are there because a result is read against them. For anyone who wants to stay healthy the top of the first row matters more than the thresholds below it.

Fasting glucose high or low: what it means

A high fasting glucose almost always comes out of the pattern of insulin resistance: belly fat, little exercise, little muscle, plenty of sugar and fast carbohydrates, and the years in which that piles up. Lack of sleep and lasting stress raise the morning value too, because stress hormones set your liver to making glucose, and an infection or a bad night does the same. Some medicines, prednisone first among them, lift the value for as long as you take them. And some high values are simply not fasting long enough: less than eight hours without food, and you are measuring your late dinner. A low fasting glucose in a healthy person is usually not a problem but a favourable sign: a sensitive metabolism that gets by on little insulin, as in lean people who exercise a lot. Below the lower limit is rare in anyone not on diabetes medication. More often a low result is a matter of the tube: red blood cells keep consuming glucose after the draw, and a sample left standing too long reads lower than what was in your arm. A low number without symptoms such as trembling, sweating or blurred vision is therefore no cause for concern. The levers are those of blood sugar as a whole, and they work a little more slowly here than for triglycerides. Losing weight around the middle is the biggest: belly fat is the engine of insulin resistance, and a few kilos less already makes your liver and muscles more sensitive. Daily movement, with a walk after meals, because working muscles take up glucose without insulin being involved. Strength training, because muscle is the largest storage place for glucose. Less soft drink, juice, sweets and white carbohydrates, and more fibre from vegetables, legumes and whole grains, which slow the uptake. Enough sleep, because a short night makes you measurably more insulin resistant the next morning. In most people without diabetes that is the whole solution, and with diagnosed type 2 diabetes it remains the foundation that medication joins where needed. Read fasting glucose beside HbA1c, which gives the average over the past months, and beside triglycerides and HDL, which often give away insulin resistance before the sugar itself does. Anyone who wants to know more precisely has fasting insulin measured beside it: that rises years before glucose does. And read it as a trend: the same morning, the same hours fasting, a year later.

How does a fasting glucose blood test work?

Referral

Not needed. You can have fasting glucose measured without going through a doctor first.

Fasting

Yes, and that is the whole point of this test. At least 8 hours without food and preferably 12: nothing after dinner, water only, and an early-morning draw. Under 8 hours the result does not count as fasting.

The draw

One tube of blood from a vein in your arm, in a special tube with a substance that stops your blood cells eating the glucose on the way. That keeps the number the number of your arm and not of the tube.

The result

A single number in mmol/L, in the same unit at every Dutch lab and usually available the same day.

Repeating

One high value is never a diagnosis; that takes a second fasting measurement on another day. If you change something about your food, weight or exercise, measure again after three months, the same way: same time of day, same hours fasting.

Which test

Fasting glucose is in the comprehensive test (€229) and in core.

Where to draw

You can draw at 250+ locations near you, no referral needed. See the locations

Educational information only, not medical advice. Consult a healthcare professional for clinical decisions.