High fasting glucose while you are fit? This is what happens in the morning

You train four times a week, you are lean, and your fasting glucose reads 5.8. Just under the line, but higher than you expected. Every page you find is about prediabetes, and that does not feel like it is about you.

That is because it probably is not about you.

Your morning value is the noisiest measurement you own. Between four and eight in the morning your own hormone system pushes your glucose up, and that happens in healthy people just as much as in sick ones. Almost nobody has written it down for you.

Why is my blood sugar so high in the morning?

Because your body wakes you up with fuel. In the early morning your liver releases extra glucose under the influence of cortisol, adrenaline and growth hormone, so you get up with energy. That is called the dawn phenomenon, and it is not a disease but a daily rhythm.

You are literally booting up before you are awake.

This has been measured precisely, and not in diabetes patients. Bolli and Gerich followed eight healthy volunteers between one and nine in the morning, every half hour. After 5:30am glucose, insulin and C-peptide rose. Cortisol, adrenaline and noradrenaline rose between 4:00 and 6:30am from their nocturnal low, and growth hormone surged between 1:00 and 4:30am then fell by around half (PMID 6389230).

The researchers concluded that the morning rise in diabetes is probably an exaggeration of a normal circadian variation. Read that sentence again. The phenomenon was not found in sick people, it was discovered in healthy ones.

What else pushes your morning value up?

More things than you think, and most have nothing to do with your pancreas. Bad sleep, a heavy session yesterday, a stressful week, a bug or a low-carb diet: they all push the same button.

So one high morning value is almost never a conclusion.

Only that last row is interesting, and you cannot tell it apart from the other five with the measurement you currently have. For that you need insulin.

Why does fasting glucose rise on low carb?

Because your muscles start sparing glucose for your brain. When you eat few carbohydrates, your muscles become less eager for glucose so enough remains for organs that cannot run on fat. Your glucose then sits slightly higher without anything being broken.

That is adaptation, not derailment.

This explains why people new to keto sometimes see a higher fasting value than when they ate bread, and why that on its own need not be an alarm. It just makes your fasting number an even worse solo marker than it already was. What changes in your blood when you switch diets sits in blood testing before and after the keto diet.

When is a high fasting glucose actually interesting?

When it stays put once the noise is gone. Measure three times on quiet mornings, rested, at least two days after your last heavy session, and if you keep seeing the same raised number, you are no longer measuring noise.

Then it is a pattern, and patterns are worth something.

Picture two athletes both seeing 5.8 fasting. The first drew the morning after a heavy leg session, on five hours of sleep, mid-house-move. The second drew on a quiet Sunday after eight hours of sleep and three days without training. Same number, and only for the second is it a signal.

What else can sit behind a raised value, including outside the morning, sits in high blood sugar without diabetes.

What does your HbA1c say about this?

Something, but less than you hope, and in athletes it can reassure you at exactly the wrong moment. HbA1c assumes red blood cells lasting around 120 days. In endurance athletes that turnover can run faster, and younger cells have had less time to pick up glucose.

Your average then reads lower than it really is.

An eLife study modelled this and found a median red cell lifespan of 100 days with a spread of 83 to 102 days between people. At the shortest lifespan, lab HbA1c sat around 2 percent lower than the adjusted value (PMID 34515636). More in HbA1c in non-diabetics.

Does one morning measurement say anything at all?

Little, and that is better supported than most people know. Stanford researchers had people with normal standard values wear a sensor, and that normal group turned out to spend 15 percent of the time in the prediabetic range (PMID 30040822). Their fasting draw had shown none of it.

So the draw missed the interesting part entirely.

That cuts both ways: a fasting value can look too high while your day is fine, and look fine while your day is not. It is one point out of 1,400. The full trade-off between measurement methods sits in measuring your blood sugar, and what a sensor does and does not teach you in continuous glucose monitor without diabetes.

Thuisarts is the most level-headed Dutch place to read what your GP does with a raised value, and the contrast with the panic on the rest of the internet is stark.

What do you measure alongside it?

Insulin, and really only insulin. Your fasting glucose says where you land. Your fasting insulin says what it cost you, and only those two together show whether that raised morning number means anything.

Without insulin you keep guessing.

A 5.8 with a low insulin is a different story to a 5.8 your pancreas had to work hard for. That combination is called HOMA-IR, and the explanation sits in fasting insulin and HOMA-IR. The full overview of which numbers count together sits in blood sugar values when you train. The individual value sits at fasting glucose.

What do you do next?

Draw again on a quiet morning, rested, at least two days after your last heavy session, and take insulin in the same tube. One measurement under honest conditions is worth more than five measurements after busy weeks.

My advice: do not delete your old reading, but give it the context it deserves. Note what you did that week, because that is half your result.

You draw glucose and insulin together with the HOMA-IR test, or more broadly with 360 Health.

Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.

References

• Bolli GB, De Feo P, De Cosmo S, Perriello G, Ventura MM, Calcinaro F, Lolli C, Campbell P, Brunetti P, Gerich JE. Demonstration of a dawn phenomenon in normal human volunteers. Diabetes, 1984;33(12):1150-3. PMID 6389230.
• Hall H, Perelman D, Breschi A, Limcaoco P, Kellogg R, McLaughlin T, Snyder M. Glucotypes reveal new patterns of glucose dysregulation. PLoS Biology, 2018. PMID 30040822.
• Xu Y, Bergenstal RM, Dunn TC, Ajjan RA. Addressing shortfalls of laboratory HbA1c using a model that incorporates red cell lifespan. eLife, 2021. PMID 34515636.
• Diabetes Fonds. Normal blood sugar values.
• Thuisarts. I have a raised blood sugar.
• RIVM. Figures on diabetes in the Netherlands.