Blood sugar values when you train: what your glucose really says

A fasting blood sugar under 6.1 mmol/l is called normal. Between 6.1 and 6.9 it is called prediabetes, and from 7.0 your GP starts talking about diabetes. Those thresholds sit on every Dutch page about blood sugar values, and they are all correct.

They just answer a question you are not asking.

Those thresholds were built to detect diabetes. You train four times a week and want to know whether you are metabolically sharp. That is a different question, and "no diabetes" is a fairly low answer to it. Between 4.0 and 6.0 sits an enormous world that almost no Dutch page covers.

What are normal blood sugar values?

Fasting between 4.0 and 6.0 mmol/l counts as normal, and two hours after a meal glucose stays under 7.8 mmol/l in most people. The Dutch Diabetes Fonds uses those limits: up to 6.0 normal, 6.1 to 6.9 possible prediabetes, 7.0 or higher fits diabetes. Under 4.0 is called a hypo.

That is the map. Now the scale.

Those numbers come from research into who gets ill, not research into who performs well. A fasting value of 5.9 is not "almost prediabetes" and also not "fine". It is a number that only tells you something once you know how it came about, and that is exactly what the rest of this article is about.

Which four numbers do you look at?

Four values tell the story together: fasting glucose, HbA1c, fasting insulin, and your glucose after eating. On their own all four are misleading. Together they show whether your body clears glucose smoothly or has to work for it.

That last sentence is the entire point of this page.

A body that keeps glucose low using a lot of insulin looks identical on a fasting glucose test to a body that does it effortlessly. The same 5.2. Completely different metabolic situation. You only see that difference once you add insulin.

Look at that last column. Three of the four numbers move because of your training, and that is why a table written for the average person only half helps you.

Why does "no diabetes" say so little?

Because the threshold for disease is not the same as a sign of health. You can sit well below every cut-off and still have glucose spikes nobody has ever measured, simply because a fasting draw cannot see them.

That is not theory. That has been measured.

Stanford researchers had people who were normal by every standard test wear a sensor. That supposedly normal group spent 15 percent of the time in the prediabetic range and 2 percent of the time in the diabetic range (PMID 30040822). The same people whose GP had told them their blood sugar was fine.

The researchers called the patterns they found glucotypes: people fell into recognisable types based on how stable or erratic their glucose moved through the day. Two people with an identical fasting value could have completely different days.

I think that is the most interesting metabolic research of the past decade, and it appears in almost no Dutch article about blood sugar values.

What does your training do to your blood sugar?

More than most tables admit. Strength training empties your muscle glycogen and makes your muscles more sensitive to insulin afterwards, which can lower your glucose in the hours and days that follow. A heavy session can temporarily push your glucose up in the moment, because your body is releasing fuel.

Both things are true. At different moments.

That makes the timing of your draw more important than most people think. Draw the morning after a heavy leg session and you are partly measuring your session. Draw after three quiet days and you measure more of your baseline. Neither is wrong, but you need to know which of the two you are holding.

Walking after eating is the clearest example of how hard movement acts on glucose. A meta-analysis of 8 randomised trials with 116 participants found that moving after the meal lowered the glucose excursion compared with moving before the meal (standardised difference 0.47) and compared with doing nothing (0.55). Moving before eating did almost nothing (PMID 36715875).

Ten minutes of walking after your dinner does more for your curve than the supplement you wanted to take over it.

Why is HbA1c tricky in athletes?

Because HbA1c does not measure glucose but sugar-coated red blood cells, and the assumption about how long those cells live does not hold for everyone. Your HbA1c assumes your red cells last around 120 days. In endurance athletes and frequent blood donors that turnover can run faster.

Faster turnover means younger cells. Younger cells have had less time to pick up glucose. Your HbA1c then reads lower than your actual average glucose.

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

In other words: as an endurance athlete your HbA1c can reassure you while your glucose does not deserve it. The reverse happens too. That is why HbA1c is a fine population marker and a mediocre individual one, and why you place it next to your other values instead of leaning on it. The deeper explanation sits in HbA1c in non-diabetics.

What is a good fasting glucose for someone who trains?

There is no official "athlete range", and anyone giving you an exact target number is making it up. What you can do is track your own value over time instead of holding it against a table built for the general population.

Your own trend is your reference. Not the table.

If you see your fasting value creep from 5.0 to 5.7 over a year while you train the same, that is more interesting than whether 5.7 still falls inside the limit. A single measurement is a photo. Four measurements over two years are a film, and films tell better stories.

Also note: your morning value is the noisiest of them all. Your hormones push it up every morning, even if you are perfectly healthy. What exactly happens there sits in high fasting glucose while you are fit.

How do you measure your blood sugar values?

Three routes, and they answer different questions. A finger prick gives you one moment. A sensor gives you a curve across days. A lab draw gives you a calibrated value plus the numbers you cannot measure at home, like insulin.

Most people pick a method and ask the question afterwards. That is the wrong order.

If you want to know how you respond to your oats, a lab draw is useless and a sensor is perfect. If you want to know whether your insulin is working too hard, no sensor can help you. The full trade-off sits in measuring your blood sugar, and what a sensor does and does not teach you as a healthy athlete in continuous glucose monitor without diabetes.

When is a high value interesting?

When it forms a pattern, not when it stands out once. A single high reading after a bad night, a bug, a stressful week or a heavy training block says little. The same rise three times on quiet days is a different signal.

One measurement is never a conclusion.

What can sit behind a high value when you are not diabetic is covered in high blood sugar without diabetes. And if you want to know whether your spike after eating is a problem or just fuel, glucose spike after eating is about that.

What you should not take from this: that you should hunt the lowest possible number. A blood sugar that never moves is not a goal, it is an athlete who is not eating.

What if your values shift?

Then the first question is what else changed. Your sleep, your stress, your training volume, your weight and your food all land in the same total, and your glucose is one of the most honest places where that total becomes visible.

RIVM tracks how many people in the Netherlands meet the Gezondheidsraad physical activity guidelines, and it is fewer than half. If you are reading this, you are probably not in that group. That means the general lifestyle advice sitting under every blood sugar article is largely already in place for you.

Your margin sits somewhere else: in your timing, your sleep and your recovery.

The Voedingscentrum is the most level-headed Dutch source on carbohydrates, and it is striking how little of the online spike panic appears there. That says enough about the rest of the internet. What lifestyle demonstrably does sit in lowering blood sugar.

What do you measure if you want to look further?

Insulin is the value that adds the most and gets drawn the least. A fasting glucose plus a fasting insulin together show what glucose alone hides: how much effort your body puts into that tidy number. That combination is called HOMA-IR.

That is the measurement most people miss.

Picture two athletes both sitting at 5.3 mmol/l fasting. One does it with a low insulin, the other with an insulin twice as high. The same glucose, two completely different metabolic stories, and only the second athlete has something to keep an eye on. How that works sits in fasting insulin and HOMA-IR.

The individual values are explained at fasting glucose, HbA1c and fasting insulin. If you want the broader metabolic picture, measuring metabolic health is your next step, and insulin resistance if you want to know where all of this leads.

What do you do next?

Draw your fasting glucose and your fasting insulin on the same morning, after at least two quiet days, and write the date next to it. Repeat it in three months at the same point in your week. Two points are a line, and a line is more than any table can give you.

My advice stays simple: measure less often than you want to, and compare more consistently than you are used to.

If you want those two values drawn together, the HOMA-IR test is the shortest route, and 360 Health if you want to see them in a broader panel.

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

References

• 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.
• 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.
• 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.
• Engeroff T, Groneberg DA, Wilke J. After Dinner Rest a While, After Supper Walk a Mile? A Systematic Review with Meta-analysis on the Acute Postprandial Glycemic Response to Exercise Before and After Meal Ingestion in Healthy Subjects and Patients with Impaired Glucose Tolerance. Sports Medicine, 2023. PMID 36715875.
• Diabetes Fonds. Normal blood sugar values.
• Gezondheidsraad. Dutch physical activity guidelines.
• RIVM. Figures on meeting the physical activity guidelines.
• Voedingscentrum. Carbohydrates.