HbA1c: what your average blood sugar says about the next decade

Most blood tests are snapshots. Take one at eight in the morning and again at four in the afternoon and you get different answers, both accurate, neither especially informative on its own.

HbA1c is different, and that is what makes it useful. It is a three-month average, and it is one of the few numbers on a standard blood panel that tells you something about where you are heading rather than where you are.

What it measures

When glucose circulates in the blood, some of it binds irreversibly to haemoglobin inside red blood cells, forming glycated haemoglobin — HbA1c.

Red blood cells live around three months. The proportion of your haemoglobin that has been glycated therefore reflects your average glucose over roughly the preceding two to three months, weighted towards the more recent weeks.

You cannot game it by fasting the day before or eating carefully for a week. That is precisely its value.

The numbers

In the UK, results are reported in mmol/mol, following NICE and WHO thresholds:

Result	Meaning
Below 42 mmol/mol	Normal
42–47 mmol/mol	Non-diabetic hyperglycaemia — commonly called prediabetes
48 mmol/mol or above	Diagnostic of type 2 diabetes

Older results and US sources use percentages: 42 mmol/mol is 6.0%, 48 is 6.5%. Some letters print both.

For an asymptomatic person, a result of 48 or above needs a second confirmatory test before diabetes is diagnosed. A single reading is not a diagnosis. If diabetes symptoms are present, one result may be enough.

The range that matters most here

For a site about ageing, the interesting band is 42–47 — above normal, below diagnosis.

This is a genuine warning, delivered years before anything happens, and it is one of the few times medicine gives you that.

NICE guidance for this group is explicit and worth quoting in spirit: people in this range should be told they are currently at high risk, but that this does not necessarily mean they will progress to type 2 diabetes. Progression is not automatic.

The recommended follow-up is a blood test at least once a year, preferably the same type each time so results are comparable.

And in England, people in this range may be eligible for referral to the NHS Diabetes Prevention Programme — a free, structured programme covering diet, activity and weight management. It is not widely known, and it is worth asking your GP about specifically if your result lands here.

Prediabetes is frequently reversible with sustained changes to diet, physical activity and weight.

Why it belongs in a longevity section rather than a diabetes one

Glucose regulation deteriorates quietly, over years, and its consequences are not confined to diabetes.

Type 2 diabetes appears on the Lancet Commission’s list of modifiable dementia risk factors. Impaired glucose regulation is closely tied to cardiovascular risk. And the process that ends in a diagnosis at 60 typically began somewhere in the forties, with numbers drifting upward in a range nobody treated as urgent because it was not yet abnormal.

That drift is what HbA1c makes visible. A result of 39 at 45 and 44 at 52 is a trajectory, even though the first is normal and the second only just is not.

Which is the most useful way to read this test: as a direction, not a category. Keep your results. Compare them across years. A number that is stable is different information from the same number arriving on the way up.

What can make it wrong

HbA1c depends on red blood cells behaving normally, so anything affecting their lifespan distorts the result.

Falsely altered readings can occur with iron deficiency anaemia, haemolytic anaemia, haemoglobinopathies such as sickle cell trait and thalassaemia, chronic kidney disease, and recent blood transfusion or significant blood loss.

HbA1c is not appropriate for diagnosis in children, in pregnancy, where type 1 diabetes is suspected, or in people with haemoglobinopathies. In these situations, fasting glucose or an oral glucose tolerance test is used instead.

If your result seems inconsistent with everything else about you, this is the first thing to raise.

What moves it

The interventions are the ones already covered in this site’s nutrition and movement sections, which is not a coincidence.

Physical activity improves insulin sensitivity, and both aerobic exercise and resistance training contribute — muscle is the largest site of glucose disposal in the body, so having more of it helps directly.

Fibre slows glucose absorption and blunts post-meal spikes, which is one of the several reasons the 30 g target keeps appearing.

Weight, particularly around the abdomen, is closely tied to insulin resistance, and modest sustained loss produces disproportionate improvement in glucose regulation.

Reducing refined carbohydrates — the practical version of which is mostly the ultra-processed food question again.

None of this is separate from the rest of the site. The same handful of habits keep showing up against different outcomes.

A note on continuous glucose monitors

CGMs are increasingly sold to people without diabetes as a longevity tool, and the evidence for that use is thin.

They produce a great deal of data about glucose variability, whose significance in non-diabetic people is not established, and they can encourage anxious food avoidance based on spikes that are entirely normal physiology.

For anyone with diabetes, particularly on insulin, they are a genuine advance. For a healthy person curious about their metabolism, an annual HbA1c costs the NHS a few pounds and answers the question that has actual evidence behind it.

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This article is for general information and is not medical advice. If your HbA1c is raised, discuss it with your GP — do not self-diagnose or self-treat.

Sources

1. NICE. Type 2 diabetes in adults: management. NICE guideline NG28, and NICE guidance on preventing type 2 diabetes.
2. Pre-diabetes: NICE thresholds and annual monitoring guidance. GPnotebook. https://primarycarenotebook.com/pages/diabetes-and-endocrinology/pre-diabetes
3. HbA1c diagnostic thresholds, UK reference ranges and conditions affecting accuracy. https://www.boltpharmacy.co.uk/guide/hba1c-diabetes-cut-off
4. HbA1c and average glucose: ranges and interpretation. https://www.boltpharmacy.co.uk/guide/average-glucose-and-hba1c
5. Livingston G, et al. Dementia prevention, intervention, and care: 2024 report of the Lancet standing Commission. The Lancet, 2024.