Blood sugar moves all day — up after a meal, down with a walk, lower again overnight. A single glucose reading is a snapshot of one of those moments. HbA1c — glycated haemoglobin — is something more useful for thinking about ageing: a slow-moving average of where your blood sugar has actually been over the past few months. That is why it has become one of the most informative routine blood tests for the long game of how you are ageing, not just for diagnosing diabetes.
This article explains what HbA1c measures, why glucose and ageing are biologically linked, what large studies show about risk that begins below the diabetes line, how to read your own number sensibly — including a real UK-versus-US difference in the cut-offs — and what the evidence says about moving it. It is educational, not medical advice; every substantive claim is cited, and the caveats are spelled out as we go.
What HbA1c actually measures
Haemoglobin is the protein inside red blood cells that carries oxygen. When glucose is present in the blood, a small fraction of it sticks to haemoglobin in a slow, essentially irreversible reaction. The more glucose that has been circulating, the more haemoglobin ends up "glycated". Because red blood cells live for roughly three months before being replaced, the proportion of glycated haemoglobin — your HbA1c — reflects your average blood-glucose exposure over that window.
That is the quiet strength of the test. It does not care whether you happened to fast before the blood draw, and it cannot be gamed by a single good day. It integrates the highs and lows into one number, which makes it far steadier than a one-off glucose reading and far better suited to tracking a trend over months and years.
It is worth being clear about the marker's limits up front. Because HbA1c depends on red blood cells living a normal lifespan, anything that changes that lifespan can distort the result. Anaemia, recent blood loss, pregnancy, some inherited haemoglobin variants and certain kidney conditions can all make HbA1c read artificially high or low. In those situations the number is not wrong about the chemistry — it is just no longer a faithful proxy for average glucose, which is why clinicians interpret it in context rather than in isolation.
Glycation: the chemistry that links sugar to ageing
The same reaction that creates HbA1c happens throughout the body, and this is where blood sugar connects to ageing in a mechanistic way. When glucose attaches to proteins and, over time, rearranges into more stable compounds, the end products are called advanced glycation end-products, or AGEs.
AGEs matter because many of the proteins they modify are long-lived structural ones — collagen in skin, blood vessels, tendons and the lens of the eye. As AGEs accumulate, they cross-link these proteins, making tissues stiffer and less elastic, and they engage receptors that switch on inflammatory signalling. A review of the evidence by Semba and colleagues argued that this accumulation plausibly contributes to the ageing phenotype itself — the gradual loss of tissue flexibility and the rise in low-grade inflammation that show up across multiple organ systems with age (Semba et al., 2010, Journals of Gerontology: Series A). In other words, persistently higher glucose is not just a metabolic number; it leaves a slow chemical footprint on the body's structural proteins.
This is the bridge from a routine blood test to the biology of getting older: HbA1c is a readout of how much glucose has been available to drive that glycation, averaged over months.
The risk starts below the diabetes line
For a long time, glucose was treated as a roughly binary thing — you either had diabetes or you did not. The large prospective studies of the last two decades tell a more graded story, and that is what makes HbA1c interesting for healthy ageing rather than only for diabetes care.
In the Atherosclerosis Risk in Communities (ARIC) study, researchers followed more than 11,000 adults who did not have diabetes at the start. Higher HbA1c was associated with a steadily greater risk of coronary heart disease and stroke across the non-diabetic range, and the relationship held even after accounting for fasting glucose and the usual risk factors (Selvin et al., 2010, New England Journal of Medicine). The cardiovascular risk, in other words, did not switch on at the diabetes threshold — it climbed gradually well below it.
The EPIC-Norfolk study, following a large general-population cohort in the UK, found much the same shape for cardiovascular disease and all-cause mortality: risk rose continuously with HbA1c across the whole range, with the lowest risk among those at the bottom and no clean cut-off where it began (Khaw et al., 2004, Annals of Internal Medicine).
One honest nuance keeps this accurate. While the link between higher HbA1c and cardiovascular events is graded and consistent, the relationship with all-cause death is better described as J-shaped than as a straight line — risk is lowest in a middle band and is somewhat higher at both the high and the very low ends, the latter often reflecting illness or frailty rather than anything good about low glucose. So the takeaway is not "lower is always better without limit"; it is that drifting upward through the upper-normal and intermediate range is a meaningful, measurable signal worth watching.
Diabetes itself roughly doubles vascular risk
Stepping up from the marker to the condition it helps diagnose: the Emerging Risk Factors Collaboration pooled individual data from 102 prospective studies covering nearly 700,000 people. Having diabetes was associated with roughly a doubling of the risk of coronary heart disease, ischaemic stroke and other vascular deaths, independent of conventional risk factors (Sarwar et al., 2010, The Lancet).
That same analysis is also a useful corrective against over-reading glucose. The relationship between fasting glucose and vascular risk was modest and far from a smooth dose-response — risk was essentially flat across the normal fasting range and only rose appreciably at clearly elevated levels. The strong, reliable signal in that dataset is the roughly two-fold risk that comes with diabetes, not a tidy gradient for every small fluctuation in a fasting glucose reading. This is part of why an averaged marker like HbA1c, read as a trend, tends to be more informative than chasing individual glucose numbers.
How to read your own number
HbA1c is reported in two units. UK laboratories usually use mmol/mol (the IFCC scale); the older percentage scale (DCCT/NGSP) is still widely quoted. The widely used categories are:
- Normal — below about 5.7% (39 mmol/mol).
- Intermediate / "at-risk" — the band below the diabetes threshold, where glucose handling is no longer optimal.
- Diabetes range — 6.5% (48 mmol/mol) or higher, confirmed on repeat testing.
The 6.5% / 48 mmol/mol diabetes threshold comes from an International Expert Committee report that established HbA1c as a diagnostic test in its own right (International Expert Committee, 2009, Diabetes Care). The intermediate band is where a genuine UK-versus-US difference shows up, and it is worth knowing if you compare your result against figures you read online. In the United States, the American Diabetes Association defines "prediabetes" as starting at 5.7% (39 mmol/mol). In the UK, NICE defines the high-risk "non-diabetic hyperglycaemia" band as starting higher, at 42 mmol/mol (6.0%). So a result of, say, 40 mmol/mol would be labelled "prediabetes" by US criteria but sit just under the UK at-risk band — a difference of definition, not of biology.
Two practical points keep this sensible. First, a single result near a threshold is not a diagnosis; clinically, diabetes is confirmed on a repeat test, and any decision belongs with a doctor who can account for the distortions mentioned earlier. Second — and this is the part that matters most for ageing — the value of HbA1c here is as a trend you can watch. A number creeping up year on year through the upper-normal range is an early, actionable signal long before it reaches any diagnostic line; one result is a dot, but a series of them is a direction.
What the evidence says about moving it
The reassuring part of the glucose story is that the upper-normal and at-risk range is one of the most modifiable in all of medicine — and we have unusually strong trial evidence for it.
The landmark example is the Diabetes Prevention Program, a large randomised trial in people at high risk of type 2 diabetes. A structured lifestyle programme — aiming for modest weight loss of around 7% and at least 150 minutes of physical activity a week — reduced the incidence of type 2 diabetes by 58% over roughly three years, and did so more effectively than the medication metformin, which reduced incidence by 31% (Knowler et al., 2002, New England Journal of Medicine). That is one of the clearest demonstrations in the literature that the trajectory of blood sugar is not fixed: regular movement and a modest change in body weight measurably bent the curve.
The themes that recur across this evidence are familiar and unglamorous: regular physical activity (both aerobic exercise and resistance training help muscle take up glucose), a dietary pattern built on whole and minimally processed foods with less refined sugar and starch, maintaining a healthy amount of body fat, adequate sleep, and not smoking. None of these is a quick fix, and none replaces medical care for a diagnosed condition — but together they are associated with a steadier, lower glucose trajectory, and with it a slower pace of the glycation chemistry described above.
The bottom line
HbA1c turns the chaos of daily blood sugar into a single, slow-moving number — your average glucose exposure over about three months. That makes it one of the best routine windows onto a process, glycation, that quietly stiffens tissues and stokes inflammation as we age. The risk it tracks is graded and begins below the diabetes line, so the marker earns its place in a longevity picture, not just a diabetes one. Read it as a trend rather than a verdict, know that the at-risk cut-offs differ between the UK and US, interpret any single value in context, and treat an upward drift as an early, modifiable signal — because the trial evidence says the trajectory really can be changed.
Sources
- Semba RD, Nicklett EJ, Ferrucci L. Does accumulation of advanced glycation end products contribute to the aging phenotype? The Journals of Gerontology: Series A. 2010;65(9):963–975. PMID 20478906.
- Selvin E, Steffes MW, Zhu H, et al. Glycated hemoglobin, diabetes, and cardiovascular risk in nondiabetic adults. New England Journal of Medicine. 2010;362(9):800–811. PMID 20200384.
- Khaw KT, Wareham N, Bingham S, Luben R, Welch A, Day N. Association of hemoglobin A1c with cardiovascular disease and mortality in adults: the European prospective investigation into cancer in Norfolk. Annals of Internal Medicine. 2004;141(6):413–420. PMID 15381514.
- Sarwar N, Gao P, Seshasai SR, et al. (Emerging Risk Factors Collaboration). Diabetes mellitus, fasting blood glucose concentration, and risk of vascular disease: a collaborative meta-analysis of 102 prospective studies. The Lancet. 2010;375(9733):2215–2222. PMID 20609967.
- International Expert Committee. International Expert Committee report on the role of the A1C assay in the diagnosis of diabetes. Diabetes Care. 2009;32(7):1327–1334. PMID 19502545.
- Knowler WC, Barrett-Connor E, Fowler SE, et al. (Diabetes Prevention Program Research Group). Reduction in the incidence of type 2 diabetes with lifestyle intervention or metformin. New England Journal of Medicine. 2002;346(6):393–403. PMID 11832527.
