What Is HbA1c and Why Does It Matter for Diabetes Risk?

Your fasting glucose came back at 5.4 mmol/L — perfectly normal. So your blood sugar is fine, right? Not necessarily. A single fasting glucose measurement is a snapshot of one moment in time. It tells you nothing about what your blood sugar has been doing for the other 23 hours and 59 minutes of every day for the past three months. That's where HbA1c comes in — and it's a far more revealing test.

What Is HbA1c?

HbA1c stands for glycated haemoglobin. Haemoglobin is the protein inside your red blood cells that carries oxygen. When glucose circulates in your blood, some of it naturally attaches to haemoglobin — a process called glycation. The higher your average blood sugar over time, the more glucose gets stuck to your haemoglobin.

Because red blood cells live for approximately 120 days (about three months), measuring the percentage of haemoglobin that has glucose attached to it gives you a reliable picture of your average blood sugar over the preceding 2–3 months.

This is what makes HbA1c so powerful. It can't be fooled by a single good or bad day. It can't be manipulated by fasting before a test. It represents the cumulative truth of your blood sugar control.

In the UK, HbA1c is reported in mmol/mol (the international standard), though you may also see it expressed as a percentage. Both scales are used, but mmol/mol is standard in NHS practice.

How HbA1c Differs from Fasting Glucose

Fasting glucose measures the concentration of glucose in your blood after an overnight fast (usually 8–12 hours). It's useful but has significant limitations:

• It only reflects your blood sugar at that precise moment
• It can be affected by stress, illness, a poor night's sleep, or what you ate the day before
• It misses post-meal glucose spikes, which may be one of the earliest signs of metabolic dysfunction
• It can remain normal until diabetes is already well established

HbA1c captures the average exposure of your red blood cells to glucose over months. It detects pre-diabetes earlier, isn't affected by short-term fluctuations, and doesn't require fasting — making it more convenient and more reliable.

Practical takeaway: A normal fasting glucose with an elevated HbA1c is a red flag. It suggests your blood sugar is spiking after meals but recovering before the next morning's fasting test — a pattern you'd never catch without HbA1c.

HbA1c Ranges: What the Numbers Mean

Classification	HbA1c (mmol/mol)	HbA1c (%)	What It Means
Normal	Below 42	Below 6.0%	Blood sugar control is healthy
Pre-diabetes	42–47	6.0–6.4%	Increased risk of developing type 2 diabetes
Diabetes	48 or above	6.5% or above	Diagnostic threshold for type 2 diabetes

What pre-diabetes means

An HbA1c of 42–47 mmol/mol places you in the pre-diabetic range, sometimes called "non-diabetic hyperglycaemia." This is not a benign finding. According to Diabetes UK, approximately 5–10% of people with pre-diabetes progress to type 2 diabetes each year if no changes are made.

However, pre-diabetes is also the stage where intervention is most effective. Research consistently shows that lifestyle changes at this point can reduce the risk of progression by up to 58% — more effective than medication.

Optimal vs merely "normal"

While any HbA1c below 42 mmol/mol is technically normal, a growing body of evidence suggests that lower within the normal range is better. An HbA1c of 39–41 mmol/mol — though "normal" — already indicates higher average blood glucose than someone sitting at 30–35 mmol/mol. Many longevity-focused clinicians aim for an optimal HbA1c below 36 mmol/mol.

Why HbA1c Matters

Type 2 diabetes doesn't appear overnight. It develops gradually over years, progressing through a predictable sequence: normal blood sugar → insulin resistance → pre-diabetes → type 2 diabetes. By the time you receive a diabetes diagnosis, the metabolic dysfunction has typically been building for a decade or more.

HbA1c catches this progression earlier than fasting glucose alone. More importantly, it quantifies the degree of risk, allowing you and your doctor to intervene at precisely the right moment — ideally during the pre-diabetic window, when the condition is still fully reversible.

The stakes are high. Type 2 diabetes increases cardiovascular risk two to fourfold and raises the risk of kidney disease, nerve damage, and eye damage. It reduces life expectancy by an average of 6–7 years if poorly controlled. According to the NHS, 4.3 million people in the UK have a diabetes diagnosis, with an estimated 850,000 more undiagnosed.

Risk Factors for Type 2 Diabetes

You should be especially vigilant about monitoring HbA1c if you have one or more of the following:

• Family history — a parent or sibling with type 2 diabetes roughly doubles your risk
• Excess weight — particularly visceral fat. A waist circumference above 94 cm (men) or 80 cm (women) is significant
• Sedentary lifestyle — physical inactivity impairs insulin sensitivity
• Age over 40 (or over 25 for South Asian, African-Caribbean, or Black African heritage)
• Ethnicity — South Asian and Black populations have two to four times higher risk
• History of gestational diabetes — 50% lifetime risk of developing type 2 diabetes
• Polycystic ovary syndrome (PCOS) — closely linked to insulin resistance
• High blood pressure or abnormal cholesterol — both cluster with metabolic dysfunction

Practical takeaway: If you have two or more risk factors, annual HbA1c monitoring is a sensible investment in your long-term health — regardless of whether your fasting glucose looks normal.

How to Lower Your HbA1c

The good news is that HbA1c is highly responsive to lifestyle changes, particularly in the pre-diabetic and early diabetic range.

Dietary strategies

• Reduce refined carbohydrates and added sugars — white bread, pasta, rice, sugary drinks, and processed foods cause the sharpest glucose spikes
• Increase fibre intake — vegetables, legumes, whole grains, and nuts slow glucose absorption and improve insulin sensitivity
• Prioritise protein and healthy fats at every meal — these blunt post-meal glucose spikes
• Consider meal timing — eating your largest carbohydrate portion earlier in the day, when insulin sensitivity is highest, may improve HbA1c
• Limit alcohol — particularly sugary cocktails, beer, and sweet wines; alcohol also impairs liver glucose regulation

Physical activity

• Aerobic exercise (brisk walking, cycling, swimming) — 150 minutes per week reduces HbA1c by approximately 0.5–0.7 percentage points
• Resistance training (weights, bodyweight exercises) — builds muscle mass, which acts as a glucose sink, improving insulin sensitivity
• Post-meal walks — even 10–15 minutes of walking after a main meal significantly reduces post-prandial glucose spikes
• Reduce sitting time — breaking up prolonged sedentary periods improves glucose metabolism

Weight management

Losing 5–7% of body weight (roughly 5–7 kg for someone weighing 100 kg) is one of the single most effective interventions for lowering HbA1c and reversing pre-diabetes. The NHS Diabetes Prevention Programme is built on this evidence.

Sleep and stress

Poor sleep and chronic stress both impair insulin sensitivity. Aim for 7–9 hours of quality sleep and consider stress management strategies such as regular exercise, mindfulness, or simply reducing overcommitment.

When to Get Tested

The NHS doesn't routinely test HbA1c in the absence of symptoms or risk factors. The NHS Health Check for adults aged 40–74 may include a diabetes risk assessment, but this typically uses a questionnaire rather than a blood test.

Consider proactive HbA1c testing if you:

• Have any of the risk factors listed above
• Experience symptoms of high blood sugar — excessive thirst, frequent urination, unexplained weight loss, fatigue, blurred vision
• Want a baseline measurement for future comparison (this is especially valuable in your 30s and 40s)
• Have had a previous borderline result and want to track whether it's improving or worsening
• Are making lifestyle changes and want to measure their impact objectively

HbA1c testing doesn't require fasting, can be done at any time of day, and provides a far more comprehensive view of your metabolic health than fasting glucose alone. It's one of the most cost-effective, evidence-based screening tools available — and catching pre-diabetes early could be the single most impactful thing you do for your long-term health.

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