Glycated haemoglobin, HbA1c, measures the percentage of haemoglobin molecules in red blood cells that have had glucose bound to them. Since red blood cells have a lifespan of approximately 90 days, HbA1c reflects the average blood glucose concentration over the preceding 2 to 3 months, providing a far more clinically useful picture of glycaemic control than a single fasting glucose measurement taken on a single morning. It is the primary diagnostic test for type 2 diabetes and the standard monitoring tool for glycaemic management.
The Diagnostic Ranges and Their Limitations
The conventional clinical categories in the UK are: below 42 mmol/mol normal, 42 to 47 mmol/mol prediabetes, 48 mmol/mol and above type 2 diabetes. These thresholds were set primarily to identify populations at high risk of microvascular complications including nephropathy, retinopathy, and peripheral neuropathy. They were not set to optimise longevity in healthy adults, and the research on where cardiovascular risk begins to rise suggests that the curve is continuous, with risk rising even within the normal range.
What the Cardiovascular Evidence Shows
A large study published in the New England Journal of Medicine by Selvin et al. followed over 11,000 adults and found that HbA1c levels were significantly associated with cardiovascular events even in adults without diagnosed diabetes. Specifically, HbA1c above 35 mmol/mol showed progressively increasing cardiovascular risk. An HbA1c of 42 mmol/mol, the upper limit of the normal range, carries meaningfully higher cardiovascular risk than 35 mmol/mol, which would be considered excellent glycaemic health.
What Lowers HbA1c in Non-Diabetic Adults
- Reducing refined carbohydrate and ultra-processed food intake: the most direct intervention for lowering average blood glucose
- Aerobic exercise: improves insulin sensitivity and glucose clearance; each session reduces blood glucose for 24 to 48 hours
- Resistance training: improves GLUT-4 expression in muscle cells, increasing glucose uptake independently of insulin
- Weight loss: each kilogram of weight lost is associated with approximately 0.1 to 0.2 mmol/mol reduction in HbA1c
- Time-restricted eating: reducing the window of daily food intake reduces the number of daily glucose spikes
Sources
- Selvin, E., et al. (2010). Glycated hemoglobin, diabetes, and cardiovascular risk in nondiabetic adults. New England Journal of Medicine, 362(9), 800-811.
- Khaw, K.T., et al. (2004). Glycated haemoglobin, diabetes, and mortality in men in Norfolk cohort of European Prospective Investigation of Cancer and Nutrition (EPIC-Norfolk). BMJ, 322(7277), 15-18.
- Dabelea, D., et al. (2014). Prevalence of type 1 and type 2 diabetes among children and adolescents from 2001 to 2009. JAMA, 311(17), 1778-1786.