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Blood Markers

7 min read
1 May 2026

HbA1c: What the Normal Range Does Not Tell You

A result below 42 mmol/mol comes back marked 'normal' and the appointment moves on. The research tells a more precise story.
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Jon Bell

Metabolic Health Specialist · Leamington Spa

The HbA1c test measures the proportion of haemoglobin molecules in your blood that have glucose permanently attached to them. Because red blood cells live for approximately 90 days, the result reflects your average blood sugar over the preceding three months. A single number, representing a quarter of a year of metabolic function, delivered in a letter or a brief consultation.
For most GPs, the reference range ends the conversation. A result below 42 mmol/mol (6.0%) comes back marked ‘normal’ and the appointment moves on. What rarely gets discussed is that the risk curve does not begin at 42.

The Problem With 'Normal'

Population reference ranges are built to identify disease, not to define health. The prediabetes threshold of 42 mmol/mol was chosen because above that point, the incidence of type 2 diabetes begins to rise significantly. Below it, you are not labelled ‘at risk’. That does not mean the results are metabolically equivalent.

Prediabetes threshold

42 mmol/mol

The GP intervention point, where the conversation about diabetes prevention begins. Not the point at which cardiovascular risk begins to increase.

Research published in The Lancet and the New England Journal of Medicine has consistently shown that cardiovascular risk increases in a continuous, linear fashion from HbA1c levels as low as 34 to 35 mmol/mol. A result of 40 mmol/mol and a result of 30 mmol/mol are both reported as ‘normal’, but they are not the same thing. In a 2010 analysis of over 310,000 participants, adults with HbA1c in the 39 to 42 mmol/mol range had measurably higher all-cause mortality and cardiovascular event rates than those in the 31 to 35 mmol/mol range, despite both groups falling below the prediabetes threshold.

What Optimal Actually Looks Like

Optimal functional range

31–36 mmol/mol

Consistent with lowest cardiovascular and all-cause mortality risk across multiple large-scale population studies.

In clinical research, the lowest cardiovascular risk is consistently found in individuals with HbA1c between 31 and 36 mmol/mol (approximately 5.0 to 5.4%). This is the range that reflects efficient glucose metabolism, low chronic inflammation from glycation, and well-maintained insulin sensitivity. Jon Bell’s protocol uses 30 to 38 mmol/mol as its functional target. Not because 41 mmol/mol is dangerous in itself, but because arriving in the lower third of ‘normal’ means the entire metabolic system is working well: blood sugar is being managed efficiently between meals, insulin is not being chronically overproduced, and the low-grade glycation damage that accumulates silently over years is minimised.

The Levers That Move It

HbA1c does not move quickly. It is a lagging indicator, and the 90-day averaging means that meaningful dietary and lifestyle changes take 10 to 14 weeks to show fully in the result. This is one reason Jon’s programme runs for 12 weeks: it is specifically designed to produce a measurable, confirmed shift in the result within a single cycle.
  • Reducing refined carbohydrate load, particularly at breakfast and in the evening
  • Improving post-meal glucose clearance through strategic movement after eating
  • Addressing sleep quality, since poor sleep directly elevates fasting glucose via cortisol
  • Reducing visceral adiposity, which drives hepatic insulin resistance
  • Managing the chronic stress response, which maintains elevated glucose independently of diet

What Your Existing Result Already Tells You

If you have had a blood test in the last two years, you likely already have an HbA1c result. If it sits between 38 and 42 mmol/mol, you are in the zone that conventional medicine considers unremarkable but that Jon’s programme addresses directly. That four-millimole range represents a large cohort of adults whose metabolic trajectory is moving in the wrong direction, and who will not receive intervention until it crosses the threshold into prediabetes. By which point, years of opportunity have passed.

"The data you already have is not the end of the conversation. It is the starting point for a more precise one."

Frequently Asked

The Problem With 'Normal'
Population reference ranges are built to identify disease, not to define health. The prediabetes threshold of 42 mmol/mol was chosen because above that point, the incidence of type 2 diabetes begins to rise significantly. Below it, you are not labelled ‘at risk’. That does not mean the results are metabolically equivalent.
In clinical research, the lowest cardiovascular risk is consistently found in individuals with HbA1c between 31 and 36 mmol/mol (approximately 5.0 to 5.4%). This is the range that reflects efficient glucose metabolism, low chronic inflammation from glycation, and well-maintained insulin sensitivity. Jon Bell’s protocol uses 30 to 38 mmol/mol as its functional target. Not because 41 mmol/mol is dangerous in itself, but because arriving in the lower third of ‘normal’ means the entire metabolic system is working well: blood sugar is being managed efficiently between meals, insulin is not being chronically overproduced, and the low-grade glycation damage that accumulates silently over years is minimised.
HbA1c does not move quickly. It is a lagging indicator, and the 90-day averaging means that meaningful dietary and lifestyle changes take 10 to 14 weeks to show fully in the result. This is one reason Jon’s programme runs for 12 weeks: it is specifically designed to produce a measurable, confirmed shift in the result within a single cycle.
If you have had a blood test in the last two years, you likely already have an HbA1c result. If it sits between 38 and 42 mmol/mol, you are in the zone that conventional medicine considers unremarkable but that Jon’s programme addresses directly. That four-millimole range represents a large cohort of adults whose metabolic trajectory is moving in the wrong direction, and who will not receive intervention until it crosses the threshold into prediabetes. By which point, years of opportunity have passed.