High HbA1c on keto is an unexpected finding that concerns many people following a ketogenic diet for weight management or blood sugar control. A ketogenic diet — typically under 50 g of carbohydrate per day — can trigger physiological changes that influence HbA1c readings, the NHS's primary tool for diagnosing and monitoring diabetes. Whether the rise reflects a genuine metabolic concern, a dietary adaptation, or an unrelated haematological condition, it warrants proper clinical assessment. This article explains the possible causes, what UK guidance says, when to seek medical advice, and how to interpret your results alongside other glucose markers.

Why HbA1c May Rise on a Ketogenic Diet

HbA1c may rise on a ketogenic diet due to proposed metabolic adaptations such as adaptive glucose sparing, individual variation in glucose metabolism, or unrelated haematological conditions that affect HbA1c reliability.

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A ketogenic diet — characterised by very low carbohydrate intake (typically under 50 g per day), moderate protein, and high fat — is used by some people for weight management and blood sugar control. However, some individuals following this dietary pattern report an unexpected rise in their HbA1c, which can understandably cause concern.

HbA1c measures the percentage of haemoglobin molecules that have glucose attached to them, reflecting average blood glucose levels over approximately two to three months. It remains the standard NHS test for diagnosing and monitoring diabetes, as set out in NICE guideline NG28. On a ketogenic diet, several physiological changes may influence this reading, though the evidence base for some proposed mechanisms is still limited and not all explanations are well established.

Factors that may contribute to a raised HbA1c on a ketogenic diet include:

• A metabolic adaptation sometimes described as 'adaptive glucose sparing', in which muscle cells preferentially use fat and ketones for fuel, potentially leaving glucose in the bloodstream for slightly longer (discussed further below)

• Individual variation in glucose metabolism, meaning responses to very low-carbohydrate eating differ significantly between people

It is also important to be aware that HbA1c can be unreliable or misleading in certain medical conditions unrelated to diet, including:

• Iron, vitamin B12, or folate deficiency anaemia

• Haemoglobinopathies (such as sickle cell trait or thalassaemia)

• Chronic kidney disease (CKD)

• Haemolytic conditions or recent blood transfusion

If any of these conditions may apply to you, discuss this with your GP, as an alternative marker such as fructosamine or glycated albumin may be more appropriate — your GP or specialist can advise.

Any rise in HbA1c warrants clinical review. Understanding the context behind the number is essential before drawing conclusions about metabolic health.

Understanding HbA1c Readings in the Context of Low-Carb Eating

An HbA1c of 48 mmol/mol or above on two occasions is diagnostic of type 2 diabetes per NICE NG28; on a ketogenic diet, apparent discordance with glucose monitoring data exists but evidence is limited and should not replace proper clinical assessment.

HbA1c is the primary diagnostic and monitoring tool used by the NHS for diabetes. According to NICE guideline NG28, a result of 48 mmol/mol or above on two occasions is diagnostic of type 2 diabetes, whilst 42–47 mmol/mol indicates a high risk of developing diabetes (sometimes called prediabetes). The NHS provides patient-facing information on HbA1c testing and what results mean.

In standard dietary conditions, HbA1c reliably correlates with average blood glucose. In the context of a ketogenic or very low-carbohydrate diet, some clinicians and researchers have observed that fasting glucose and HbA1c can appear elevated even when glucose monitoring data suggests predominantly normal levels throughout the day. However, the evidence for this discordance is limited and largely observational; it should not be used to dismiss a raised HbA1c without proper assessment.

It is important to understand what HbA1c does and does not measure:

• It does not directly capture glucose variability or post-meal spikes

• It does not account for changes in red blood cell lifespan or haematological conditions (see above)

• It does provide a clinically validated long-term average that remains the standard NHS monitoring tool

HbA1c should be interpreted in the context of the individual's clinical picture. If there is a genuine reason to suspect HbA1c is unreliable — for example, due to a haematological condition — a GP or diabetes team can arrange alternative testing. Healthcare professionals are best placed to decide whether additional investigations are warranted alongside HbA1c, rather than replacing it.

Physiological Insulin Resistance and Its Effect on Blood Sugar Markers

Adaptive glucose sparing — a proposed mechanism where muscle cells preferentially use fat and ketones — may modestly raise fasting glucose and HbA1c on keto, but this remains a hypothesis and should not be assumed benign without clinical evaluation.

One explanation sometimes proposed for a raised HbA1c on a ketogenic diet is a metabolic adaptation variously described as 'adaptive glucose sparing' or 'physiological insulin resistance'. This is not a term used in NICE or MHRA guidance, and the evidence for its clinical significance remains mixed. It is therefore important to treat this as a hypothesis rather than an established fact.

The proposed mechanism is that, after an extended period of very low carbohydrate intake, muscle cells preferentially use fatty acids and ketones for energy, reducing their uptake of glucose. As a result, glucose may remain in the bloodstream for slightly longer, potentially contributing to modestly raised fasting glucose and HbA1c. Some researchers distinguish this from pathological insulin resistance, which is associated with:

• Elevated fasting insulin levels

• High triglycerides and low HDL cholesterol

• Central adiposity and features of metabolic syndrome

• Progressive worsening of glucose tolerance

However, it is important to note that ketogenic diets can also raise LDL cholesterol in some individuals, which has implications for cardiovascular risk. Lipid monitoring — including total cholesterol, LDL, HDL, and triglycerides — is recommended as part of routine diabetes and cardiovascular risk management per NICE NG28.

Some clinicians use fasting insulin measurement to help contextualise a raised HbA1c, but this is not a routine NHS primary care test and is not recommended by NICE for standard clinical decision-making. Reference ranges vary between laboratories, and interpretation requires specialist input. If you are concerned about your results, your GP or diabetes team is best placed to advise on whether additional investigations are appropriate.

A modestly raised HbA1c on a ketogenic diet should always be assessed in full clinical context — it should not be assumed to be benign without proper evaluation.

When to Seek Advice from Your GP or Diabetes Team

Seek GP advice promptly if HbA1c rises above 48 mmol/mol, if you take insulin or SGLT2 inhibitors, or if you experience symptoms of hyperglycaemia or DKA — call 999 immediately for severe DKA symptoms.

Whilst some degree of HbA1c elevation on a ketogenic diet may reflect metabolic adaptation, there are circumstances where professional medical advice should be sought promptly. Always discuss unexpected changes in blood glucose markers with your GP or diabetes care team, particularly if you have a pre-existing diagnosis of type 1 or type 2 diabetes, prediabetes, or are taking glucose-lowering medicines.

Contact your GP or diabetes team if:

• Your HbA1c rises above 48 mmol/mol (6.5%) for the first time

• You have a known diagnosis of type 2 diabetes and your HbA1c is rising despite dietary changes

• You experience symptoms such as excessive thirst, frequent urination, unexplained weight loss, or persistent fatigue

• Your fasting glucose is consistently above 7.0 mmol/L

• You are taking medicines such as metformin, SGLT2 inhibitors, or insulin — a ketogenic diet can significantly alter medication requirements and must not be started without clinical supervision in these circumstances

Important safety information — SGLT2 inhibitors and DKA:

SGLT2 inhibitors (such as dapagliflozin and empagliflozin) carry a known risk of diabetic ketoacidosis (DKA), including euglycaemic DKA (where blood glucose may appear near-normal). This risk is increased by very low-carbohydrate or ketogenic diets. The MHRA has issued Drug Safety Updates on this risk. Do not start a ketogenic diet if you are taking an SGLT2 inhibitor without first discussing this with your prescriber. During acute illness, surgery, or prolonged fasting, SGLT2 inhibitors should be temporarily stopped in line with sick-day guidance — your GP or diabetes team can advise.

Do not stop insulin without medical advice — this can be dangerous. If you are using insulin and wish to change your diet significantly, seek guidance from your diabetes team first.

Symptoms of DKA — seek urgent help:

DKA is a medical emergency. Symptoms include nausea or vomiting, abdominal pain, rapid or laboured breathing, drowsiness or confusion, and a fruity or acetone smell on the breath. If you experience these symptoms:

• Call 999 or go to A&E immediately if symptoms are severe

• Call NHS 111 or contact your GP or diabetes team the same day if you are concerned but symptoms are mild

• Check blood ketones if you have a meter — a reading above 3.0 mmol/L requires urgent medical attention

You can report suspected side effects from any medicine, including SGLT2 inhibitors or insulin, to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk.

Even for those without a diabetes diagnosis, a rising HbA1c warrants investigation to rule out underlying conditions and to ensure the dietary approach is genuinely beneficial for long-term health.

Interpreting HbA1c Results Alongside Other Glucose Measures

A comprehensive assessment may include fasting glucose, lipid profile, and — where clinically indicated — CGM or alternative glycaemic markers such as fructosamine, all guided by your GP or diabetes team.

Given the limitations of HbA1c as a standalone marker in certain clinical contexts, a more comprehensive assessment of glucose metabolism may sometimes be appropriate. This should be guided by your GP or diabetes team, who can determine which investigations are clinically indicated.

Investigations that may be considered, depending on clinical context:

• Fasting glucose: Assessed alongside dietary pattern and time since last meal; a fasting glucose of 7.0 mmol/L or above on two occasions is diagnostic of diabetes per NICE NG28

• Lipid profile: Monitoring of total cholesterol, LDL, HDL, and triglycerides is recommended as part of cardiovascular risk management; note that LDL cholesterol may rise on a ketogenic diet in some individuals

• Continuous glucose monitoring (CGM) or flash glucose monitoring: These devices can provide additional information about glucose patterns throughout the day. Access to CGM or flash monitoring on the NHS is subject to NICE eligibility criteria (see NICE NG17 for type 1 diabetes; NICE NG28 for type 2 diabetes) and is not routinely available to all patients

• Fasting insulin and HOMA-IR (Homeostatic Model Assessment of Insulin Resistance): These are not routine NHS primary care tests and are not recommended by NICE for standard clinical decision-making. If a specialist considers them appropriate, interpretation should be done in the context of local laboratory reference ranges and the full clinical picture

• C-peptide: Useful in distinguishing between type 1 and type 2 diabetes if there is diagnostic uncertainty, and can be arranged by your GP or diabetes team

• Alternative glycaemic markers: When HbA1c is known to be unreliable (for example, due to haemoglobinopathy, CKD, or haemolytic anaemia), fructosamine or glycated albumin may be considered as alternatives, on the advice of your GP, diabetes team, or local laboratory

Interpreting these results together allows for a more complete clinical picture. Context is always important — the same HbA1c value may carry very different implications depending on the individual's overall metabolic profile, medical history, and medicines.

NHS and NICE Guidance on Low-Carb Diets and Long-Term Metabolic Health

NICE NG28 supports individualised dietary advice, including low-carbohydrate approaches, for type 2 diabetes, but recommends professional supervision, regular monitoring of HbA1c and lipids, and medication review if dietary changes alter glucose control.

NICE guideline NG28 (Type 2 Diabetes in Adults: Management) recognises that a range of dietary approaches, including low-carbohydrate diets, may be appropriate for some individuals with type 2 diabetes, and emphasises the importance of individualised dietary advice delivered by a qualified healthcare professional or registered dietitian. Diabetes UK's position statement on low-carbohydrate diets similarly supports an individualised approach, noting that low-carbohydrate eating may benefit some people with diabetes but is not suitable for everyone.

It is important to distinguish between a low-carbohydrate diet (typically under 130 g carbohydrate per day) and a ketogenic diet (under 50 g per day). NICE guidance supports individualised, professionally supervised dietary advice rather than prescribing a single dietary pattern. The long-term evidence base for ketogenic dieting beyond one to two years remains limited, and ongoing monitoring is important.

NICE also recommends structured self-management education for people with diabetes — programmes such as DESMOND (for type 2 diabetes) and DAFNE (for type 1 diabetes) can support informed dietary decision-making alongside clinical care.

For long-term metabolic health, the following principles are broadly supported by NHS and NICE guidance:

• Regular monitoring of HbA1c, lipids (including LDL cholesterol), renal function, and blood pressure, at intervals recommended by your care team

• Dietary review at least annually with a healthcare professional or registered dietitian

• Medication review if dietary changes significantly alter glucose or blood pressure control — do not adjust doses without clinical advice

• Avoiding unsupervised dietary extremes, particularly in those with existing health conditions or those taking insulin or SGLT2 inhibitors

A raised HbA1c on a ketogenic diet should prompt a thoughtful, evidence-informed conversation between patient and clinician. With appropriate investigations and professional support, it is possible to determine whether the reading reflects a genuine metabolic concern or a well-understood adaptation — and to make safe, informed decisions about dietary management going forward.

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