HbA1c Above or Below Normal: Causes in Athletes and What to Do
Glycated haemoglobin, better known as HbA1c, is one of the most widely ordered blood tests in the UK. It is the standard way to diagnose and monitor type 2 diabetes, and many athletes now see it in private "performance panels". Yet in people who train hard, the number can be misleading in both directions. Our editorial team explains what HbA1c measures, why athletes may sit outside the usual range, and what sensible next steps look like.
What HbA1c measures
Haemoglobin, the oxygen-carrying protein inside red blood cells, slowly and irreversibly binds glucose from the blood. The higher the average blood glucose, the greater the proportion of haemoglobin that becomes glycated. Laboratories report this proportion as HbA1c.
Because red blood cells typically live for around 120 days, HbA1c reflects average glucose over the previous two to three months, with recent weeks contributing most. This is why it is so useful clinically: a single fasting glucose reading reflects one moment, while HbA1c integrates many weeks of fluctuations.
That strength is also its weakness. HbA1c depends not only on glucose but on how long red cells survive. Anything that shortens or lengthens red cell lifespan, or changes the age profile of circulating cells, can shift HbA1c independently of glucose. Athletes, with their high red cell turnover, are a classic example.
Researchers have mapped HbA1c to an "estimated average glucose", most notably in the ADAG study led by Nathan and colleagues. The relationship is strong at population level but shows considerable individual scatter, which is another reason not to over-interpret small differences in one person.
Reference ranges in the UK
In the UK, HbA1c is reported in millimoles per mole (mmol/mol), following the IFCC standard, although some reports still show the older percentage (DCCT) units. Thresholds used by the NHS are based on World Health Organization recommendations.
| HbA1c (mmol/mol) | Approximate % | Usual interpretation |
|---|---|---|
| Below 42 | Below 6.0 | Normal range |
| 42–47 | 6.0–6.4 | Increased risk (non-diabetic hyperglycaemia, "prediabetes") |
| 48 or above | 6.5 or above | Consistent with diabetes; confirmatory test needed if no symptoms |
| Unusually low (e.g. below about 20) | Below about 4 | Consider red cell or haemoglobin factors |
A diagnosis of diabetes is never made on one borderline result in someone without symptoms. NHS practice is to repeat the test, and in some situations to use glucose-based tests instead of, or alongside, HbA1c.
The WHO guidance on HbA1c specifically lists conditions in which the test is unreliable, including haemoglobin variants, anaemias, recent blood loss or transfusion, and pregnancy. Several of these are directly relevant to athletes.
Why HbA1c may be low in athletes
A lower-than-expected HbA1c in an endurance athlete is common and usually reflects red cell biology rather than unusually good glucose control. When red cells are destroyed and replaced faster than normal, the circulating population is younger and has had less time to accumulate glucose.
- Exercise-induced haemolysis: repeated foot strike in running, as shown by Telford and colleagues, and intense muscle contraction can break down red cells.
- Increased erythropoiesis: altitude training camps and recovery from blood loss produce a surge of young red cells.
- Recent blood donation or bleeding: the body replaces lost cells, temporarily lowering HbA1c.
- Iron therapy: correcting iron deficiency stimulates new red cell production.
- Haemoglobin variants: some inherited variants interfere with certain laboratory methods.
Genuine low average glucose is also possible, for instance in athletes with low energy availability or those following very low-carbohydrate diets, and recurrent hypoglycaemia deserves attention in its own right. An unexpectedly low HbA1c in anyone using insulin or other glucose-lowering drugs without medical supervision is a serious warning sign: insulin misuse can cause severe hypoglycaemia, seizures and death.
Why HbA1c may be high
The first possibility to take seriously is real hyperglycaemia. Being fit does not make anyone immune to type 2 diabetes, especially with a strong family history, South Asian or African-Caribbean heritage, or a history of gestational diabetes. Type 1 diabetes can also appear in adulthood and occasionally presents in athletes as unexplained weight loss and fatigue.
Iron deficiency, very common in female and endurance athletes, can falsely raise HbA1c. A systematic review by English and colleagues (2015) found that iron deficiency anaemia was associated with higher HbA1c values, likely because red cell production slows and the circulating cells are older. Deficiencies of vitamin B12 or folate may act similarly.
Certain substances raise blood glucose. Systemic glucocorticoids, sometimes prescribed for asthma flares or injuries, can elevate glucose for days or weeks. Growth hormone misuse and some anabolic agents are associated with insulin resistance, and dietary supplements adulterated with undeclared drugs have been documented.
Other non-glycaemic factors include kidney disease, removal of the spleen, some haemoglobin variants and high-dose vitamin C or aspirin affecting certain assays. Radin's review of HbA1c pitfalls gives a detailed overview of these interferences.
Lastly, laboratory variation exists. Results from different analysers can differ slightly, so comparing a private sports panel with an NHS result requires caution.
What to do with an unexpected result
An out-of-range HbA1c is a reason for a conversation with your GP, not for self-treatment. Practical steps usually include:
- Repeat the test, ideally in an accredited laboratory, rather than reacting to one value.
- Request a full blood count, ferritin, B12, folate and kidney function to identify red cell or iron issues.
- Add a fasting plasma glucose or an oral glucose tolerance test if HbA1c may be unreliable.
- Consider a reticulocyte count where haemolysis or high red cell turnover is suspected.
- Share your training load, recent altitude exposure, blood donation and all supplements or medicines with the clinician.
Continuous glucose monitors (CGMs) are increasingly marketed to athletes. They can show how training and meals affect glucose, but the healthy-population data needed to interpret them are still developing. They are not a substitute for a medical assessment of a high HbA1c.
If dysglycaemia is confirmed, sport remains strongly beneficial. Many athletes with diabetes compete at the highest level with good management and specialist input, including advice on carbohydrate periodisation and hypoglycaemia prevention around training.
Conclusions of the editorial team
HbA1c is an excellent test for most people, but in athletes it depends heavily on red cell turnover. Low values are often explained by haemolysis or increased red cell production, while iron deficiency can push results upward.
A high result should never be dismissed as an "athlete artefact" without checking. Genuine diabetes and prediabetes do occur in active people, and glucose-based tests can clarify the picture.
Interpreting HbA1c alongside a full blood count, iron studies and context about training is the sensible approach.
Our editorial team also recommends our articles on ferritin and iron deficiency in athletes, interpreting a full blood count, and low energy availability in sport.
References
- World Health Organization. Use of glycated haemoglobin (HbA1c) in the diagnosis of diabetes mellitus: abbreviated report of a WHO consultation. Geneva: WHO; 2011.
- Nathan DM, Kuenen J, Borg R, et al. Translating the A1C assay into estimated average glucose values. Diabetes Care. 2008;31(8):1473–1478.
- English E, Idris I, Smith G, et al. The effect of anaemia and abnormalities of erythrocyte indices on HbA1c analysis: a systematic review. Diabetologia. 2015;58(7):1409–1421.
- Radin MS. Pitfalls in hemoglobin A1c measurement: when results may be misleading. J Gen Intern Med. 2014;29(2):388–394.
- Telford RD, Sly GJ, Hahn AG, et al. Footstrike is the major cause of hemolysis during running. J Appl Physiol. 2003;94(1):38–42.
