Most runners don't think about blood tests. You train, you manage your diet, you deal with injuries as they come. Blood tests feel like something for people who are ill, or for elite athletes with sports medicine teams. But for recreational runners logging consistent mileage, targeted blood testing can reveal — and fix — problems that no amount of training will solve.
This isn't a comprehensive guide to every test your GP can order. It's a practical guide to the handful of markers that are specifically relevant to runners, commonly suboptimal in this population, and where knowing your number makes a meaningful practical difference.
Important caveat before we start: this article is for informational purposes. If you suspect a deficiency, or if you act on any of these markers, do so under the guidance of your GP or a qualified clinician. RunStrong is a daily maintenance supplement, not a clinical treatment.
Why Runners Need Different Reference Ranges
Standard NHS laboratory reference ranges are designed for the general population, not for people doing 40+ miles of training per week. A result that sits within the "normal" range for the average person may be functionally suboptimal for an endurance athlete whose physiological demands are significantly higher.
Ferritin is the clearest example — more on this below. Vitamin D is another. Understanding that "normal" for a sedentary adult and "optimal for performance" in a trained runner are not the same thing is the key context for this whole discussion.
Test 1: Iron Panel — Ferritin, Haemoglobin, Transferrin Saturation
Iron is the most critical blood marker for distance runners, and it's also the most frequently misunderstood. The standard NHS test for "anaemia" measures haemoglobin — the protein in red blood cells that carries oxygen. But for runners, haemoglobin alone tells you surprisingly little.
Why You Need Ferritin, Not Just Haemoglobin
Ferritin is the storage form of iron. It's a more sensitive early marker of iron status because stores deplete before circulating haemoglobin begins to fall. By the time your haemoglobin is low enough to flag clinical anaemia, your iron stores have typically been running on empty for weeks or months — during which time your aerobic capacity has been quietly declining.
Research makes this point clearly. A study by Hinton et al. (2000) in the Journal of Applied Physiology found that iron-depleted, non-anaemic women showed significant improvements in endurance following training when supplemented with iron — despite having haemoglobin levels that remained within the normal range throughout. The implication: ferritin depletion alone was impairing performance, independently of haemoglobin status.
A companion study by Brownlie et al. (2004) in the American Journal of Clinical Nutrition confirmed the finding: marginal iron deficiency without anaemia meaningfully impairs aerobic adaptation in untrained women, and supplementation restores it.
The Sports Anaemia Complication
Endurance training causes plasma volume to expand faster than red blood cell mass — which means haemoglobin concentrations can look low even in healthy, well-nourished athletes. This is called "sports anaemia" or dilutional pseudoanaemia. It's an adaptation, not a deficiency.
The practical consequence: a standard FBC (full blood count) in a trained runner can be misleading in both directions. Haemoglobin may appear low when iron status is fine (sports anaemia). Haemoglobin may appear normal while ferritin is depleted and performance is suffering. This is why asking specifically for ferritin alongside haemoglobin is essential — not just requesting a general "blood test" or "anaemia check".
What to Ask For
- Full blood count (FBC) — haemoglobin, MCV, red cell morphology
- Serum ferritin — iron stores (ask for this explicitly; GPs don't always include it routinely)
- Transferrin saturation — measures how much of your transport protein is carrying iron; useful if ferritin is borderline
What the Numbers Mean for Runners
Standard NHS ferritin reference ranges are typically 15–300 μg/L for women and 30–400 μg/L for men. These are population ranges — not performance targets for endurance athletes. Sports medicine research and practitioner consensus (Sim et al., 2019) generally applies a higher threshold for functional performance:
- Below 30 μg/L — suboptimal for aerobic performance; supplementation or dietary intervention warranted
- 30–40 μg/L — borderline; monitor closely, particularly if training volume is increasing
- 40–75 μg/L — range often recommended by sports medicine practitioners as functionally adequate for endurance athletes
- Above 75 μg/L — well-stocked; no intervention needed
If your ferritin is severely depleted (below 15–20 μg/L), see your GP for a therapeutic prescription dose. RunStrong contains 5mg of iron bisglycinate per serving — a daily maintenance dose designed to support healthy iron levels in runners, not to correct a clinical deficiency requiring medical-grade intervention. Why runners specifically lose iron →
Test 2: Vitamin D (25-hydroxyvitamin D)
Vitamin D testing matters for UK runners specifically because the UK's geography means we cannot synthesise adequate Vitamin D from sunlight between October and March. If you're training through autumn and winter — which most marathon-training runners are — your Vitamin D levels are likely declining through the same months when your training load is increasing.
Research by Close et al. (2013) in the British Journal of Sports Medicine specifically examined Vitamin D status in UK-based athletes and found widespread deficiency, recommending routine supplementation strategies for those training in northern latitudes. This applies directly to UK runners.
Vitamin D is relevant to runners not just for bone health and stress fracture prevention (Lappe et al., 2008 found that calcium and Vitamin D supplementation significantly reduced stress fracture incidence in Navy recruits under high physical training loads) but also for immune function and, as noted in Willis et al. (2012), its association with inflammatory biomarkers — lower Vitamin D correlates with higher systemic inflammation.
What to Ask For
- Serum 25-hydroxyvitamin D (25(OH)D) — the standard Vitamin D test
What the Numbers Mean
NHS/NICE classification:
- Below 25 nmol/L — deficient; supplementation required
- 25–50 nmol/L — insufficient; supplementation recommended
- 50–75 nmol/L — adequate for most purposes
- 75–200 nmol/L — optimal range; many sports medicine practitioners target this for athletes
The NHS recommends that all UK adults supplement Vitamin D (10μg/400 IU daily) from October to March as a baseline. Runners may benefit from supplementing year-round due to the combination of high physical demands and inconsistent outdoor exposure. RunStrong contains 10μg of vegan Vitamin D3 (from algae) per serving — the NHS daily reference intake. Vitamin D for runners →
Test 3: Thyroid Function (TSH)
Thyroid function often gets overlooked in runners because its symptoms — persistent fatigue, feeling cold, heavy legs, unexplained weight changes, poor recovery — are almost identical to the symptoms of overtraining or iron deficiency. Hypothyroidism (underactive thyroid) is relatively common, particularly in women, and is completely manageable with treatment — but only if you know it's there.
A single TSH (thyroid stimulating hormone) test is a reasonable screen. If TSH is elevated (suggesting the thyroid is struggling), your GP will follow up with T3/T4 tests to confirm the picture.
Thyroid testing isn't runner-specific in the way iron and Vitamin D are — but it deserves a mention because a runner who is genuinely hypothyroid will feel exactly like a runner with iron deficiency or overtraining syndrome, and no amount of supplementation or training adjustment will fix it. If you've addressed iron and Vitamin D and still feel inexplicably flat, thyroid is worth checking.
Test 4: B12 and Folate (If Plant-Based or High-Risk)
B12 and folate are essential for red blood cell production and neurological function. Deficiency causes a specific type of anaemia (megaloblastic anaemia) characterised by large, poorly-functioning red cells that cannot carry oxygen efficiently.
For most omnivorous runners eating a varied diet including meat, dairy, and eggs, B12 deficiency is uncommon and routine testing isn't generally warranted. The groups who should consider testing:
- Vegan and vegetarian runners — B12 is found almost exclusively in animal products. Without regular supplementation, depletion is almost inevitable over time and can take years to manifest clinically because stores are substantial.
- Runners over 50 — gastric acid production declines with age, reducing B12 absorption from food
- Runners on long-term metformin (diabetes medication) — metformin interferes with B12 absorption
- Anyone with unexplained fatigue or tingling in hands/feet — these can be early B12 deficiency symptoms
The ACSM/AND/DC joint position statement on nutrition and athletic performance (Thomas et al., 2016) recommends targeted rather than routine B12 testing — but specifically identifies plant-based athletes as a group where monitoring is appropriate.
Test 5: Magnesium (With Caveats)
Magnesium is involved in over 300 enzymatic reactions, including muscle contraction, energy production, and protein synthesis — all of relevance to runners. Exercise increases magnesium requirements and losses through sweat. In principle, testing makes sense.
In practice, there's a complication: standard serum magnesium tests are a poor indicator of body magnesium status. Only around 1% of total body magnesium circulates in the blood; the rest is stored in bone and muscle. Serum magnesium can appear normal even when tissue stores are depleted, and vice versa. Nielsen and Lukaski (2006) specifically highlight this limitation in a review of magnesium and exercise.
The practical implication: if you're experiencing muscle cramps, fatigue, and sleep problems, dietary magnesium intake is worth examining regardless of what a blood test shows. Good dietary sources include dark leafy greens, nuts, seeds, legumes, and wholegrains. Magnesium glycinate or bisglycinate are well-tolerated supplemental forms if needed.
A serum magnesium test is cheap and broadly available via your GP, so it's not unreasonable to include if you're already getting bloodwork done — just be aware it will only tell you if you're severely deficient, not whether tissue stores are at performance-optimal levels.
How to Get These Tests in the UK
Via Your GP
The most important test — ferritin — requires a specific request. Don't just ask for "a blood test" or an "anaemia check." Say explicitly: "I'd like a full blood count including ferritin and serum Vitamin D." GPs will often run haemoglobin alone unless prompted; ferritin and Vitamin D are separate requests in most NHS systems and may not be automatically included. UK GPs can order these routinely, though some may ask about symptoms or clinical justification.
Private Testing
If you'd prefer not to rely on a GP referral — or if you want a more comprehensive panel without a consultation — private home blood testing services are widely available in the UK. Companies like Medichecks and Thriva offer finger-prick tests posted to a lab, with results online within a few days. A basic runner's panel (ferritin, Vitamin D, FBC, thyroid) typically costs £40–70. These are convenient for runners who want baseline data before symptoms appear, or who want to monitor their numbers more frequently than GP visits allow.
When to Test
The most useful time to test is:
- Before a major training block — establishes a baseline before your iron and Vitamin D are further depleted by high mileage
- After a period of unexplained underperformance — if you've been sluggish, recovering poorly, or feeling flat for more than 2–3 weeks despite sensible training and sleep, it's worth investigating before changing your training plan
- In October — before UK Vitamin D levels start declining and as marathon season training begins
Annual testing is a sensible approach for any runner doing more than 25 miles per week.
The Bottom Line
For most runners, the three highest-yield tests are ferritin (not just haemoglobin), Vitamin D, and thyroid function. Ferritin depletion is common in high-mileage runners — particularly women — and can measurably impair VO2 max and running economy long before clinical anaemia develops. Vitamin D insufficiency affects the majority of UK adults in winter, with direct implications for bone health, immunity, and inflammation in runners. Thyroid problems mimic overtraining exactly and are only diagnosable through testing.
Getting tested takes 20 minutes and a GP appointment (or a home test kit). The information it gives you is the kind you can actually act on.
References
- Hinton PS, Giordano C, Brownlie T, Haas JD. (2000). Iron supplementation improves endurance after training in iron-depleted, nonanemic women. Journal of Applied Physiology, 88(3), 1103–1111. https://doi.org/10.1152/jappl.2000.88.3.1103
- Brownlie T, Utermohlen V, Hinton PS, Haas JD. (2004). Marginal iron deficiency without anemia impairs aerobic adaptation among previously untrained women. American Journal of Clinical Nutrition, 79(3), 437–443. https://doi.org/10.1093/ajcn/79.3.437
- Sim M, Garvican-Lewis LA, Cox GR, Govus A, McKay AK, Stellingwerff T, Peeling P. (2019). Iron considerations for the athlete: a narrative review. European Journal of Applied Physiology, 119(7), 1463–1478. https://doi.org/10.1007/s00421-019-04096-2
- Close GL, Leckey J, Patterson M, Bradley W, Owens DJ, Fraser WD, Morton JP. (2013). Strategies for the treatment and prevention of vitamin D deficiency in UK based athletes: time for a paradigm shift? British Journal of Sports Medicine, 47(7), 440–447. https://doi.org/10.1136/bjsports-2012-091735
- Lappe J, Cullen D, Haynatzki G, Recker R, Ahlf R, Thompson K. (2008). Calcium and vitamin D supplementation decreases incidence of stress fractures in female navy recruits. Journal of Bone and Mineral Research, 23(5), 741–749. https://doi.org/10.1359/jbmr.080102
- Willis KS, Smith DT, Broughton KS, Larson-Meyer DE. (2012). Vitamin D status and biomarkers of inflammation in runners. Open Access Journal of Sports Medicine, 3, 35–42. https://doi.org/10.2147/OAJSM.S31022
- Lippi G, Schena F, Franchini M. (2011). Prevalence, etiology and prevention of anemia in athletes. British Journal of Sports Medicine, 45(5), 416–421. https://doi.org/10.1136/bjsm.2009.063727
- Thomas DT, Erdman KA, Burke LM. (2016). Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. Journal of the Academy of Nutrition and Dietetics, 116(3), 501–528. https://doi.org/10.1016/j.jand.2015.12.006
- Nielsen FH, Lukaski HC. (2006). Update on the relationship between magnesium and exercise. Magnesium Research, 19(4), 180–189. PMID: 17172008.