As an endurance athlete you have abnormal blood values more often than the average person, and that is down to your sport. A lot of training volume costs iron, loads your recovery and changes markers such as ferritin, CRP and cortisol. Most of those shifts come with hard training, but a few you really want to catch in time.
In our experience, runners and cyclists only go looking for answers once their legs have felt heavy for weeks. That is late, because by then the simplest clue is already in your blood.
Why endurance athletes have different blood values
Endurance training uses up iron and triggers mild inflammation, which colours your recovery markers. Through footstrike, sweat and impact you lose iron, while inflammation blunts its absorption. As a result ferritin often drops quietly, before you notice anything (Sim, 2019).
Female and young endurance athletes carry the most risk. In them, low iron is one of the most commonly missed causes of unexplained fatigue.
Which blood values to check as an endurance athlete
For recovery and performance it comes down to iron status, inflammation and your stress-recovery balance. Together these markers tell you whether your sport is building you up or slowly draining you.
| Marker | What it tells you |
|---|---|
| Ferritin | Your iron stores, the earliest clue to a deficiency |
| Transferrin saturation | How much iron is available for transport |
| CRP and hs-CRP | Inflammation and recovery load |
| Cortisol | Chronic stress and possible overload |
| Magnesium | Muscle function, cramp and sleep |
Iron: the best known pitfall for endurance athletes
In endurance athletes, iron deficiency is the most common nutrition-related cause of declining performance. You need iron for oxygen transport in your blood, so a deficiency hits your endurance directly. Ferritin often falls before your haemoglobin does.
You read the full explanation in iron deficiency in athletes.
Overtraining and recovery
Too little recovery leaves traces in your blood, though no single test proves overtraining. The international consensus describes overtraining as a combination of symptoms and markers, not as a stand-alone value (Meeusen, 2013). Cortisol, CRP and a persistent heavy feeling all belong to the picture.
How to recognise this, you read in recognising overtraining through your blood values. Magnesium plays a role in muscle function and recovery (Zhang, 2017); see magnesium for athletes.
How to test your blood values as an endurance athlete
Test on a calm day and not right after your hardest training, because exertion and mild inflammation skew your picture temporarily (Peeling, 2008). Plan your measurement in a rest week for the fairest result.
To track your iron, inflammation and recovery on purpose, compose your own panel with the markers that fit your sport. You do that through our custom blood test. If you also train with weights, read blood values for strength athletes.
My advice: treat your blood values like training data. Measure at rest, follow the trend across seasons, and discuss a persistent deviation or symptoms with your GP.
References
- Sim M, Garvican-Lewis LA, Cox GR, et al. Iron considerations for the athlete: a narrative review. European Journal of Applied Physiology. 2019. PMID: 31055680.
- Meeusen R, Duclos M, Foster C, et al. Prevention, diagnosis, and treatment of the overtraining syndrome: joint consensus statement of the European College of Sport Science and the American College of Sports Medicine. Medicine and Science in Sports and Exercise. 2013. PMID: 23247672.
- Zhang Y, Xun P, Wang R, et al. Can magnesium enhance exercise performance? Nutrients. 2017. PMID: 28846654.
- Peeling P, Dawson B, Goodman C, et al. Athletic induced iron deficiency: new insights into the role of inflammation, cytokines and hormones. European Journal of Applied Physiology. 2008. PMID: 18365240.
Disclaimer
Every blood test result includes a professional assessment by a BIG-registered doctor. This article gives general information and is not a substitute for medical advice. A blood test is a tool to walk into the conversation with your GP better informed, not a diagnosis in itself. For treatment decisions, discuss your results with your GP.
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