Your GPS watch logged every kilometre. Your nutrition is dialled, macros tracked, meal timing optimised. You added a third session per week in January and a fourth by March. And yet your 5K time hasn't moved in six months. Your lifts have stalled. Your triathlon splits are identical to last season's.
This is the performance paradox: more input, no output. And the reason is often not in the training plan.
Training is a stimulus. Your body's ability to respond to that stimulus, to adapt, recover, and improve, depends on biological systems that operate entirely beneath the level of perception. You can feel tired. You cannot feel your ferritin dropping. You cannot feel your cortisol-to-testosterone ratio shifting catabolic. You cannot feel a vitamin D level that has slid below the range associated with good muscle function.
This article covers the five biomarker categories that most directly relate to athletic output in trained amateurs, and why the standard blood test your GP orders tells you almost nothing about any of them. In the second half, we look at what the research says specifically about endurance athletes: triathletes, Ironman competitors, and ultramarathon runners, whose training demands push these systems to extremes that recreational athletes rarely approach, but whose lessons apply to anyone training seriously.
The goal is not to add supplements to your stack. It is to understand which biological systems may be limiting you, and to measure them.
The Five Biomarker Categories That Directly Limit Athletic Output
Performance adaptation is not a single process. It is the product of several interdependent biological systems functioning in parallel. When any one of them is compromised, training adaptation can slow or stop, regardless of how well the others are functioning.
The five categories that matter most for trained amateur athletes are iron status, the cortisol and testosterone balance, vitamin D, magnesium, and the haematological markers.
None of these appear in full on a standard German großes Blutbild. None are tested in the Gesundheits-Check-up 35. And none can be meaningfully assessed by feel alone, which is precisely what makes them so easy to miss.
Ferritin: The Oxygen Bottleneck Nobody Talks About
Why haemoglobin alone misses the problem
Most athletes who get blood work done receive a haemoglobin reading and are told their iron is fine. This is a meaningful result, but it is not the whole result. Haemoglobin reflects clinical anaemia, the endpoint of iron depletion. Ferritin, by contrast, reflects iron stores, the reserve your body draws on before haemoglobin starts to fall.
The sequence matters: ferritin depletes first, while haemoglobin can remain normal. An athlete can be training with low iron stores and be told by a basic blood test that their iron is fine, because the wrong marker is being measured.
Ferritin is the marker that catches the problem before anaemia develops. And for athletes, the reference ranges used by most labs are not fit for purpose. Swiss sports medicine guidance states that in healthy male and female athletes over 15, ferritin values below 15 µg/L are equivalent to empty stores and values from 15 to 30 µg/L to low stores, so a cut-off of 30 µg/L is appropriate. It adds that in adult elite sport a ferritin value of 50 µg/L should be attained before altitude training, because iron demands in those situations are higher. [1]
What the evidence says about ferritin and aerobic capacity
A 2024 systematic review of 23 studies covering 669 high-level female athletes found that iron deficiency reduced endurance performance by 3 to 4 percent. Endurance performance improved by 2 to 20 percent when iron-deficient athletes were treated with supplemental elemental iron, and maximal aerobic capacity improved by 6 to 15 percent. The authors also noted that most included studies had fewer than 20 athletes per group, which limits how firmly the numbers can be read. [2]
The mechanism is direct: iron is a core component of haemoglobin and myoglobin, the proteins that carry and store oxygen in blood and muscle respectively. When iron stores fall, oxygen delivery to working muscle can be compromised. The athlete trains the same hours but gets less oxygen where it is needed.
Critically, iron supplementation only helps athletes who are actually low. The same Swiss guidance is explicit that long-term daily oral or intravenous iron in the presence of normal or high ferritin values does not make sense and may be harmful. [1] This is why testing before supplementing is not optional, it is the only way to know whether iron is the limiting variable.
What athletes should target, rather than the lab's default range
The gap between not clinically deficient and performing well is wide, and the 30 µg/L athlete cut-off sits well above the 12 to 15 µg/L lower limit printed on many European lab reports.
Why are athletes prone to iron depletion in the first place? Several mechanisms operate together. Foot strike haemolysis, the mechanical destruction of red blood cells from the repetitive impact of running, was tested directly in a 2003 study in which ten male triathletes ran and cycled for one hour each at the same oxygen uptake. Plasma free haemoglobin rose after both, but the increase was fourfold greater after running, and the authors concluded that footstrike is the major contributor to haemolysis during running. [3] Sweat and urinary losses add to the deficit. And the hormone hepcidin, which rises after intense exercise, blocks iron absorption from the gut, which means the harder you train, the less iron you absorb from food at precisely the time your body most needs it. [1] [4]
There is also a timing consideration that most athletes are unaware of: ferritin is an acute-phase protein and rises with inflammation, so it can read artificially high in the days after a hard session, an Ironman, or a marathon. Swiss guidance recommends excluding an acute phase reaction by history and by measuring C-reactive protein before interpreting a ferritin result. [1] Testing immediately post-race gives a misleading result. Test in a rested, recovered state, ideally at least a week after a hard race or training block.
If you have a tendency to plateau despite consistent training, if your RPE at a given pace or power has been rising, or if you experience fatigue that sleep does not resolve, ferritin is a sensible first place to look. Our article on iron and energy biomarkers covers the full diagnostic picture, including the distinction between serum iron, transferrin saturation, and ferritin, and what each tells you.
Aniva tests the complete iron panel as standard. The panel covers ferritin, serum iron, transferrin saturation, and haemoglobin. Because a single haemoglobin reading is not a performance baseline. See the full biomarker list →
The Cortisol/Testosterone Ratio: Your Internal Overtraining Signal
Anabolic vs. catabolic: what the ratio is meant to measure
Testosterone drives tissue repair, protein synthesis, and adaptation to training. Cortisol drives energy mobilisation and the stress response. Both are necessary. The relationship between them, the testosterone-to-cortisol ratio (TCR), is intended to describe the net anabolic or catabolic state of the body at a given moment.
The theory is that when training load is appropriate and recovery is adequate, the ratio stays relatively stable, and that when training stress exceeds recovery capacity, cortisol rises and testosterone falls. A frequently quoted rule of thumb holds that a fall of more than 30 percent in resting TCR from an athlete's own baseline indicates overreaching.
It is worth being sceptical about that rule. The most thorough systematic review of hormonal markers in overtraining syndrome, covering 38 studies, found that basal hormone levels were mostly normal in athletes with overtraining syndrome, functional overreaching or non-functional overreaching compared with healthy athletes. Its conclusion was that basal hormone levels are not good predictors of the condition, and that blunted ACTH and growth hormone responses to stimulation testing may be better ones. [5] So a resting TCR is not a diagnostic test for overtraining, and it should not be read as one.
How to read the signal
The ratio has no fixed normal value that applies universally. Whatever value it has comes from tracking it against your own history, not from a single measurement. What is your resting TCR when you are fully recovered and performing well? And how has it shifted as your training load has increased?
Read that way, a declining TCR across a training block is a prompt to look harder at recovery, not a diagnosis. Given the review evidence above, treat it as one input alongside performance data, sleep, and how the sessions actually feel, rather than as an early warning system that can be relied on by itself.
The problem with training by feel
Overreaching feels like many things. Persistent fatigue. Motivation that has quietly disappeared. Sessions that feel harder than they should. A mood that has gone flat. These are real symptoms, but they overlap with normal training fatigue, life stress, poor sleep, and half a dozen other causes.
The cortisol and testosterone picture does not tell you how to train. But it adds an objective layer to a judgement that is otherwise entirely subjective. Our cortisol guide covers the full HPA axis picture, including what circadian rhythm disruption means for training adaptation and why a single cortisol reading is almost never sufficient.
Vitamin D: The Performance Hormone Most Endurance Athletes Are Missing
The muscle function connection is not just about bones
Vitamin D is classified as a vitamin but functions as a hormone. Beyond bone health, recent research points to a role in skeletal muscle growth, immune and cardiopulmonary function, and inflammatory modulation, all of which bear on athletic performance. Vitamin D also interacts with extraskeletal tissues in ways that appear to affect injury recovery and infection risk. [6]
The honest summary is that the mechanistic case is good and the performance case is still being built. What is not in doubt is that low levels are common in athletes, and that a level is easy to measure.
How bad is the deficiency problem in German-latitude athletes?
Common enough to be worth checking. A 2020 review of vitamin D in athletes reported that athletes share the general population's predisposition to low vitamin D, with the majority of concentrations below 20 ng/mL across a wide range of sports, especially in the winter months. [6] A 2023 supplementation trial run during the winter in Germany had no difficulty recruiting 90 athletes who were already below 30 ng/mL. [7]
The underlying reason is latitude. At German and Austrian latitudes, cutaneous vitamin D synthesis falls away sharply through the winter months regardless of time spent outdoors, because the sun angle is too low.
The standard clinical threshold for sufficiency is 20 ng/mL. In sports medicine the working target is higher: the 2023 German trial above notes that although exact requirements for athletes have not been established, maintaining a 25(OH)D level of at least 40 ng/mL is considered beneficial. [7]
Why supplementing without testing is just guessing
The standard over-the-counter vitamin D supplement is 1,000 IU per day. Depending on your baseline, that dose might maintain sufficiency, or be nowhere near enough. In the German trial, insufficient athletes received either 2,000 IU per day or a loading dose of 4,000 IU per day followed by 1,000 IU per day maintenance, and the individualised loading approach reached the 40 ng/mL target while the standardised one did not. [7] The practical lesson is that dosing from a measured baseline beats dosing from a rule of thumb.
Testing your 25(OH)D level, the metabolite that reflects status, before supplementing gives you the information needed to dose correctly. Our vitamin D guide covers the cofactor question in full, including the interaction with magnesium and K2 that most supplementation advice ignores.
Aniva tests 25(OH)D alongside magnesium, calcium, and the markers needed to interpret your vitamin D status in context. An isolated reading without cofactors is half an answer. Get started with Aniva →
Magnesium: The Recovery Mineral That Depletes Fastest When You Train Hardest
Energy production, muscle contraction and recovery
Magnesium is an intracellular cation involved in the biochemical reactions behind energy production and storage, neuronal and vasomotor control, cardiac excitability and muscle contraction. Magnesium deficiency may result in impaired physical performance. [8]
Athletes have higher magnesium requirements than sedentary individuals. A 2024 systematic review in the Journal of Translational Medicine concluded that individuals engaged in intense exercise should have a magnesium requirement 10 to 20 percent higher than sedentary people, taken about two hours before training, and suggested maintaining levels in the recommended range during the off-season. [8] Worth noting: that review found only four eligible studies, so this is a thin evidence base and the type, timing and dosage of magnesium intake are not settled.
The European RDA of 350 mg per day for men and 300 mg per day for women was established for a sedentary population. For an athlete training six to ten hours per week, that figure is likely to be an underestimate.
Serum vs. red cell magnesium, why the standard test misleads athletes
A technical nuance applies here: the standard serum magnesium test that most GPs order is a limited marker of true magnesium status. Serum magnesium reflects only a small fraction of total body magnesium, and the body tightly regulates that fraction by drawing on intracellular stores. An athlete can be depleted at the cellular level while showing a normal serum magnesium result.
Red blood cell magnesium, or ionised magnesium, gives a closer view of tissue-level status, but these are rarely ordered in standard panels. The practical implication: a normal serum magnesium in an athlete with cramps, poor sleep and slow recovery should be interpreted with caution rather than taken as a clean bill of health.
The same 2024 review reported that magnesium supplementation reduced muscle soreness, improved performance and recovery, and had a protective effect on muscle damage in the studies it examined. [8]
Sleep, cortisol, and the recovery loop
Magnesium and cortisol are often described as operating in a bidirectional feedback relationship, where low magnesium raises cortisol output and elevated cortisol depletes magnesium through urinary excretion. That model is plausible and widely repeated, but it is a mechanism rather than a proven clinical loop, so treat it as a reason to measure rather than a reason to supplement blind.
Haematocrit and Haemoglobin Mass: Your Endurance Ceiling
What the haematological markers actually predict
Haematocrit, the percentage of blood volume made up of red blood cells, is the classic endurance marker and the number most athletes associate with oxygen-carrying capacity.
The most comprehensive analysis of this question, a 2023 systematic review and meta-analysis covering 384 studies, found positive associations between absolute VO2max and haemoglobin concentration, haemoglobin mass and haematocrit alike, in both observational and interventional data. Changes in haemoglobin concentration and haemoglobin mass were both positively associated with changes in VO2max. [9] So haematocrit is not useless. What it is not is a complete picture, because a percentage says nothing about the absolute size of the oxygen-carrying system behind it.
Plasma volume expansion: the training adaptation that looks like a problem
In the early weeks of an endurance training block, something counterintuitive can happen: haematocrit appears to drop. This is often not a sign of deterioration. Endurance training expands plasma volume, the liquid fraction of blood, as an early cardiovascular adaptation. Because haematocrit is a ratio, a larger denominator dilutes the red cell percentage even if absolute red cell mass is unchanged or increasing.
This phenomenon, sometimes called sports pseudoanemia, is commonly misinterpreted. Athletes or their practitioners see a falling haematocrit early in a training cycle and assume iron deficiency or overtraining, when the cause is beneficial plasma expansion.
What training-load variation means for your baseline
Haematocrit and haemoglobin values fluctuate with hydration status, time of day, altitude, and training phase. A single haematocrit reading, taken at an arbitrary point in the training cycle, tells you little on its own. The same reading compared to your values at the same phase in the previous training year, or measured consistently across multiple testing points, begins to reveal real information. This is why the timing and regularity of testing matters as much as the test itself.
When the Distance Gets Extreme: Ironman, Triathlon, and Ultramarathon Demands
The biomarker principles above apply to any trained amateur. But for athletes preparing for or recovering from events at the longer end of the endurance spectrum, such as Ironman triathlons, ultramarathons, or multi-day stage races, the demands on these biological systems are qualitatively different, and the research is striking.
Iron depletion: a structural problem for endurance specialists
A retrospective study of routine blood test data from 38 elite runners and triathletes, taken between 2009 and 2015, found that female triathletes experienced at least one episode of iron deficiency in 60.0 percent of cases and male triathletes in 37.5 percent, compared with values reported in the wider endurance literature of 20 to 50 percent in women and 0 to 17 percent in men. The same study found that male triathletes and runners had a higher incidence of iron deficiency anaemia than their female teammates, and that even with monitoring and oral iron treatment in place, iron deficiency remained a significant concern. [10]
The multi-discipline nature of triathlon plausibly creates compound iron stress. Cycling imposes high metabolic demands without foot strike losses. Running adds foot strike haemolysis on top. An Ironman triathlete completing 3.8 km of swimming, 180 km of cycling, and 42.2 km of running in a single day is loading iron metabolism across all three disciplines in one session.
Ultramarathon runners face a related challenge. A study of 25 male finishers of a 100 km ultramarathon found that haemoglobin and erythropoietin rose immediately post-race and then dropped sharply at 24 hours, and that red blood cell count and haematocrit were significantly lower at 24 hours than before the race. Serum iron fell immediately post-race. Ferritin, by contrast, increased both immediately and at 24 hours, alongside interleukin-6, TNF-alpha and hs-CRP, which reflects the acute-phase inflammatory response rather than any improvement in iron stores. [11] This is exactly why testing ferritin in the days after a long race gives a falsely reassuring reading.
The practical implication for endurance athletes: test ferritin at the start of each training block, before your heaviest training weeks, not after them. An off-season baseline and a pre-competition block reading give you the information needed to intervene before a deficit affects training quality.
The hormonal cost of extreme endurance
The cortisol and testosterone dynamics described above take on a different character when the event lasts 8 to 17 hours. A study of 42 well-trained male triathletes found that cortisol rose significantly and testosterone fell significantly immediately after an Ironman. Cortisol then dropped below pre-race values by one day post-race, while creatine kinase, myoglobin, interleukin-6 and hs-CRP were still significantly elevated five days out, and myoglobin and hs-CRP remained slightly but significantly raised at 19 days. [12] The systemic inflammation resolves faster than the muscle damage does.
Research on ultramarathon runners tells a more extreme version of the same story. A study of 12 men who completed the 161 km Western States Endurance Run found significant decreases in testosterone, luteinising hormone and SHBG, with a marked elevation in cortisol immediately post-race. All four were still significantly different from pre-race one day later, and the testosterone-to-cortisol ratio was depressed both immediately post-race and at day one. [13] In a separate study of 16 men running the 246 km Spartathlon, testosterone and IGF-1 had only partially recovered at 48 hours, to about half of their pre-race values, even though cortisol and DHEAS had already returned to normal. [14]
What does this mean for the amateur Ironman or ultramarathon runner? Two things. First, the recovery window after an event of this duration is longer than most athletes allow. Reproductive and anabolic hormones can remain suppressed for days after cortisol has normalised. Second, an athlete who completes multiple long events in a season, or returns to training quickly after an extreme event, may be stacking hormonal suppression that never fully resolves between cycles. Testing resting testosterone and cortisol before and after competition blocks is one way to see whether that is happening.
Magnesium and haematocrit in extreme endurance contexts
Sweat losses during an Ironman or a 100 km ultramarathon are not comparable to a 90-minute training session. Athletes completing events of 8 to 36 hours lose substantial sodium and magnesium through sweat and urine, on top of the metabolic demands of sustained energy production. Given the elevated requirement in intense exercise noted above, that is worth planning for rather than improvising. [8]
On haematocrit: the pseudoanemia phenomenon is particularly relevant in the early weeks of Ironman-specific training blocks, where swim volume, bike intervals and long runs create a strong stimulus for plasma volume expansion. Athletes who test during this window and see falling haematocrit without context may incorrectly conclude they have iron deficiency. This is another argument for testing at consistent, standardised time points relative to your training calendar, rather than reactively when you feel something is wrong.
The Quarterly Testing Framework for Serious Amateur Athletes
A single blood test taken at a random point in your training year is of limited use for performance purposes. What creates value is a structured testing cadence that tracks the same markers at the same relative phases of your annual training plan. Four windows make sense for most serious amateurs:
Window 1: Off-season baseline (October to November). Test at full recovery, at least two weeks after your last hard race or training block. This is your true resting baseline, the number you compare everything else against. Markers to prioritise: ferritin, resting testosterone, cortisol, vitamin D, magnesium, full iron panel, haematocrit.
Window 2: Pre-competition block (January to February). Test before the heaviest training period begins. This confirms you are starting from a replenished state, identifies any deficiencies that have developed over the winter, and establishes a pre-load baseline before training stress accumulates. Vitamin D is particularly worth checking at this window for athletes in Northern Europe, as January levels are typically at their annual low.
Window 3: Mid-season check (April to May). Test in the middle of your main competition build, ideally at a planned recovery week. Compared against window 2, this shows whether your body is absorbing the training load or accumulating deficit. Falling ferritin or a declining TCR at this point is a prompt to look harder, with enough runway to act before race season.
Window 4: Post-season recovery (August to September). Test 3 to 4 weeks after your last major race. This tells you what the season cost. Many athletes, particularly Ironman and ultramarathon competitors, are surprised to see how much ferritin, testosterone, and vitamin D have moved across a full competition year. This reading informs your off-season recovery plan and sets up the next year's baseline.
The value of this framework is not any single data point. It is the trend. An athlete with ferritin at 45 µg/L in November and 22 µg/L in April, with no supplementation in between, has a clear actionable story. An athlete with a single April reading of 22 µg/L has a number without context. Our guide to comprehensive blood testing in Germany explains the full landscape of what standard panels cover, and what they miss.
Aniva's annual membership, €199/year, is structured around this kind of baseline-and-track philosophy. 100+ biomarkers, ISO 15189-certified German laboratory, personalised results report. Start your membership →
Ask Your Coach. Then Ask Your Blood.
Every serious amateur athlete has a coach, a programme, or a structured approach to training. The science of how to train, covering periodisation, progressive overload, zone training and race-specific intervals, has never been more accessible or better understood at the amateur level.
What the training programme cannot tell you is what state the biological machinery is in. Your coach can see your power numbers, your pace, your RPE. They cannot see your ferritin. They cannot see whether your resting cortisol has climbed since January. They cannot see that your vitamin D dropped below 20 ng/mL in February.
The athlete who trains hard and also tracks the biological systems that underpin adaptation has an advantage over the athlete who trains equally hard and guesses. Not because testing replaces training, which it does not, but because it helps answer the question training alone cannot answer: why is this not working the way it should?
For triathletes entering their Ironman build, for ultrarunners preparing a 100 km peak, for CrossFitters chasing a two-year-old PR, the answer to that question is rarely "train more." It is often somewhere in these five systems. And the only way to find out is to measure.
Aniva's comprehensive biomarker panel covers all five performance-relevant categories: iron status, hormonal balance, vitamin D, magnesium, and haematological markers. That is 100+ biomarkers in total, tested annually from a single blood draw at a certified German laboratory, with a personalised report and action plan. For €199/year. See what is included →
Key Takeaways
Training adaptation requires biological systems to function in parallel, and the five categories covered here are largely invisible to standard blood panels.
Sports medicine guidance puts the ferritin cut-off for athletes at 30 µg/L, well above the 12 to 15 µg/L printed on most lab reports, and at 50 µg/L before altitude training. Haemoglobin alone does not catch depletion early enough. The cortisol and testosterone balance describes the anabolic and catabolic state, but the systematic review evidence shows basal hormone levels are not reliable predictors of overtraining syndrome on their own, so read the ratio as one input among several. Low vitamin D is common in athletes, especially in winter at Northern European latitudes, and the working target in sports medicine, at least 40 ng/mL, sits well above the clinical sufficiency threshold of 20 ng/mL. Athletes engaged in intense exercise need an estimated 10 to 20 percent more magnesium than sedentary people, and standard serum testing understates depletion. Haemoglobin concentration, haemoglobin mass and haematocrit are all positively associated with VO2max, but a single percentage reading without a trend behind it says little.
For endurance athletes in particular: Ironman, triathlon, and ultramarathon training pushes all five systems to extremes that recovery by feel cannot reliably track. Testing at structured points in the training year, not reactively when performance has already deteriorated, is the difference between managing your biology and being managed by it.
Aniva tests all five categories, 100+ biomarkers, annually. Explore the panel →
Sources
- [1] Swiss Medical Weekly, Iron deficiency in sports, definition, influence on performance and therapy, 2015
- [2] Journal of Sport and Health Science, Iron deficiency, supplementation, and sports performance in female athletes, a systematic review, 2024
- [3] Journal of Applied Physiology, Footstrike is the major cause of hemolysis during running, 2003
- [4] Nutrients, The IRONy in athletic performance, 2023
- [5] BMC Sports Science, Medicine and Rehabilitation, Hormonal aspects of overtraining syndrome, a systematic review, 2017
- [6] Nutrients, Role of vitamin D in athletes and their performance, current concepts and new trends, 2020
- [7] Nutrients, Effects of an individualized versus standardized vitamin D supplementation on the 25(OH)D level in athletes, 2023
- [8] Journal of Translational Medicine, Effects of magnesium supplementation on muscle soreness in different type of physical activities, a systematic review, 2024
- [9] PLOS ONE, The relationship between hemoglobin and VO2max, a systematic review and meta-analysis, 2023
- [10] Clinical Journal of Sport Medicine, Incidence of iron deficiency and iron deficient anemia in elite runners and triathletes, 2017
- [11] Journal of the Chinese Medical Association, Early changes of the anemia phenomenon in male 100-km ultramarathoners, 2015
- [12] European Journal of Applied Physiology, Recovery after an Ironman triathlon, sustained inflammatory responses and muscular stress, 2008
- [13] Wilderness and Environmental Medicine, The impact of an ultramarathon on hormonal and biochemical parameters in men, 2014
- [14] Annals of Research Hospitals, Effect of an ultra-distance foot race on the hypothalamic-pituitary testicular and adrenocortical axes and insulin growth factor levels, 2019
This article is for informational and educational purposes only. It is not medical advice, and should not be used as a substitute for professional medical consultation. Blood test results should always be interpreted in the context of your individual health history by a qualified healthcare professional. Aniva Health does not provide medical diagnoses or treatment recommendations.
