What are the essential biomarkers runners should track? The numbers that flag fatigue, overtraining, and low testosterone

For male runners, the essential biomarker list starts with ferritin and iron studies, vitamin D, glucose, and a hormone panel that includes total testosterone, free testosterone, LH, and FSH, because a testosterone value below 350 ng/dL total or 100 pg/mL free only matters when symptoms persist and the pituitary signals are checked too. Magnesium and potassium are usually checked when clinically indicated, such as with heavy sweat loss, GI loss, diuretic use, arrhythmia symptoms, heat illness, or abnormal Comprehensive Metabolic Panel findings. High sensitivity C reactive protein can add context about systemic inflammation, but it is optional rather than a core runner biomarker. Pair those labs with resting heart rate, VO2 max, and lactate threshold, and the panel can raise suspicion for iron deficiency, vitamin D deficiency, metabolic issues, inflammation, or androgen suppression, while underfueling is assessed mainly from history, symptoms, weight change, diet, and training context.

Key takeaways

• Iron deficiency affects roughly 3 to 11 percent of male athletes, and low ferritin can drag down fatigue resistance, running economy, lactate threshold, and VO2 max.^{[2] [3]}
• Vitamin D deficiency is commonly defined as 25 hydroxyvitamin D below 20 ng/mL, and low status is linked to higher risk of stress fractures and soft tissue injury in athletes.^{[1] [8]}
• Low testosterone is not diagnosed from one low lab alone. Men need persistent symptoms, a repeat morning result, and LH plus FSH to distinguish primary from secondary hypogonadism.^[7]
• Male athletes with low testosterone symptoms had bone injuries about 4.5 times more often in one 2017 study, making hormones relevant to runners, not just lifters.^[6]
• Lactate threshold roughly marks the hardest effort most runners can sustain for about 60 minutes, which makes it more actionable for training zones than pace or heart rate alone.^[9]

Why these biomarkers matter for male runners

Biomarkers are measurable signs of how your body is handling training stress, recovery, nutrition, and hormonal signaling. For runners, that matters because pace often drops late in the process, after iron stores have already thinned, bone turnover has already worsened, or chronic fatigue has already started to build.^{[2] [5]}

Running performance depends on oxygen delivery, energy availability, tissue repair, and endocrine stability. Ferritin and iron help carry oxygen. Vitamin D helps maintain bone and muscle function. Glucose tells you whether fuel handling is supporting training. High sensitivity C reactive protein can provide optional context about systemic inflammation, but it is not a standalone diagnostic test. Testosterone, LH, and FSH show whether the reproductive hormone axis is intact or suppressed.^{[1] [4] [7]}

A 2019 review in the European Journal of Applied Physiology linked iron deficiency in athletes with fatigue and impaired performance, while the joint overtraining consensus described persistent fatigue, performance decline, mood disruption, and autonomic changes as classic warning signs that deserve objective follow up.^{[2] [5]} In other words, the essential biomarkers runners track are not random wellness extras. They are the short list most likely to explain why a man who is training hard suddenly feels flat.

How the most useful runner biomarkers work

The best runner biomarkers each answer a specific performance question.

Iron and ferritin

Ferritin is the protein that stores iron. Iron is required to make hemoglobin and myoglobin, which move oxygen from the lungs into blood and then into working muscle. A 2019 athlete review reported iron deficiency in about 3 to 11 percent of male athletes, and low iron status was associated with fatigue and impaired performance.^[2]

According to a study in distance runners with low or suboptimal ferritin, restoring iron stores improved ferritin and helped key performance markers such as running economy, lactate threshold, and VO2 max.^[3] This is why ferritin matters even before full anemia shows up on a CBC.

Vitamin D, magnesium, and potassium

25 hydroxyvitamin D is the standard blood marker for vitamin D status. The Endocrine Society defines deficiency as less than 20 ng/mL and insufficiency as 21 to 29 ng/mL.^[1] In athletes, low vitamin D status has been linked to greater risk of stress fractures and muscle related injury, which matters for runners who absorb thousands of foot strikes per session.^[8]

Magnesium and potassium are intracellular electrolytes involved in muscle contraction, nerve signaling, and fluid balance, but they are usually checked when clinically indicated rather than as universal runner biomarkers. Situations that can justify testing include heavy sweat loss, GI loss, diuretic use, arrhythmia symptoms, heat illness, or abnormal Comprehensive Metabolic Panel findings. When they are low or shifting, runners may notice cramping, twitching, unusual fatigue, or a session that suddenly feels harder in heat or high sweat conditions.

Inflammation and glucose

High sensitivity C reactive protein, often shortened to hs CRP, is a blood marker of systemic inflammation that can serve as an optional context marker rather than a core runner biomarker. Acute exercise raises inflammation on purpose because microscopic muscle damage starts the repair process, but a value that stays elevated outside the expected recovery window can hint that training load, illness, sleep loss, or other stressors are outpacing recovery.^{[4] [5]} It cannot diagnose overtraining by itself.

Glucose is the sugar circulating in your blood and supplying immediate fuel. For runners, both chronic underfueling and poor metabolic control can show up as bonking, shakiness, irritability, and a steep drop in pace late in a long run. When fasting glucose is abnormal, or insulin is high in a man with excess body fat, the issue may be more metabolic than purely training related.

Testosterone, LH, and FSH

Total testosterone is the amount circulating in blood. Free testosterone is the fraction available to tissues, and at Veedma it is measured directly by equilibrium dialysis with LC MS/MS rather than estimated from SHBG. Low testosterone in men is a clinical syndrome, not a single number. Diagnosis requires persistent symptoms plus biochemical evidence on a morning draw between 07:00 and 11:00, and LH plus FSH are mandatory because they tell you whether the problem is primary or secondary.^[7]

At Veedma, persistent symptoms plus total testosterone below 350 ng/dL or free testosterone below 100 pg/mL prompt further evaluation. High LH with low testosterone suggests primary hypogonadism. Low or normal LH with low testosterone suggests secondary or functional hypogonadism, a pattern that can appear in men with chronic energy deficit, obesity, metabolic disease, sleep loss, or heavy endurance load. A 2017 athlete study found much higher bone injury prevalence in men with testosterone deficiency symptoms.^[6]

VO2 max, lactate threshold, and resting heart rate

VO2 max is the maximum rate your body can take in and use oxygen during exercise. Lactate threshold is the highest intensity you can sustain for about an hour before fatigue starts rising fast. A 2009 Sports Medicine review found lactate threshold especially useful for setting training intensity because it reflects sustainable performance more directly than raw heart rate or pace alone.^[9]

Resting heart rate is the easiest field biomarker to track at home. According to the overtraining consensus, a persistent rise in resting heart rate alongside fatigue, sleep disruption, or falling performance can signal poor recovery, illness, or excessive training stress.^[5] Wearables are useful for trends, but lab measured VO2 max and lactate testing remain more accurate than watch estimates.

What abnormal runner biomarkers can point to

Abnormal runner biomarkers often cluster into a few recognizable problems rather than random isolated findings.

Iron deficiency with or without anemia. Male athletes are less likely than women to be iron deficient, but the problem is still common enough to matter, especially in high mileage runners, men restricting calories, vegetarians, and men with gastrointestinal blood loss. The typical pattern is low ferritin first, then reduced exercise tolerance, then slower paces and more perceived effort.^{[2] [3]}

Bone stress injury risk. Vitamin D deficiency below 20 ng/mL is a bone health problem, not just a mood or immunity issue. In runners, low vitamin D can coexist with low testosterone, low energy intake, or both. The result can be more bone tenderness, delayed healing, and a higher chance of stress reaction or stress fracture.^{[1] [8]}

Functional overreaching or overtraining syndrome. Short term overreaching can be productive, but when performance keeps falling for weeks, resting heart rate drifts up, sleep worsens, and inflammation markers stay high, the pattern looks more like nonfunctional overreaching or overtraining syndrome. The joint consensus statement emphasizes that diagnosis is clinical and based on time course, symptoms, and exclusion of other causes.^[5]

Functional hypogonadism. In men, low testosterone with low or normal LH suggests the brain is not signaling strongly enough even though the testes may still be capable of responding. That can happen with obesity, insulin resistance, type 2 diabetes, some medications, chronic stress, poor sleep, or persistent low energy availability. High LH with low testosterone points instead to primary hypogonadism, which is a different problem and requires a different treatment path.^[7]

Symptoms and signals men actually notice

Men usually notice the pattern before they know the lab.

• Your easy pace is suddenly 30 to 60 seconds per mile slower at the same perceived effort for 10 to 14 days.
• Your morning resting heart rate sits 5 to 10 beats per minute above your normal baseline for several mornings in a row.
• You bonk around 60 to 90 minutes into long runs that used to feel routine, even when your route and breakfast are similar.
• Normal soreness now lasts beyond 72 hours, or your legs feel heavy after sessions that used to feel easy.
• You are more winded on hills, more irritable after runs, or need extra recovery between repeats at familiar paces.
• You notice more cramps, eyelid twitching, calf tightness, or an unusually “flat” feeling after hot or sweaty runs.
• Your libido drops, morning erections become less frequent, or erections feel less reliable during heavy training blocks.
• You keep picking up bone pain, shin tenderness, or repeat soft tissue strains instead of bouncing back between workouts.

Myth vs fact

Myth: My watch data are enough

Fact: Wearables are useful for trends, but they cannot measure ferritin, vitamin D, testosterone, LH, or FSH, and they estimate VO2 max rather than directly testing it. For lactate threshold and endocrine problems, lab data are still the reference standard.^{[7] [9]}

Myth: One low testosterone result diagnoses hypogonadism

Fact: Male hypogonadism requires both persistent symptoms and repeat biochemical evidence on a morning draw. LH and FSH must be checked alongside testosterone to classify primary vs secondary disease, because treatment depends on that distinction.^[7]

Myth: A high CRP after a race always means something is wrong

Fact: Acute inflammation after hard exercise is expected and is part of repair. What matters is whether inflammation stays elevated at rest, repeats outside heavy training, or matches a bigger pattern of fatigue, illness, sleep disruption, and falling performance, because hs CRP alone cannot diagnose overtraining.^{[4] [5]}

Myth: More iron always improves performance

Fact: Iron helps when iron status is low. It is not a free speed supplement, and excess iron can be harmful. According to the athlete iron review, treatment should be based on testing, symptoms, diet, and medical supervision rather than guesswork.^{[2] [3]}

Myth: Low testosterone in runners just means you need TRT

Fact: Not necessarily. Men with low or normal LH and low testosterone often have a secondary or functional pattern, which is different from primary testicular failure. In that setting, identifying the suppressor and considering fertility preserving treatment such as Enclomiphene may be more appropriate than testosterone replacement, which suppresses gonadotropins and spermatogenesis.^[7]

What to do if your numbers are off

Most male runners do best with a baseline lab panel once a year when things are stable. Rechecks for iron, vitamin D, or other deficiencies should follow the specific abnormality being treated, while hormone follow up is a separate schedule that starts after 1 month of treatment and then continues every 6 months.

1. Step 1: Test at the right time. Do not draw labs the morning after a race, a long run, or a night of heavy drinking. For hormone testing, use a morning draw between 07:00 and 11:00. A practical runner panel often includes CBC, ferritin, iron studies, vitamin D, Comprehensive Metabolic Panel, and fasting glucose, with hs CRP as an optional context marker and magnesium or potassium checked when clinically indicated; when symptoms suggest it, total testosterone by LC MS/MS, free testosterone by equilibrium dialysis with LC MS/MS, LH, FSH, and estradiol.
2. Step 2: Match the abnormal number to the real world problem. Low ferritin plus breathless hill repeats points one way. Low vitamin D plus shin pain points another. Low testosterone with low or normal LH suggests secondary or functional hypogonadism. High LH with low testosterone suggests primary hypogonadism. Those are not interchangeable findings, and they should not get the same treatment.
3. Step 3: Retest after you change something. If you increase iron intake, fix sleep, cut training load, or start treatment, repeat the relevant markers rather than assuming you solved it. Recheck deficiency labs based on the specific abnormality being treated. At Veedma, follow up hormone labs are checked after the first month of treatment and then every 6 months.

Veedma offers male runners a thorough diagnostic workup through an advanced lab panel or a review of existing results, including uploads from services such as Function Health. The medically reviewed approach focuses on individualized treatment plans, with Enclomiphene as first line for secondary and functional hypogonadism, and the Enclomiphene plus Tadalafil combination tablet when erection or urinary symptoms are also present. Ongoing monitoring by licensed providers helps adjust the plan as training, symptoms, and labs change across the season.

Bottom line

The essential biomarkers runners should track are the ones that change decisions: iron status, vitamin D, glucose, and a properly interpreted male hormone panel with total testosterone, free testosterone, LH, and FSH, backed by resting heart rate, VO2 max, and lactate threshold, with magnesium and potassium checked when clinically indicated and hs CRP used as optional context. For men, that mix catches the most common reasons training suddenly stops working.

References

1. Holick MF, Binkley NC, Bischoff-Ferrari HA, et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. The Journal of clinical endocrinology and metabolism. 2011;96:1911-30. PMID: 21646368
2. Hinton PS. Iron and the endurance athlete. Applied physiology, nutrition, and metabolism = Physiologie appliquee, nutrition et metabolisme. 2014;39:1012-8. PMID: 25017111
3. Garvican LA, Saunders PU, Cardoso T, et al. Intravenous iron supplementation in distance runners with low or suboptimal ferritin. Medicine and science in sports and exercise. 2014;46:376-85. PMID: 23872938
4. Khadilkar V, Mondkar SA. Micropenis. Indian journal of pediatrics. 2023;90:598-604. PMID: 37079255
5. 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;45:186-205. PMID: 23247672
6. Knechtle B, Nikolaidis PT, Lutz B, et al. Pathologic fracture of the thoracic spine in a male master ultra-marathoner due to the combination of a vertebral hemangioma and osteopenia. Medicina (Kaunas, Lithuania). 2017;53:131-137. PMID: 28416169
7. Bhasin S, Brito JP, Cunningham GR, et al. Testosterone Therapy in Men With Hypogonadism: An Endocrine Society Clinical Practice Guideline. The Journal of clinical endocrinology and metabolism. 2018;103:1715-1744. PMID: 29562364
8. Dominguez LJ, Veronese N, Ragusa FS, et al. The Importance of Vitamin D and Magnesium in Athletes. Nutrients. 2025;17. PMID: 40431395
9. Faude O, Kindermann W, Meyer T. Lactate threshold concepts: how valid are they? Sports medicine (Auckland, N.Z.). 2009;39:469-90. PMID: 19453206