What Biomarkers Should You Test When Training for a Marathon or Triathlon?

Training data can tell you how fast, far and hard you are working, but it cannot always explain why your performance, energy or recovery has changed. This article explores the blood biomarkers most relevant to marathon, triathlon and endurance-event training, including iron stores, blood health, vitamins, inflammation, metabolic health, thyroid function and organ health. You will also learn when to test, how strenuous exercise can temporarily affect certain results, and why a comprehensive health baseline may be more useful than checking a single performance marker.

Training for a marathon, triathlon or endurance event usually comes with plenty of data. You measure your pace, heart rate, distance, power, sleep, cadence and recovery. You know how many kilometres you completed last week and exactly how much slower the final interval felt. But most fitness trackers cannot tell you whether your body has enough iron to transport oxygen efficiently, whether your vitamin levels support normal red blood cell production, or whether your liver, kidneys and metabolic health are responding well to the demands of training. That is where biomarker testing can add another layer to your training plan.

A blood test cannot predict your finishing time or replace a well-designed training programme. Not every biomarker directly affects sporting performance, either. Some results may help explain changes in energy, endurance or recovery, while others provide a broader assessment of the health of the person completing the training. Both matter.

Whether you are preparing for a marathon, triathlon, HYROX, cycling event, long-distance swim or your first major fitness challenge, testing can provide something every athlete needs: a useful biological baseline.

Why blood testing matters during endurance training

Endurance training places coordinated demands on the cardiovascular, metabolic, musculoskeletal, immune and endocrine systems. Oxygen must be transported to working muscles, nutrients must be converted into energy, and tissues must recover between sessions. When iron status, energy availability, vitamin levels or another part of this system is compromised, exercise capacity and recovery may also be affected (Lukaski, 2004; Mountjoy et al., 2023).

Some biomarkers are closely connected to exercise performance. Haemoglobin, for example, is essential for carrying oxygen, while ferritin provides information about the iron stores needed to support haemoglobin production.

Other markers, including cholesterol and HbA1c, are less likely to explain whether you missed your target pace during last Sunday’s long run. They are still important because training for an event does not make someone immune to cardiovascular, metabolic, thyroid, liver or kidney conditions.

Blood testing can therefore serve two purposes:

1. It may identify biological factors affecting energy, endurance and recovery.

2. It can provide a broader health assessment before the body is placed under increasing physical demand.

What biomarkers should runners and endurance athletes test?

There is no single “performance biomarker”. Athletic performance is produced by several interconnected biological systems, and results must be considered alongside training load, nutrition, sleep, stress and medical history.

The most useful markers can be grouped into three broad areas:

Performance-related biomarkers

These may have a more direct relationship with oxygen transport, energy metabolism and exercise capacity. Examples include haemoglobin, red blood cells, ferritin, vitamin B12 and folate.

Recovery and training-context biomarkers

These may provide supporting information about inflammation, physiological stress and recent exercise. Examples include hs-CRP, cortisol, white blood cells and certain liver or kidney-associated measurements.

Wider health biomarkers

These are not necessarily predictors of sporting performance, but they can identify important health risks or conditions that should not be overlooked simply because someone is physically active. Examples include cholesterol, HbA1c, thyroid function, liver function and kidney function.

1. Full blood count

A full blood count examines the different cells circulating in your blood, including red blood cells, white blood cells and platelets.

For athletes, some of the most relevant measurements include:

Haemoglobin

Haemoglobin is the protein within red blood cells that transports oxygen from the lungs to working tissues. Haemoglobin concentration is particularly relevant to endurance exercise because it influences the blood’s oxygen-carrying capacity. Reductions in haemoglobin associated with nutritional deficiency or anaemia can therefore impair endurance work capacity (Lukaski, 2004).

Low haemoglobin may contribute to fatigue, breathlessness and reduced exercise capacity. However, the cause must be properly investigated rather than assumed to be related to training.

Red blood cell count and haematocrit

Red blood cell count measures the number of red blood cells present, while haematocrit measures the proportion of blood made up of red blood cells. These results provide additional context alongside haemoglobin and can help build a broader picture of oxygen-carrying capacity.

Training, hydration and changes in plasma volume may influence these measurements, so an isolated result should not be used as a direct score of fitness.

Mean cell volume

Mean cell volume, or MCV, describes the average size of red blood cells. Changes in MCV can sometimes provide clues about iron, vitamin B12 or folate deficiency, although the result must be interpreted alongside other blood count and nutrient markers.

White blood cells

White blood cells form part of the immune system. Prolonged and intensive exercise can produce temporary changes in circulating immune cells. However, a white blood cell count cannot determine whether an athlete is adequately recovered or diagnose overtraining (Nieman, 1999).

It is best viewed as a broader blood and immune health marker rather than a performance score.

2. Ferritin and iron status

Ferritin reflects the amount of iron stored within the body. Iron supports haemoglobin production, oxygen transport, mitochondrial energy production and several enzymatic processes involved in exercise metabolism. Iron deficiency is relatively common among endurance athletes, particularly menstruating women, and may reduce aerobic capacity and endurance even before iron-deficiency anaemia develops (DellaValle, 2013; Pengelly et al., 2024).

A 2024 systematic review involving 669 athletes found that iron deficiency was associated with an estimated 3–4% reduction in endurance performance. Performance improved in some iron-deficient athletes following appropriately managed iron treatment, although the benefit depended on baseline iron status and study design (Pengelly et al., 2024).

Athletes who may be at greater risk of low iron include:

• Menstruating women
• Runners and endurance athletes
• Vegetarians and vegans
• People training at altitude
• Athletes restricting calories
• People with gastrointestinal symptoms
• Athletes with a previous iron deficiency

Ferritin is one of the biomarkers with the clearest potential relationship to endurance performance. However, it is also an acute-phase protein and may rise during inflammation. It should therefore be assessed alongside haemoglobin, other iron markers, inflammatory status, symptoms and medical history.

Iron supplements should not be taken simply because training feels difficult. Excess iron can also be harmful, so supplementation should be based on appropriate testing and professional guidance.

3. Vitamin B12 and folate

Vitamin B12 and folate are required for normal red blood cell production. Severe deficiency can cause megaloblastic anaemia and reduce endurance work capacity, although there is limited evidence that supplementation improves performance in athletes who already have adequate levels (Lukaski, 2004; Woolf and Manore, 2006).

People following vegan diets may be at greater risk of inadequate vitamin B12 intake unless they consume fortified foods or suitable supplements (Bakaloudi et al., 2021).

Testing vitamin B12 and folate alongside a full blood count provides more useful context than looking at either nutrient in isolation. These are performance-relevant primarily when a deficiency is present. Higher results do not necessarily translate into faster race times.

4. Vitamin D

Vitamin D contributes to calcium regulation, bone health, immune function and normal muscle function. Athletes who train indoors or receive limited sunlight may be at increased risk of low vitamin D status (von Hurst and Beck, 2014).

Research has associated vitamin D deficiency with impaired muscle function and an increased risk of some musculoskeletal problems. However, supplementation has not consistently improved strength or sporting performance in athletes who are not deficient (Owens et al., 2018; Sist et al., 2023).

Testing is therefore most useful for identifying and correcting genuine insufficiency rather than using vitamin D as a general performance enhancer. For runners and other athletes completing large volumes of impact-based exercise, vitamin D also matters as part of a wider assessment of bone and musculoskeletal health.

5. Kidney function and electrolytes

The kidneys regulate fluid balance, remove waste products and help control electrolytes.

Relevant markers may include:

• Creatinine
• Estimated glomerular filtration rate, or eGFR
• Urea
• Sodium
• Potassium

These markers are primarily measures of health and physiological function rather than direct predictors of endurance performance.

However, endurance exercise, dehydration, muscle breakdown and changes in blood volume may temporarily affect creatinine, urea and electrolyte results. Creatinine can also be influenced by muscle mass, recent training and creatine supplementation.

Strenuous endurance exercise has been shown to cause temporary changes in several laboratory markers associated with kidney function and fluid regulation (Tiller et al., 2023).

An unexpected result does not automatically indicate kidney disease, particularly when the sample was collected after demanding exercise. It should still be interpreted carefully and followed up where appropriate.

6. Liver function

Liver profiles commonly include:

• Alanine aminotransferase, or ALT
• Aspartate aminotransferase, or AST
• Alkaline phosphatase, or ALP
• Gamma-glutamyl transferase, or GGT
• Bilirubin
• Albumin

The liver helps process nutrients, regulate energy metabolism and remove substances from the blood. These tests primarily assess general health. They do not tell you how quickly you will run, cycle or swim. However, AST and ALT are not completely specific to the liver. Both are also present in skeletal muscle and may rise following strenuous exercise or muscle damage.

In one controlled study, AST and ALT remained significantly elevated for at least seven days after intensive weightlifting in healthy men, alongside increases in creatine kinase and myoglobin (Pettersson et al., 2008).

More recent research has similarly shown that exercise-related muscle damage can produce apparently abnormal liver-associated enzyme results in otherwise healthy athletes (Tiller et al., 2023).

Testing immediately after a race or hard training session may therefore capture recent muscular stress rather than a person’s usual baseline.

7. Inflammation markers

C-reactive protein, or CRP, is produced by the liver in response to inflammatory signalling. High-sensitivity CRP, often written as hs-CRP, can detect lower concentrations of this protein. Strenuous or prolonged exercise can temporarily increase inflammatory markers. This means hs-CRP should not be treated as a direct recovery score or a standalone test for overtraining.

An elevated result could reflect:

• Recent strenuous exercise
• Infection
• Injury
• Chronic inflammation
• Another health condition

hs-CRP may provide useful context when recovery feels unusually slow, but it cannot explain athletic performance by itself.

Persistent elevation should be assessed by a qualified healthcare professional, particularly when accompanied by symptoms or other abnormal results.

8. HbA1c and blood sugar control

HbA1c reflects average blood glucose exposure over approximately the previous eight to twelve weeks.

It is principally used to screen for, diagnose and monitor diabetes rather than assess short-term sports nutrition or race fuelling (Diabetes UK, 2026).

Regular physical activity generally supports metabolic health, but physically active people can still develop impaired glucose regulation. Genetics, diet, sleep, medication, body composition and other health conditions may all contribute.

HbA1c is therefore best understood as a wider health marker.

It is unlikely to tell you whether you consumed enough carbohydrate before this morning’s training session. What it can do is identify longer-term metabolic patterns that may need attention.

For somebody preparing to place significant physical demands on their body, understanding blood sugar regulation forms part of a responsible health baseline.

9. Cholesterol and triglycerides

A lipid profile may include:

• Total cholesterol
• LDL cholesterol
• HDL cholesterol
• Non-HDL cholesterol
• Triglycerides
• Cholesterol ratios

These measurements relate primarily to long-term cardiovascular health rather than immediate exercise performance.

Regular exercise can favourably influence lipid metabolism, but athletes are not exempt from inherited or acquired dyslipidaemia (Christou et al., 2017; Muscella et al., 2020).

Someone may have excellent cardiovascular fitness while still having raised LDL cholesterol, elevated triglycerides or a strong family history of cardiovascular disease.

Cholesterol testing is therefore not about finding a shortcut to a personal best. It is about assessing the long-term health of the cardiovascular system supporting every training session.

10. Thyroid function

Thyroid hormones contribute to metabolic regulation, cardiovascular function, temperature control and neuromuscular function.

Both an underactive and overactive thyroid can produce symptoms that overlap with training fatigue, including:

• Changes in energy
• Weight changes
• Muscle weakness
• Changes in heart rate
• Poor temperature tolerance
• Sleep disruption
• Mood changes
• Menstrual changes

Thyroid testing commonly begins with thyroid-stimulating hormone, or TSH, and may include free T4 and free T3 where appropriate.

Exercise itself can also produce temporary changes in thyroid hormone concentrations, particularly during intensive training or periods of low energy availability (Kanaka-Gantenbein, 2005).

Thyroid results should not be used as a sporting performance score. Their value lies in identifying a health condition that may otherwise be mistaken for poor fitness, overtraining or lack of motivation.

11. Cortisol

Cortisol helps regulate the stress response, energy availability, blood pressure and the sleep-wake cycle.

Moderate-to-high-intensity exercise can acutely increase circulating cortisol, making this a normal component of the exercise response rather than automatically a sign that something is wrong (Hill et al., 2008).

Cortisol also varies according to:

• Time of day
• Sleep
• Illness
• Psychological stress
• Food intake
• Recent exercise

A single cortisol result cannot diagnose overtraining or determine whether an athlete needs a rest day.

It may provide supporting context when interpreted alongside symptoms, training load, recovery and other health information, but it should not be marketed as a standalone measure of athletic readiness (Urhausen, Gabriel and Kindermann, 1995).

12. Sex hormones and low energy availability

Athletes need enough energy not only to complete training, but also to support normal physiological function.

Low energy availability occurs when dietary energy intake is insufficient to support both exercise and the body’s essential processes. It may be intentional, such as during aggressive weight loss, or unintentional when training volume rises without a sufficient increase in food intake.

Persistent low energy availability can contribute to Relative Energy Deficiency in Sport, or RED-S.

RED-S can affect female and male athletes and may impair:

• Reproductive function
• Bone health
• Immunity
• Metabolism
• Cardiovascular health
• Psychological wellbeing
• Athletic performance

Potential warning signs include menstrual disruption, reduced libido, recurrent bone stress injuries, fatigue, frequent illness, mood changes and declining performance (Mountjoy et al., 2023).

There is no single blood test that confirms RED-S. Hormone results may form part of a broader assessment, but symptoms, diet, training load, bone health and medical history must also be considered.

What is the best blood test for marathon or triathlon training?

A sports-focused panel may be useful when the main concern is oxygen transport, iron status, vitamins or blood health. However, training for an endurance event places demands on the whole body, not simply the muscles and cardiovascular system.

A comprehensive assessment may therefore be more useful for someone who wants to understand both:

• Biomarkers that may affect energy, endurance and recovery
• Wider health markers that are important regardless of sporting performance

The distinction is important. Ferritin, haemoglobin, vitamin B12 and folate may have a relatively direct connection with exercise capacity when they are low. Cholesterol, HbA1c, liver function and kidney function are less likely to explain whether you hit a personal best. They may still identify significant health risks that should not be ignored simply because you are active.

Why choose the Vitall Total Health Check?

Vitall’s Male and Female Total Health Checks provide a broad snapshot of the biological systems supporting both physical activity and general health.

The panels assess key areas including:

• Full blood count and blood health
• Ferritin and relevant nutrients
• Liver function
• Kidney function
• Cholesterol and triglycerides
• HbA1c and blood sugar control
• Inflammation
• Vitamins and minerals
• Cortisol
• Relevant hormone markers

The purpose is not to promise a faster race. it's to help identify measurable factors that may affect how you train, recover and feel, while also checking important areas of health that might not produce obvious symptoms. A comprehensive test provides greater context than checking one or two isolated markers.

For example, low ferritin becomes more meaningful when viewed alongside haemoglobin, red blood cell indices and inflammation. An unusual liver-associated enzyme result becomes easier to interpret when recent exercise and wider liver markers are considered.

Testing before a major training block also creates a personal baseline. Future results can then be compared with your own previous measurements, rather than relying only on population reference ranges.

When should you get a blood test during training?

Before the main training block

Testing before training volume peaks can help establish a baseline and identify areas that may require attention early. Ideally, testing should take place several weeks or months before the event rather than during race week. This leaves time to discuss unexpected results and make evidence-based changes where appropriate.

During a long training programme

Repeat testing may be useful when training continues for several months, particularly if you have:

• A history of iron deficiency
• A restrictive diet
• Thyroid disease
• Menstrual changes
• Persistent fatigue
• Declining performance
• Poor recovery
• Recurrent illness or injury

The appropriate frequency depends on your health, symptoms and clinical guidance.

After the event

Testing immediately after a marathon, triathlon or demanding race may capture the acute effects of exercise rather than your usual health.

Strenuous exercise can temporarily alter:

• AST and ALT
• Creatinine and urea
• Inflammatory markers
• White blood cells
• Hydration-sensitive measurements

Testing before a hard session, rather than immediately afterwards, generally provides a cleaner baseline (Pettersson et al., 2008; Tiller et al., 2023).

Always follow the preparation instructions provided and disclose recent exercise, illness, medication and supplements when discussing unexpected results.

Can a blood test help you achieve a personal best?

Blood testing cannot manufacture fitness. It cannot replace consistent training, adequate sleep, appropriate nutrition, hydration, sensible pacing or recovery. Some biomarkers may identify factors that are directly relevant to performance. Low haemoglobin or depleted iron stores, for example, may compromise oxygen transport and endurance capacity. Other biomarkers serve a different purpose. They assess the long-term health of the cardiovascular, metabolic and organ systems supporting your training.

A comprehensive health check therefore does more than search for a hidden performance advantage. It helps you understand whether the body being asked to train harder is broadly healthy, adequately supported and responding as expected. Your next personal best will still be built through repeated effort. Your biomarkers can help show whether anything measurable is standing in the way.

Frequently asked questions

What blood tests should runners have?

Useful tests may include a full blood count, ferritin, vitamin B12, folate, vitamin D, kidney function, liver function, HbA1c, cholesterol, thyroid function and inflammation markers. Some of these markers relate more directly to performance, while others assess general health. The appropriate tests depend on your symptoms, diet, medical history and training load.

Is ferritin important for marathon runners?

Yes. Ferritin reflects stored iron, which supports haemoglobin production, oxygen transport and cellular energy metabolism. Low iron stores may affect endurance before severe anaemia develops. Ferritin should be interpreted alongside haemoglobin, inflammation and other iron markers.

Can overtraining show up in a blood test?

There is no single diagnostic blood marker for overtraining syndrome. Hormones, inflammation, blood count and nutrient status may provide supporting information, but none can diagnose overtraining independently. Training performance, symptoms, sleep, nutrition, psychological stress and recovery history must also be considered (Urhausen, Gabriel and Kindermann, 1995).

Should I test before or after a marathon?

A baseline test is generally more useful before the hardest stage of your training programme. Testing immediately after a marathon may produce temporary changes caused by strenuous exercise, dehydration, muscle damage and inflammation.

Does being fit mean my blood results will be healthy?

No. Regular exercise supports health, but physically active people can still experience nutritional deficiencies, raised cholesterol, thyroid disease, abnormal blood sugar control and other health conditions. Fitness and health overlap, but they are not identical.

Which biomarkers are most closely related to endurance performance?

Haemoglobin, red blood cell measurements, ferritin, vitamin B12 and folate may have a relatively direct relationship with oxygen transport and exercise capacity, particularly when results are low. Other markers, such as hs-CRP and cortisol, provide supporting context but cannot independently measure recovery or readiness.

Which Vitall test is best for endurance training?

The Vitall Total Health Check provides the broadest overall assessment for someone who wants to examine both performance-related biomarkers and wider health. A focused sports test may suit somebody mainly interested in blood health, iron and nutrients. The Total Health Check offers greater context across blood health, metabolic function, organ health, inflammation and cardiovascular risk.

Test the body behind the training

You already track the miles, minutes and metres. Now look beneath them. The Vitall Total Health Check examines important biomarkers associated with blood health, energy metabolism, recovery and long-term health, helping you enter your next training block with a clearer understanding of your baseline.

It will not complete the training for you, but it may show whether anything measurable is making that training harder than it needs to be. Your next PB might be hiding in your biomarkers.

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Medical disclaimer

This article provides general health information and is not a substitute for clinical assessment, diagnosis or treatment. Blood test results should be considered alongside your symptoms, medical history, training load and any medicines or supplements you take. Speak to a qualified healthcare professional if you are concerned about your results or health.