Essential biomarkers runners should track to run faster and avoid injury

Your watch can tell you how far and how fast you run, but your bloodwork reveals whether your body can keep up. Here are the essential biomarkers runners should track to build speed, stay healthy, and avoid burnout.

The relationship

Biomarkers are measurable signs in blood, urine, or tissues that show how your body’s systems are working. For runners, essential biomarkers include nutrients like iron and vitamin D, hormones like cortisol and testosterone, markers of blood sugar, and signals of inflammation and recovery.

Training is controlled stress. Every long run, speed session, and hill repeat forces your body to adapt. When recovery, nutrition, and stress management are on point, biomarkers stay in healthy ranges. When something is off, those same markers start to drift, often before you feel anything obvious in your legs or lungs.

Research shows that deficiencies in key nutrients are common in endurance athletes. Vitamin D inadequacy is widespread among outdoor and indoor athletes, even in sunny climates, and is linked with higher stress fracture risk.^[1] Low iron stores are also frequent in distance runners and can quietly erode endurance long before full anemia appears. By focusing on a small group of essential biomarkers runners can move from guessing to making targeted changes in fueling, training load, and recovery.

How it works

Most of the essential biomarkers runners should track fall into five buckets: blood and oxygen delivery, bone and muscle health, metabolism, hormones, and inflammation or recovery. Here is how each category connects directly to your performance and injury risk.

Blood and oxygen biomarkers: iron, ferritin, and hemoglobin

Iron is a mineral your body uses to make hemoglobin, the protein inside red blood cells that carries oxygen to working muscles. Ferritin is an iron-storage protein in your cells; low ferritin usually means your iron stores are running low even if hemoglobin is still normal. Studies in endurance athletes show that low ferritin and iron deficiency, even without anemia, can reduce VO₂ max, increase perceived exertion, and slow time-trial performance.

Many sports-medicine clinicians flag ferritin below about 30–35 ng/mL in endurance athletes as a sign to look closer at diet, gut health, and training load, because runners lose iron through sweat, footstrike hemolysis, and sometimes gastrointestinal bleeding.

Bone and muscle biomarkers: vitamin D and magnesium

Vitamin D is a hormone-like vitamin that helps your gut absorb calcium and supports bone strength, immune function, and muscle performance. In athletes, low blood levels of 25-hydroxyvitamin D, the main vitamin D biomarker, are linked with a higher risk of stress fractures and other bone injuries.^[1] Many sports medicine groups aim for levels of at least 30 ng/mL for bone health in runners.

Magnesium is a mineral that helps enzymes produce energy, supports muscle contraction and relaxation, and stabilizes heart rhythm. Reviews suggest that marginal magnesium deficiency can impair exercise capacity and increase muscle cramps and arrhythmias, especially when sweat losses are high and diets are low in whole grains, nuts, and leafy greens.^[4]

Metabolic biomarkers: blood sugar and lipids

Fasting blood glucose is your blood sugar level after not eating overnight, and hemoglobin A1c (HbA1c) reflects your average blood sugar over about three months. While regular exercise improves insulin sensitivity, runners are not immune to prediabetes or type 2 diabetes, especially if there is a family history or central weight gain. Position statements from diabetes and sports organizations show that good blood sugar control lowers fatigue, supports training consistency, and reduces long-term cardiovascular risk.^[8]

Blood lipids include LDL cholesterol, HDL cholesterol, and triglycerides. Even endurance athletes can have elevated LDL or triglycerides due to genetics or diet, which raises lifetime heart attack and stroke risk. Checking these biomarkers lets you catch problems early and tailor food choices, like fiber and unsaturated fats, rather than assuming that running alone is enough.

Hormone biomarkers: cortisol, testosterone, and thyroid

Cortisol is your main stress hormone, released by the adrenal glands to mobilize energy. Short bursts during workouts are normal, but chronically elevated cortisol is tied to overtraining, poor sleep, mood changes, and more frequent infections in endurance athletes.^[2] Testosterone is the primary male sex hormone; in men it supports muscle mass, red blood cell production, bone density, mood, and sex drive. High-volume endurance training, especially with low energy intake, can suppress testosterone, creating what some researchers call the “exercise-hypogonadal male condition.”^[5]

Guidelines and meta-analyses suggest that symptomatic men with total testosterone below about 350 ng/dL (≈12 nmol/L) are most likely to benefit from testosterone replacement, and that free testosterone below 100 pg/mL supports a diagnosis of hypogonadism when total levels are borderline.^[6],[7] Thyroid hormones, especially thyroid-stimulating hormone (TSH) and free thyroxine (free T4), regulate metabolism and heart rate; abnormal values can cause fatigue, weight changes, and altered heat tolerance that blunt training gains.

Inflammation and recovery biomarkers: C-reactive protein and heart rate variability

C-reactive protein (CRP) is a protein made by the liver when inflammation rises. High-sensitivity CRP (hs-CRP) detects low-grade chronic inflammation and is used to estimate future cardiovascular risk. Regular, moderate exercise tends to lower baseline CRP, but very hard or excessive training can spike levels along with muscle damage and illness risk.^[3] Heart rate variability (HRV) is the tiny variation in time between each heartbeat; higher resting HRV usually signals better recovery and parasympathetic (“rest and digest”) activity, while sustained drops in HRV can flag accumulated fatigue or illness.^[5]

Conditions linked to it

When essential biomarkers drift out of range, runners are more likely to face specific health and performance problems. Some show up quickly in training. Others build over years.

• Iron deficiency and iron-deficiency anemia: Low ferritin and iron can cause heavy legs, shortness of breath at familiar paces, headaches, and ultimately anemia. In controlled studies, correcting iron deficiency in athletes improves VO₂ max and time-trial performance, especially in those who were clearly low to begin with.
• Bone stress injuries: Low vitamin D, inadequate calcium, and low testosterone in men all increase the risk of stress reactions and stress fractures in weight-bearing bones. Military and athletic cohorts with lower 25-hydroxyvitamin D levels have significantly more stress fractures than those with higher levels.
• Overtraining and relative energy deficiency in sport (RED-S): Chronic high cortisol, low sex hormones, and suppressed immune markers characterize overtraining syndrome and RED-S, conditions where training load and energy intake are out of balance. Consensus statements note persistent fatigue, mood changes, and performance decline despite rest as key features.^[2]
• Cardiometabolic disease: Elevated fasting glucose, HbA1c, LDL cholesterol, triglycerides, and CRP increase the risk of type 2 diabetes, heart attack, and stroke over time, even in physically active people.^[3],[8]
• Exercise-associated hypogonadism in men: In some male endurance athletes, long-term high mileage combined with low energy intake lowers testosterone into the hypogonadal range, with symptoms like low libido, reduced morning erections, and weaker bones.^[5]

Limitations note: Many of these links come from observational studies, which show associations but cannot always prove cause and effect. Individual responses vary: one runner may tolerate low-normal ferritin or vitamin D better than another. Biomarkers should always be interpreted in context, alongside symptoms, performance trends, and medical history.

Symptoms and signals

You do not need to chase every lab test. Instead, watch for patterns that suggest it is time to check the essential biomarkers runners rely on to stay healthy.

• Persistent fatigue or “heavy legs” that does not improve with a week of lighter training
• Slower paces at the same heart rate or effort, or an unexplained drop in race performance
• Shortness of breath or pounding heart on hills that used to feel easy
• Frequent colds, sinus infections, or lingering respiratory illnesses
• Stress fractures, recurring shin pain, or bone tenderness that worsens with impact
• Muscle cramps, twitching, or unusual heart flutters during or after runs
• Unintentional weight loss, low appetite, or feeling cold all the time
• Low mood, irritability, poor sleep, or waking unrefreshed most days
• For men: reduced morning erections, lower sex drive, or loss of strength in the gym
• Resting heart rate higher than your normal baseline for several days, or a sustained drop in HRV if you track it

One bad week is normal. When several of these signs line up for more than 2–3 weeks, it is smart to talk with a clinician about checking key biomarkers rather than doubling down on “mental toughness.”

What to do about it

The goal is not to medicalize every mile, but to use lab data as one more tool, along with your training log and how you feel. Here is a simple three-step approach.

1. Get a targeted testing plan. Work with a primary care doctor or sports medicine physician who is comfortable with endurance athletes. Ask about a baseline panel that usually includes:
   • Complete blood count (CBC), iron, ferritin, and transferrin saturation for oxygen-carrying capacity
   • 25-hydroxyvitamin D and serum magnesium for bone and muscle health
   • Fasting glucose, HbA1c, and a lipid panel for metabolic health
   • TSH and free T4 for thyroid function
   • Morning total testosterone, and if needed free testosterone, for men with low energy, libido, or strength
   • High-sensitivity CRP; plus your HRV trend if you track it with a watch or chest strap

   For most healthy runners, repeating this set once a year, or before and after a heavy training block, is enough. If you are correcting a known deficiency, your doctor may recheck every 8–12 weeks.

2. Make targeted lifestyle and treatment changes. Once you know your numbers, adjust your inputs.
   • Nutrition: Emphasize iron-rich foods like lean red meat, dark poultry, beans, and lentils, paired with vitamin C sources to boost absorption. If ferritin or iron is low, your clinician may prescribe iron supplements, which should always be monitored because excess iron can damage organs.
   • Bone and muscle support: If vitamin D is low, your doctor may recommend a specific dose of vitamin D₃ to reach at least 30 ng/mL, plus adequate calcium and magnesium from foods like dairy, nuts, seeds, and leafy greens.^[1],[4]
   • Training and recovery: Use a periodized plan with hard, moderate, and easy days, and at least one full rest day per week. Consensus guidelines on overtraining stress the importance of sleep, simple carbs around key workouts, and lower-intensity aerobic time to keep cortisol and inflammation in check.^[2]
   • Metabolic health: If glucose, HbA1c, or lipids are borderline, tighten up portions of added sugars and refined carbs, add more fiber and unsaturated fats, and include 2–3 strength sessions weekly. These steps improve insulin sensitivity and lipid profiles in people with and without diabetes.^[8]
   • Hormone care: For men with symptoms and low testosterone, address sleep, caloric intake, and training volume first. If levels stay below about 350 ng/dL and free testosterone is under 100 pg/mL with ongoing symptoms, guidelines suggest that testosterone therapy may be considered under specialist care.^[6],[7]
3. Monitor, don’t obsess. Track how changes affect your energy, mood, paces, and heart rate or HRV over 8–12 weeks. Retest the key biomarkers you and your clinician are following, then adjust. The aim is stable, healthy ranges and consistent training, not hitting some “perfect” lab number.

Myth vs Fact: Biomarkers and running performance

• Myth: Only elite runners need to care about lab tests.
  Fact: Studies show iron and vitamin D issues in recreational and competitive athletes alike. You do not need a pro contract to benefit from checking essential biomarkers.
• Myth: If you eat “clean,” your labs are automatically fine.
  Fact: Even very healthy diets can fall short on iron or vitamin D, especially for runners who avoid red meat or live at northern latitudes.
• Myth: More supplements always mean better performance.
  Fact: Too much iron, vitamin D, or testosterone can be dangerous. Doses should be personalized and supervised, not based on social media advice.
• Myth: One blood test tells you everything you need to know.
  Fact: Trends over time matter more than a single snapshot. Context from symptoms, training logs, and repeat labs is key.
• Myth: Normal lab ranges mean you cannot have a problem.
  Fact: “Normal” lab ranges are broad. A ferritin of 15 ng/mL may be technically normal but still too low for a marathoner logging 60 miles per week.

Bottom line

GPS data shows what happened on the road. Biomarkers explain why it felt the way it did. By focusing on a manageable set of essential biomarkers runners can uncover hidden iron or vitamin D issues, spot early signs of overtraining or hormonal imbalance, and protect long-term heart and bone health. Work with a clinician who understands endurance sports, test with a purpose, and then let the numbers guide smarter choices about food, training, and recovery rather than chasing every new lab or supplement trend.

References

1. Farrokhyar F, Tabasinejad R, Dao D, et al. Prevalence of vitamin D inadequacy in athletes: a systematic-review and meta-analysis. Sports medicine (Auckland, N.Z.). 2015;45:365-78. PMID: 25277808
2. 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
3. Kasapis C, Thompson PD. The effects of physical activity on serum C-reactive protein and inflammatory markers: a systematic review. Journal of the American College of Cardiology. 2005;45:1563-9. PMID: 15893167
4. Nielsen FH, Lukaski HC. Update on the relationship between magnesium and exercise. Magnesium research. 2006;19:180-9. PMID: 17172008
5. Hackney AC. Effects of endurance exercise on the reproductive system of men: the “exercise-hypogonadal male condition”. Journal of endocrinological investigation. 2008;31:932-8. PMID: 19092301
6. 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
7. Mulhall JP, Trost LW, Brannigan RE, et al. Evaluation and Management of Testosterone Deficiency: AUA Guideline. The Journal of urology. 2018;200:423-432. PMID: 29601923
8. Colberg SR, Sigal RJ, Fernhall B, et al. Exercise and type 2 diabetes: the American College of Sports Medicine and the American Diabetes Association: joint position statement. Diabetes care. 2010;33:e147-67. PMID: 21115758
9. Plews DJ, Laursen PB, Kilding AE, et al. Heart rate variability in elite triathletes, is variation in variability the key to effective training? A case comparison. European journal of applied physiology. 2012;112:3729-41. PMID: 22367011