Endurance is the engine. Recovery is the pit crew. If you want to go long and bounce back strong, you need both running smoothly. Blood and wearable data won’t win a race for you, but they can tell you how your oxygen delivery, fuel systems, and repair crews are actually performing. What should you measure, when should you measure it, and how do you make sense of the results without getting lost in the noise?
Let’s translate lab values and physiology into plain English so your training story has a clear plot line. Ready to see what your biomarkers are really saying?
The physiology of endurance and recovery, in plain language
Endurance is about moving oxygen to hungry muscles, delivering the right fuel at the right time, and clearing waste so you can keep going. Recovery is the cleanup and rebuild phase that restores glycogen, quiets inflammation, and repairs microscopic damage. Think of a long run or ride as a controlled stress test. Your body adapts by upgrading the network: stronger heart, more capillaries, more mitochondria, smarter fuel use. The biomarkers below map those upgrades and the friction points that slow them down. Which of these levers might be holding you back?
Biomarkers that map your oxygen delivery system
Hemoglobin and hematocrit
Hemoglobin carries oxygen. Hematocrit is the fraction of your blood made up by red blood cells. Together they set your oxygen delivery ceiling. Lower numbers mean less oxygen per heartbeat and can feel like running in soft sand. Dehydration can fake a high hematocrit by shrinking plasma volume, while post-exercise hemodilution can nudge it down. For actionable signals, test when well hydrated and not immediately after a hard session. Endurance at altitude, menstruation, and age all shape these values — so does genetics.
Ferritin and transferrin saturation
Ferritin is your iron storage protein. Transferrin saturation reflects how much circulating iron is available for red blood cell building. Endurance athletes who train hard, sweat a lot, or lose iron through menstruation frequently land in low ferritin territory. Performance hits tend to show up when ferritin is low even before anemia develops, often around the 20 to 30 ng/mL range in clinical practice. Transferrin saturation adds context: low storage plus low saturation points to iron scarcity, while normal saturation with low ferritin may reflect recent stress or inflammation. Exercise acutely raises hepcidin — a hormone that blocks iron absorption for several hours — so timing matters if you’re repeating tests.
Reticulocyte hemoglobin and red cell indices
Reticulocyte hemoglobin shows how well your newest red blood cells are being loaded with iron. It changes sooner than hemoglobin when iron supply drops. Mean corpuscular volume and red cell distribution width can hint at chronic iron issues versus other causes of anemia. Foot-strike hemolysis in runners and GI blood loss can also nudge these markers. A pattern across several values tells the real story. What does your pattern suggest?
Biomarkers that reflect muscle damage and repair
Creatine kinase
Creatine kinase rises when muscle fibers are stressed and leaky. After hill repeats or heavy eccentric work, it can spike for a day or two. The range is wide and influenced by sex, body size, and ancestry, so context is everything. A trend that keeps climbing despite easy days can signal you are not repairing between sessions. A moderate bump that settles quickly can be normal training adaptation. Which pattern looks like yours this season?
AST and ALT nuances
These “liver enzymes” are actually found in muscle too. After hard endurance bouts, AST often rises more than ALT because skeletal muscle leaks it. That’s not liver injury in a healthy athlete; it’s a mirror of muscle stress. Persistent elevation with other signs of illness or performance decline deserves a deeper look, but isolated post-race bumps are a known training artifact.
Myoglobin and high-sensitivity troponin
After marathons and ultras, transient bumps in myoglobin and even high-sensitivity cardiac troponin have been documented in healthy athletes. The current view in sports cardiology is that these short-lived increases reflect membrane stress and turnover rather than heart damage in people without symptoms. These are not routine training markers, but they explain why testing right after a big day can send confusing signals. If you test, do it on a rested day unless there’s a clinical reason not to wait.
Inflammation and recovery readiness
High-sensitivity C-reactive protein
hs-CRP is a sensitive barometer of systemic inflammation. It rises with infection, tissue damage, and hard efforts. A low baseline is the target for long-term cardiovascular health. Elevated values right after hard training are expected; persistently high values at rest can blunt recovery, impair sleep, and chip away at performance. In studies, hs-CRP tends to normalize within days after acute exertion, so use a rested morning sample to see your true baseline. What is your baseline saying about your recovery environment?
White blood cell count
A surge after races is common, driven by stress hormones and tissue signaling. A low or high count at rest, especially with fatigue or illness, can point to infection or other stressors. It’s a blunt instrument but useful when paired with symptoms and training logs.
Hydration and electrolyte balance
Serum sodium, potassium, chloride, and bicarbonate
Hydration is more than water. Sodium shapes fluid shifts and nerve signals, potassium is critical for muscle contraction, chloride and bicarbonate reflect acid-base balance. Hyponatremia happens when fluid intake outpaces sodium replacement during long events, leading to headaches, confusion, or worse. On the flip side, hemoconcentration from dehydration can push sodium up. Testing at rest won’t mirror on-course values, but it flags baseline issues and guides how your body handles fluid and salt. Heavy sweaters, heat exposure, and long durations change the math.
Osmolality, BUN, creatinine, and urine specific gravity
Serum osmolality aggregates the solutes that pull water across membranes. Blood urea nitrogen and creatinine rise with dehydration and catabolic stress; their ratio can hint at protein breakdown versus fluid shifts. Urine specific gravity gives a quick snapshot of concentration. None of these alone proves your hydration strategy is ideal, but together they tell you if your kidneys and fluid balance are cruising or compensating. How stable are yours across a training block?
Energy and metabolic biomarkers
Fasting glucose, HbA1c, and fasting insulin
Endurance training improves insulin sensitivity for most people, but low energy availability and chronic stress can push glucose up. HbA1c shows the three-month average but can be confounded by red cell turnover in athletes. Fasting insulin adds context on baseline insulin demand. A low resting insulin and normal glucose usually reflects efficient glucose handling. If fasting values trend high at rest, sleep, stress, or nutrition patterns may be driving it.
Triglycerides and lipid profile
Lower fasting triglycerides and higher HDL are common in trained individuals, reflecting better fat oxidation and lipoprotein turnover. Very low triglycerides with fatigue and weight loss can point to underfueling. Changes here track your metabolic mix, not just heart risk.
Lactate and lactate dynamics
Resting lactate is often normal. The gold is in how lactate rises with workload and clears in recovery. Field or lab step tests map your lactate threshold — the tipping point where carbohydrate dominates and fatigue accelerates. Blood lactate strips, properly used, translate feel into physiology. They’re sensitive to temperature, timing, and technique, so controlled protocols matter for clean data. What does your curve look like as you progress?
Ketones in specific contexts
Training low on carbohydrates or long fasted sessions can raise blood ketones, indicating a shift toward fat use. Ketone levels are not a performance badge by themselves. They simply reflect the fuel strategy you’re imposing. Consistency and context are key if you track them.
Hormones and load tolerance
Thyroid panel
TSH with free T4 assesses thyroid drive. Low thyroid function slows heart rate response, reduces heat tolerance, and makes efforts feel heavier. In low energy availability, free T3 can fall even with normal TSH, signaling a conservation mode. Thyroid tests are susceptible to assay differences and biotin interference, so use the same lab and avoid high-dose biotin for at least a day before sampling unless advised otherwise.
Cortisol and DHEA-S
Cortisol is your stress mobilizer. Morning levels are naturally higher and best for comparison. Chronically elevated cortisol at rest, with poor sleep, mood changes, and flat training response, can point toward too much load and too little recovery. DHEA-S trends the other way over the long term and can offer a counterpoint. These are not standalone “overtraining” tests, but they add color to the picture when combined with symptoms and performance data.
Sex hormones and energy availability
Testosterone in men and estradiol in women intersect with muscle repair, bone health, and energy. In women, menstrual regularity is a frontline biomarker. Amenorrhea or cycle disruption with hard training raises a flag for low energy availability and iron issues. In men, a sustained drop in morning testosterone with fatigue and declining output can signal the same imbalance. These patterns deserve thoughtful interpretation alongside diet, training load, and iron status.
Micronutrients that move the needle
Vitamin D
Vitamin D influences bone remodeling and muscle function, though performance effects are modest in well-nourished athletes. Levels swing with season and sun. If you train indoors or at higher latitudes, a winter check can be informative. The sweet spot varies slightly by guideline, so interpret in context with bone stress history and overall nutrition.
Vitamin B12 and folate
These are the co-factors for red blood cell production. Low levels can mimic iron deficiency with fatigue and poor endurance. Vegans and older adults are at higher risk for B12 deficiency. Measuring both clarifies the picture when red cell indices drift.
Magnesium
Magnesium supports muscle contraction and energy enzymes, but serum levels often look normal even when tissue levels are low. Red blood cell magnesium or ionized magnesium may track status better, though assays vary. If cramps, fatigue, or arrhythmia-like palpitations show up, magnesium status is a reasonable piece of the puzzle.
Cardiorespiratory fitness markers you can track without a needle
Resting heart rate
As your heart gets stronger and stroke volume rises, resting heart rate usually falls. A sudden morning bump for several days can flag insufficient recovery, illness brewing, or poor sleep. Single spikes happen. Trends tell the truth.
Heart rate variability
HRV tracks the balance between sympathetic and parasympathetic tone. Higher morning HRV at your personal baseline often pairs with better readiness, while suppressed HRV can reflect stress load. Devices differ in algorithms, sampling windows, and noise handling, so compare against your own baseline rather than someone else’s numbers.
VO2max and threshold estimates
Lab VO2max is the gold standard for aerobic capacity, but field-test estimates and device models are useful for trending. Threshold pace or power is arguably more actionable, linking to how long you can sustain a hard effort. Pair these with lactate or heart rate data and you get a tighter map of your engine.
Timing, context, and assay quirks that change results
Post-exercise effects
Hard efforts change labs for hours to days. CK, AST, creatinine, lactate, white cells, and hs-CRP can all rise transiently. Plasma volume shifts can push hematocrit up or down. If you want baseline data, sample in the morning, well hydrated, after 24 to 48 hours without hard training.
Hydration and heat
Dehydration concentrates blood. Overhydration dilutes it. Heat training expands plasma volume, often nudging hematocrit lower while improving performance. Those shifts reflect adaptation, not pathology.
Menstrual cycle, contraception, and iron
Bleeding patterns change iron needs. Oral contraceptives can flatten cycle-related variability and may lower monthly iron losses. Perimenopause often brings erratic bleeding and iron swings. These aren’t side notes; they materially change ferritin and hemoglobin trajectories.
Altitude
Short stays raise heart rate and may drop plasma volume. Weeks at altitude stimulate erythropoietin and new red cells. Ferritin can fall as you build hemoglobin. Plan testing around these cycles to understand whether changes reflect adaptation or deficiency.
Laboratory methods and interferences
Reference ranges differ by lab. Hemolyzed samples can falsely elevate potassium and lactate dehydrogenase. High-dose biotin supplements can skew certain immunoassays, including thyroid and troponin. Whenever possible, use the same lab, time of day, and pre-test routine to keep your data comparable. How tight is your sampling protocol?
Building a sensible testing cadence
Start with a rested baseline that includes iron status, complete blood count, basic metabolic panel, hs-CRP, and selected hormones based on your history. Add CK or lactate if you are experimenting with load or threshold testing. Recheck at strategic points in a training cycle rather than every week. When you change altitude, diet, or training volume, anchor the change with a before-and-after sample to isolate the effect. Pair labs with device trends, workouts, and how you actually feel. Which levers will you choose to track consistently?
Putting biomarkers to work: real-world snapshots
The slow fade on long runs
An athlete feels fine at easy paces but fades early in workouts. Ferritin comes back low, transferrin saturation low-normal, hemoglobin slightly down, hs-CRP normal. This pattern points toward iron-limited oxygen delivery rather than inflammation. With iron repletion guided by a clinician, athletes in studies often regain durability within weeks to months, once new red cells mature. Which fuel system is limiting your durability right now?
The heavy legs after hill repeats
Another athlete hits weekly hills and sees creatine kinase jump for 48 hours, AST up a bit, ALT steady, but performance returns by day three. That pattern looks like normal eccentric muscle remodeling. If CK stays high and sessions feel worse, the signal shifts toward excess load without repair. What does your recovery curve look like between hard sessions?
The heat wave wobble
During a hot spell, resting heart rate climbs, HRV dips, sodium sits at the low end of normal, and body mass drops after long runs. On cooler weeks, markers normalize. This is a hydration and heat adaptation story, not a mystery illness. Gradual heat exposure expands plasma volume and steadies those numbers. How does your data shift when the environment changes?
The restless plateau
A plateauing athlete shows rising morning cortisol, falling HRV, poor sleep, and elevated hs-CRP at rest despite modest training. Thyroid panel is normal. The physiology points toward cumulative stress load rather than a gland problem. Adjusting recovery inputs typically moves these markers back toward baseline in observational data. Which recovery inputs are most underpowered for you?
What not to over-interpret
Single values rarely tell the story. Athletes can sit slightly outside population “normal” and be perfectly healthy. CK that scares a nonathlete can be routine after a long run. HbA1c can read lower or higher depending on red cell lifespan rather than glucose alone. hs-CRP can spike after a big day and settle quickly. Trends, context, symptoms, and performance are the truth serum. Which trends feel trustworthy in your own data?
The bottom line
Endurance and recovery are systems problems, and biomarkers are system signals. Use them to map oxygen delivery, fuel handling, tissue stress, and inflammation. Sample under consistent conditions, watch trends, and interpret in context with training logs and how you feel. When data and experience rhyme, adaptation accelerates. What signal will you choose to track first to make your next block smarter?
Join Superpower today to access advanced biomarker testing with over 100 lab tests.
