Track These 5 Blood Factors for Better Running

October 01, 2025 Tereza Zaťková

Do you feel fatigued while running despite regular training? The issue may not be your plan - it could be what’s happening in your blood. Modern research shows that many blood deficiencies go undetected, yet they significantly affect athletic performance [1].

Why standard blood tests aren’t enough

Most physicians check only basic parameters that reveal severe deficiencies. For runners, it’s crucial to track subtle changes that can influence oxygen delivery to muscles, energy metabolism, and recovery [5].

1) Hidden iron deficiency (without anemia)

What it is: You can have normal hemoglobin but low iron stores (ferritin).
How it affects performance: Iron is essential for oxygen transport and ATP production in mitochondria. Low stores mean reduced oxygen‑carrying capacity and slower recovery [4].
What to monitor:

Serum ferritin: ideally 50–150 μg/L for runners
Transferrin saturation: above 20%
Soluble transferrin receptor
Symptoms: Fatigue during high‑intensity sessions, slower recovery, difficulty maintaining pace.

2) Suboptimal vitamin D levels

What it is: Vitamin D acts more like a hormone than a vitamin and affects muscle function.
How it affects performance: Low vitamin D is linked to reduced muscle strength, higher injury risk, and slower bone recovery [8].
What to monitor:

25‑hydroxyvitamin D: ideally 75–150 nmol/L (30–60 ng/mL)
Aim to stay in the upper part of this range
Symptoms: Frequent muscle cramps, slow healing of micro‑traumas, greater susceptibility to stress fractures.

3) Vitamin B12 and folate deficiency

What it is: These vitamins are essential for red blood cell production and neurological function.
How it affects performance: Even mild deficits can reduce erythrocyte production and impair cognition during hard training [7].
What to monitor:

Vitamin B12: above 400 pmol/L (540 pg/mL)
Red blood cell folate: above 900 nmol/L
Homocysteine: below 10 μmol/L
Symptoms: Mental fatigue on long runs, concentration problems, “brain fog.”

4) Chronic inflammation (elevated CRP and IL‑6)

What it is: Persistent low‑grade inflammation, often not caused by infection.
How it affects performance: Inflammation raises energy demands and slows recovery. It can signal overtraining or inadequate rest [6].
What to monitor:

C‑reactive protein (CRP): below 1.0 mg/L
Interleukin‑6 (IL‑6): below 2 pg/mL
Neutrophil‑to‑lymphocyte ratio

Symptoms: Longer recovery times, more frequent minor illnesses, performance stagnation.

5) Electrolyte imbalance (magnesium and potassium)

What it is: Deficiency of minerals vital for muscle contraction and nerve conduction.
How it affects performance: Magnesium supports 300+ enzymatic reactions; potassium regulates muscle contractions and fluid balance.
What to monitor:

Erythrocyte magnesium: 1.65–2.95 mmol/L
Serum potassium: 3.5–5.0 mmol/L
Sodium‑to‑potassium ratio

Symptoms: Muscle cramps, irregular heart rhythm during exertion, increased fatigue.

What to do next

Comprehensive testing: Ask your doctor for an extended panel - ferritin, vitamin D, B12, folate, CRP, and erythrocyte electrolytes.
Optimal vs. normal values: Athletes often need higher targets than the general population [1].
Regular monitoring: Test at least every 6 months, ideally at different phases of your training cycle.