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In Threshold #145, we explored how hemoglobin and red blood cells form the foundation of your oxygen delivery system. But there’s a critical element underpinning that entire process — iron. Without sufficient iron, your body can’t build hemoglobin efficiently, and no amount of VO₂ max intervals or long runs will close that gap.

Ferritin, the storage form of iron, is the most telling biomarker endurance athletes should monitor. Chronically low ferritin can sap your speed, blunt training adaptation, and leave you feeling flat no matter how disciplined your workouts are.

So, how does iron support endurance, why ferritin is such a vital number, and how do you optimise your levels for sustained performance?

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The Main Feature

Leg 1: The Science of Iron and Endurance Performance

Iron is a cornerstone mineral in endurance physiology. At the most basic level, it’s the central atom in hemoglobin, binding oxygen in the blood and transporting it from lungs to muscles. It’s also present in myoglobin, which stores oxygen within muscle tissue, ensuring a steady supply during intense efforts.

But its role doesn’t end there. Iron is crucial for the function of mitochondrial enzymes, such as cytochromes, that drive oxidative phosphorylation — the process your cells use to generate ATP from oxygen and nutrients. Without adequate iron, mitochondrial efficiency plummets, and your capacity to sustain aerobic effort collapses.

Low ferritin means your iron reserves are depleted, making it harder to maintain hemoglobin synthesis. Even if your hemoglobin levels are still in the “normal” range, depleted ferritin can limit your ability to adapt to higher training loads because you have no buffer to meet increased oxygen demands.

This is why elite endurance coaches track ferritin as closely as training metrics. A runner with a ferritin level under 30 µg/L, for example, may struggle with chronic fatigue, poor interval performance, and slower recovery — even if they’re otherwise healthy.

T1: Mental Preparation

Leg 2: How Low Ferritin Impacts Training and Racing

When ferritin drops, the first sign is often subtle — workouts feel harder than they should. Athletes report “dead legs,” higher-than-usual heart rates for given paces, and a loss of top-end power. Recovery becomes sluggish, and immune resilience takes a hit.

Physiologically, low ferritin undermines performance in three major ways:

1. Reduced Oxygen Transport: With less iron available, hemoglobin production slows, lowering the oxygen-carrying capacity of the blood. That means less oxygen reaches the muscles during high-demand scenarios.

2. Impaired Energy Production: Iron-dependent enzymes in the mitochondria can’t function optimally without adequate stores. Even at submaximal efforts, you’ll burn through energy faster and fatigue sooner.

3. Slower Adaptation: Training signals like those from VO₂ max intervals or long Zone 2 rides only work if your body can respond by building more hemoglobin and improving oxidative enzymes. Low ferritin blunts that adaptation.

These effects are magnified in female athletes, who have higher iron turnover due to menstruation, and in those training at high volumes where microtrauma and sweat loss steadily chip away at iron stores.

Leg 3: Testing, Nutrition, and Supplementation Strategies

The only way to truly know your ferritin status is through a blood test. For most endurance athletes, a ferritin range of 50–150 µg/L supports optimal performance. Below 30 µg/L, symptoms often appear; under 20 µg/L, performance decline is almost guaranteed.

Testing Strategy:

• Test ferritin at least twice a year — pre-season and mid-season — or more often if training volume is very high.

• Pair ferritin testing with hemoglobin and hematocrit for a full oxygen transport profile.

Nutrition:

• Prioritise heme iron sources (red meat, liver, shellfish) for best absorption.

• Include non-heme iron from lentils, spinach, pumpkin seeds — and pair with vitamin C-rich foods to improve uptake.

• Avoid tea, coffee, and calcium-rich foods with high-iron meals as they inhibit absorption.

Supplementation:

• Oral iron (ferrous bisglycinate or ferrous fumarate) can raise ferritin, but dosing should be guided by blood tests to avoid overload.

• Take iron on an empty stomach or with vitamin C for best absorption.

• For athletes with stubbornly low levels, an intravenous infusion may be medically appropriate — but only under supervision.

The aim isn’t to load iron indiscriminately, but to keep your reserves topped up so you can capitalise on training stress without hitting an oxygen delivery bottleneck.

Conclusion

Iron isn’t just a box to tick in your nutrition plan — it’s a fundamental driver of endurance performance. By keeping ferritin in the optimal range through smart nutrition, guided supplementation, and regular testing, you ensure your oxygen transport system is never the weak link in your race-day performance.

Coaches Corner

Workout of the Week: Oxygen Transport Accelerator

Goal: Stimulate hemoglobin production while enhancing aerobic efficiency.

Warm-Up (15 min):

• Zone 1–2 aerobic pace with 3 x 30s strides.

Main Set (35 min):

• 6 x 3 minutes at Zone 5 (95–100% VO₂ max)

• 3 minutes recovery between efforts.

• Focus on strong, rhythmic breathing and tall posture.

Cool Down (10 min):

• Zone 1 easy pace, controlled exhale breathing.

Why it works: This interval format creates an acute oxygen deficit, prompting the body to enhance oxygen transport capacity — a stimulus that, when paired with optimal iron status, amplifies red blood cell production.