Your cardio fitness is not a property of your heart alone. It is a chain: air comes in, oxygen steps across into your blood, your heart pumps that blood around, and your muscles pull the oxygen back out. The weakest link sets the whole.
In trained athletes that link sits in the blood surprisingly often.
I have spoken to athletes who spent half a year training harder to solve a problem that was visible in two tubes of blood.
What is cardio fitness exactly?
Cardio fitness is the ability of your heart, blood and muscles to deliver oxygen together during prolonged effort. In the lab it is expressed as VO2max, in millilitres of oxygen per kilo per minute. In practice you notice it in how heavy a pace feels that you held easily last month.
The chain has four links, and training only moves two of them firmly.
Joyner and Coyle summarised endurance performance as the interplay of VO2max, lactate threshold and efficiency (PMID 17901124). Your maximum oxygen uptake is the ceiling in that model, and that ceiling is set partly by how much oxygen every litre of blood can carry.
That is exactly why two athletes on the same plan drift apart.
Which link is the brake for you?
That cannot be settled without measuring, but each link has its own signal in training and its own blood value. The table below lists them, from the lungs to the muscle. Read it as a map, not as a diagnosis.
| Link in the chain | What happens | Related blood value | Possible signal in training |
|---|---|---|---|
| Oxygen uptake into the blood | oxygen binds to your red blood cells | haemoglobin, haematocrit | the same pace feels heavier at the same heart rate |
| Red blood cell production | your iron store supplies the raw material | ferritin, transferrin saturation | intervals collapse, recovery takes longer |
| Pump function and drive | your heart shifts blood per beat | TSH, free T4 | flat heart-rate response, heavy legs |
| Uptake in the muscle | your muscles extract oxygen from the blood | hsCRP as a measure of inflammation | performance drops with no training reason |
These links come from group-level research. An abnormal value does not prove that link is your brake.
How much does a low iron store really cost?
In athletes with a low store but no anaemia the effect is measurable and moderate. Burden and colleagues pooled seventeen studies in endurance athletes. Iron treatment improved iron status substantially and had a moderate effect on VO2max, with an effect size of 0.61 (95% CI 0.40 to 0.82) (PMID 25361786).
In those studies the cut-off for a low store usually sat around 35 micrograms per litre of ferritin.
Note what that says and what it does not. The effect applied to athletes with a deficiency. In athletes with normal stores, extra iron does nothing there, and too much iron carries its own risks.
What makes iron deficiency in athletes so persistent sits in iron deficiency in athletes. The value itself is explained under ferritin.
Why does your GP see nothing while you feel it?
Because a normal result and an optimal result are not the same thing. Reference ranges are derived from the general population, not from people who load their oxygen transport to the limit four times a week. A ferritin of 25 falls inside normal in many labs.
Imagine two men with exactly the same ferritin of 25 micrograms per litre. One sits behind a desk most of the day; the other runs 70 kilometres a week. For the first that number is unremarkable, for the second it can be tight.
That is not a criticism of the GP. It is a difference in question: one asks whether there is disease, the other asks whether there is room. Both are legitimate, and they give different answers to the same result.
So discuss your results with your GP before acting on them. Supplementing on your own off the back of a number is exactly the mistake this piece does not want to encourage.
What does a slow thyroid do to your fitness?
Thyroid hormone drives your metabolism and your heart-rate response. If that drive runs slow, your heart rate under effort can stay lower than you are used to and the same pace feels heavier. That pattern looks a lot like overtraining, and it is often confused with it.
The difference sits in the recovery.
With overtraining things pick up after a rest week. With a thyroid running slow, little changes after that week. That is a clue and not proof, because fatigue has dozens of causes.
How thyroid values and performance relate sits in thyroid and your metabolism. If you suspect you have simply stacked too much, spotting overtraining through your blood values is the better entry point.
How do you improve your cardio fitness?
With a combination of easy distance kilometres and hard intervals, and with a blood side that is not in the way. Intervals at 90 to 95 percent of your maximum heart rate lift your ceiling; easy kilometres build the base beneath it. The full protocol sits in increasing your VO2max.
The order matters.
Train hard for eight weeks against a deficiency and it costs you a block in which you mostly get tired. The Dutch Hartstichting holds a simple line that fits well here: how you feel and how you progress weigh more than one number on your screen.
What counts as a good outcome for your age sits in what is a good VO2max.
Why does this matter beyond your training plan?
The American Heart Association called fitness a clinical vital sign in 2016, because low fitness is associated with a higher risk of cardiovascular disease and mortality, and because fitness sharpens risk classification (PMID 27881567). That makes your fitness more than a sports figure.
It remains an association in large groups.
What it means in practice: every link you free up counts twice. You go harder on the road, and you sit higher on the curve that falls with the years.
What do you do next?
Put your training data and your blood side next to each other before you rebuild your plan. Note your pace at a fixed heart rate for four weeks, and see whether the line drops while your training load stays level. If it drops, you have a question a measurement can answer.
My suggestion: do not change two things at once, or you will learn nothing.
Iron, haemoglobin, thyroid and inflammation markers sit together in InsideTracker. For endurance athletes specifically, the full list sits in blood values for endurance athletes.
Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.
References
- Burden RJ, Morton K, Richards T, Whyte GP, Pedlar CR. Is iron treatment beneficial in iron-deficient but non-anaemic endurance athletes? A systematic review and meta-analysis. British Journal of Sports Medicine, 2015;49(21):1389-97. PMID 25361786.
- Joyner MJ, Coyle EF. Endurance exercise performance: the physiology of champions. The Journal of Physiology, 2008;586(1):35-44. PMID 17901124.
- Ross R, Blair SN, Arena R, et al. Importance of assessing cardiorespiratory fitness in clinical practice: a case for fitness as a clinical vital sign. Circulation, 2016;134(24):e653-e699. PMID 27881567.
- Hartstichting. Exercise and your heart rate.
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