Iron, Vitamin D and B12: The Bloodwork Athletes Should Actually Check

There is a pattern that plays out constantly among endurance athletes, and it costs people entire seasons.

Training gets harder. Familiar paces feel wrong. Heart rate sits higher than it should. Recovery between sessions stops working. So you do the sensible thing — deload, sleep more, eat more, back off the volume — and it does not help. Then you conclude you are overtrained, or getting older, or that the programme is wrong.

Sometimes it is none of those. Sometimes it is a blood test.

The one that matters most: ferritin

Iron deficiency is the most common nutritional deficiency worldwide, it is markedly more common in endurance athletes, and its symptoms are almost perfectly disguised as overtraining.

Why athletes lose iron faster

Several mechanisms stack:

• Foot-strike haemolysis — repeated impact damages red blood cells in the feet. Real, and it accumulates over high running volume
• Sweat losses, small per session but not trivial across a heavy training week in heat
• Gastrointestinal losses, which increase with hard endurance training
• Menstrual losses, which is why iron deficiency is substantially more common in menstruating athletes
• Reduced absorption after hard exercise, driven by a hormone called hepcidin that rises with training-induced inflammation and temporarily blocks iron uptake

That last one is a genuinely awkward mechanism: training itself makes iron harder to absorb, for hours afterwards.

Why haemoglobin is not enough

This is the single most useful thing in this article.

Iron depletes in stages. Stored iron goes first — that is ferritin. Only once stores are exhausted does haemoglobin start to fall, which is the point at which you are formally anaemic.

Performance is affected in the first stage. Iron is not only in haemoglobin; it is central to the mitochondrial machinery that produces aerobic energy. An athlete with depleted stores and normal haemoglobin can have measurably impaired endurance capacity.

Which means the common experience of being told "your bloods are normal" is frequently a haemoglobin result, when the relevant number was never checked. Ask for ferritin specifically.

The reference ranges on lab reports are population ranges and their lower bounds are set for the general population, not for endurance athletes. Many sports medicine practitioners work with higher thresholds for athletes. What counts as adequate for you is a conversation with a doctor who knows you train — but you cannot have that conversation without the number.

One important caveat: ferritin is an acute-phase reactant, meaning it rises with inflammation and infection. A test taken shortly after a hard session or during illness can read misleadingly high. Test rested, ideally not within a day or two of a hard workout.

Vitamin D

Widely deficient, particularly in people who train indoors, live at higher latitudes, cover up in the sun, or have darker skin.

Why it matters for athletes: bone health primarily, with roles in muscle function and immune function. Low status is associated with higher stress fracture risk, which is the injury that ends seasons rather than weeks.

Worth knowing: the supplementation literature is less impressive than the deficiency literature. Correcting a genuine deficiency helps. Supplementing when already sufficient does not appear to improve performance. This is the standard shape for micronutrients — fixing a deficit works, topping up does not.

Cheap to test, cheap to correct.

Vitamin B12

Most relevant if you are vegetarian or vegan. B12 occurs naturally in meaningful amounts almost exclusively in animal products, and deficiency produces fatigue, poor concentration and, if prolonged, neurological problems.

Vegans should supplement B12 as a matter of course rather than testing and hoping. Vegetarians who eat dairy and eggs are usually adequate but not reliably so, and are worth testing. The wider picture for plant-based athletes is in the vegetarian hybrid athlete.

The rest of a sensible panel

Full blood count. Catches anaemia, gives red and white cell picture, and is bundled with almost every test anyway.

Thyroid function. Worth including when fatigue is persistent and unexplained, since an underactive thyroid produces exactly the symptom picture of chronic under-recovery.

Basic metabolic panel — kidney and liver function, electrolytes. Broad, cheap, and occasionally catches something unrelated.

Hormonal markers, if the cycle has become irregular or absent, or in male athletes with persistent fatigue and loss of drive at high training volumes. Both can indicate low energy availability, which is a serious finding rather than a training quirk — see the discussion in training through the menstrual cycle.

That is the whole list. The expansive athlete panels sold direct-to-consumer add dozens of markers with no established action attached to them, and the main outcome is anxiety about numbers nobody knows how to interpret.

When to test

Test if:

• Fatigue has persisted for more than two or three weeks despite genuine rest
• Familiar paces feel disproportionately hard, or your heart rate at those paces has climbed
• You are a menstruating endurance athlete, especially with heavy bleeding
• You are vegetarian or vegan and training seriously
• You are training at high volume and something feels off
• You have been recovering poorly for a while and cannot explain why

Do not test:

• Reflexively, every eight weeks, chasing optimisation
• Immediately after a hard session or during illness, when several markers are distorted
• Instead of addressing sleep and food, which are more likely to be the problem than any deficiency

The thing not to do

Do not supplement iron without testing.

Iron is one of the genuinely dangerous nutrients to take unnecessarily. The body has no efficient mechanism for excreting excess, and iron overload causes real organ damage. A meaningful minority of people carry a genetic predisposition to accumulate it.

There is also a large group of athletes taking iron because a forum suggested it, with no test, no follow-up, and gastrointestinal side effects they have accepted as normal. If a test shows depletion, correct it under medical guidance — including the practical detail of when and how to take it, since absorption is affected by timing relative to training, coffee, tea and calcium.

This is the exception to the general principle in the four supplements with actual evidence: most supplements are wasted money, and iron is one where being wrong is worse than wasted.

The summary

If you are inexplicably tired and training is going backwards, the reflex is to change the training. Sometimes the answer is a ferritin result and eight weeks of correction.

Ask for ferritin, not just haemoglobin. Add vitamin D and B12. Test rested. Take the results to a doctor rather than a forum, and do not supplement iron on a hunch.

It is an inexpensive test that occasionally explains an entire lost season — and it is the first thing worth ruling out before concluding that your body has simply stopped responding to training.