What a sports nutritionist actually reads in your blood work

A runner came to me after four months of feeling slow. Her easy pace had turned into work, her legs were heavy on climbs, and her doctor had run blood work and told her it was all normal, so she assumed the problem was her training, or her age, or her head. Her ferritin was 15. For a general adult that reads as inside the range. For an endurance runner it is low enough to explain every symptom she described, and nobody had connected the number to how she felt.

That gap is the whole reason blood work is worth understanding as an athlete. The lab compares you to the general population. Your training does not care about that comparison. A few numbers that look fine on the report can quietly explain why your sessions feel flat, and the most common one gets waved through because the reference range was never drawn for someone who trains.

TL;DR: Read through a sports lens, a handful of blood markers explain a lot of unexplained fatigue in athletes: iron (ferritin), vitamin D, and B12. The catch is that "normal" on a lab report is set for the general population, so a number can sit in range and still be low enough to hold back your training.

"Normal" and "good for training" are not the same thing

The single most useful thing to know about your blood work is that a normal flag means "not a medical problem for an average person," not "optimal for an athlete." Iron is the clearest example. Sports researchers stage iron status roughly as sufficient stores above 100 micrograms per litre, a functional shortfall between 30 and 99, and outright deficiency below 30 (Sim et al., 2019). Many labs only flag ferritin as low below about 15, so a runner at 20 or 30 is told she is fine while sitting in the range where performance and energy commonly suffer.

This is why the same frustration turns up again and again in running forums: "It's wild how often it gets brushed off as 'still in range'." The number inside the range can be the reason you feel slow. The range was built for the population, not for your training. That is not a knock on your doctor, whose job is to catch disease. It is just a different question from the one an athlete is asking, which is why the results deserve a second read with training in mind.

Iron: the number behind most athlete fatigue

If one marker explains unexplained tiredness in athletes, it is iron. "Iron plays a significant role in the body, and is specifically important to athletes, since it is a dominant feature in processes such as oxygen transport and energy metabolism," write Sim and colleagues (2019). Iron deficiency affects roughly 15 to 35% of female athletes, far more than the general population, and endurance runners are especially exposed through sweat, foot-strike, and monthly losses (Sim et al., 2019).

What it feels like is specific, and athletes describe it the same way: "Anxiety, legs feel heavy. Going up a flight of stairs or up a hill makes you winded. Lethargy all the time." If that is you, the markers to look at are ferritin, haemoglobin, and transferrin saturation together, not ferritin alone. One timing detail matters and is easy to get wrong: ferritin rises with inflammation and can read falsely high for up to 72 hours after a long or hard session. Test on a rested day, first thing, or you may get a reassuring number that hides a real shortfall.

Vitamin D and B12: the quieter two

Two more markers earn a place because deficiency is common and both affect how you feel and recover. Vitamin D insufficiency shows up in around 30% of adult elite athletes and higher in adolescents (Harju et al., 2022), and it climbs in anyone who trains indoors or lives somewhere with dark winters, which describes a lot of athletes in Europe. Low vitamin D links to fatigue, weaker immune defence, and stress-fracture risk.

B12 and folate matter because they build the red blood cells that carry oxygen. Run low and you get the same heavy-legged tiredness as low iron, from a different cause. B12 is worth a look especially for plant-based athletes, since it comes reliably only from animal foods, fortified foods, or a supplement. None of these three markers is exotic. They are just the ones most likely to explain a training slump, which is why they are the ones I look at first.

What a nutritionist does with the numbers, and what needs a doctor

Reading blood work as a sports nutritionist is about translation, not diagnosis. My job is to connect a marker to how you train and eat, then build the plan around it: more iron-rich food paired with vitamin C, better timing of meals and recovery, a look at whether you are simply eating enough to support your training. When a result points to something clinical, that goes to a doctor, and I say so plainly.

Iron is the one place to be careful, because more is not better and too much is harmful. Never start an iron supplement on a hunch. Test first, and if it is low, sort the dose and the follow-up with a professional rather than guessing from a forum. Blood work gives you a map. It does not replace the doctor who reads the parts of it that are theirs to read.

Before you order a panel

You do not need the biggest test menu on the site. You need the few markers that matter, drawn at the right time and read with training in mind.

• Test the ones that explain fatigue first: ferritin, haemoglobin, and transferrin saturation for iron, plus vitamin D and B12. Add more only with a reason.
• Time it right: rested day, morning, not in the 72 hours after a long or hard session, so inflammation does not inflate your ferritin.
• Read results against training, not just the lab flag: a "normal" ferritin of 20 to 30 in a symptomatic endurance athlete deserves a closer look.
• Take clinical findings to a doctor, and never self-prescribe iron.

Frequently asked questions

What blood tests should athletes get?

Start with the markers most likely to explain fatigue: ferritin, haemoglobin, and transferrin saturation for iron status, plus vitamin D and B12. That short panel covers the common causes of unexplained tiredness in athletes. Add hormonal or other tests only when symptoms or a doctor give a specific reason.

Can low iron cause fatigue if I'm not anaemic?

Yes. You can have normal haemoglobin and still have low iron stores, and that alone is linked to fatigue, heavy legs, and reduced endurance in athletes. This is why ferritin matters, not just the haemoglobin that defines anaemia. Test both, and read ferritin against athlete ranges rather than the general population flag.

What's a good ferritin level for runners?

Many sports practitioners want to see ferritin comfortably above 30 micrograms per litre, and often higher for endurance runners, rather than just above the lab's low flag near 15. Ranges and goals vary by person, so use a symptomatic low-but-"normal" result as a reason to look closer with a professional, not to self-treat.

How often should athletes get blood work?

For most healthy athletes, once or twice a year is plenty, or when symptoms like persistent fatigue appear. Endurance athletes and those with a history of low iron may benefit from checking iron markers more regularly. More testing is not automatically better, and each result is most useful when read alongside your training.

Your blood work is more than a disease screen. Read with training in mind, a few markers explain the tiredness that willpower cannot, and "normal" is not the same as good for your sport. If you want someone to read your results in the context of how you train and eat, and build a plan around what they show, book an online consultation and bring your recent labs. Message Bela on WhatsApp to book a consultation and review your blood work together.

This article is educational and is not a substitute for individual medical advice, diagnosis, or treatment. Discuss your results and any supplements with a qualified health professional.