If you train regularly and have ever had a blood test, there is a good chance at least one result came back flagged, and a good chance the flag meant nothing. Exercise changes blood. It shifts the fluid in your veins, burns through iron, nudges cholesterol and moves hormones around the clock. A doctor reading your results without knowing you ran 40 km last week can see problems that are not there, and miss ones that are.
I read lab reports for a living, mostly for people who lift and run. These are the five blood markers that come up again and again, what they actually tell you, and the one thing about each that training changes.
1. Ferritin: Your Iron Store, And The First Thing Endurance Training Empties
Ferritin is the protein that stores iron, and the blood test for it is the earliest warning of iron deficiency, long before anaemia shows up. Active people run short more often than most. A 2019 review put iron deficiency at roughly 15 to 35 percent of female athlete groups and 5 to 11 percent of male ones, and pointed to a hormone called hepcidin, which rises after hard exercise and blocks iron absorption for hours, as a big part of the reason [1].
What it feels like: heavy legs, a resting heart rate that creeps up, a pace that will not come back however hard you try. What to do: if you are a woman who trains, a runner, or a vegetarian, ask for ferritin with your next test. If it is low, the fix is dietary iron or a supplement under a GP’s guidance, and timing it away from training sessions so hepcidin is not in the way.
2. Haematocrit: The One That Training Lowers On Purpose
Haematocrit is the share of your blood made up of red cells. Lifters worry it is too high. Endurance athletes are told it is too low. Both usually misread the same thing.
Regular aerobic training expands your blood volume, and in the first two to four weeks nearly all of that expansion is plasma, the watery part [2]. Same red cells, more fluid, so the percentage falls. This is why a fit runner can show a haematocrit a point or two below the reference range and be perfectly healthy. It is sometimes called sports anaemia, and it is not anaemia at all. The reverse is also true: test after a hard session or a hot day, dehydrated, and the number reads high because the plasma has temporarily gone.
What to do: give it a rest day and a normal morning’s water before any blood test, and read haematocrit next to haemoglobin and ferritin rather than on its own. If you want the full picture of how to read those three together, this guide to haematocrit and haemoglobin walks through it.
3. Vitamin D: The British Problem
In the UK this is less a sports marker than a weather one. A meta-analysis of 23 studies covering 2,313 athletes found 56 percent had inadequate vitamin D, with the UK among the worst-affected locations, and the risk nearly doubled in winter and spring and rose again for indoor sports [3].
The honest caveat is that the link between vitamin D and injury or performance is weaker than the supplement adverts suggest. A 2018 review noted that data have consistently failed to show a relationship between vitamin D levels and bone health in athletes, that very high-dose supplementation may itself be a problem, and that without symptoms or a specific condition, routine screening may not be needed [4].
What to do: from October to March, follow the NHS advice of a daily 10 microgram supplement, which costs pennies. If you are going to test, do it in late winter when it is lowest, and treat a low result as a reason to take the small daily dose consistently, not a reason to take a huge one.
4. Cholesterol: Training Helps, But Not In The Way The Headline Says
Most people know their total cholesterol and nothing else. The useful numbers sit underneath it: LDL, HDL and triglycerides, and in the UK they come in mmol/L. Exercise improves the profile, but a review of 13 studies found the pattern depends on the type: aerobic work, resistance training and the two combined move different parts of the panel, and the right dose for cholesterol has not been pinned down [5]. The consistent finding is that regular activity of any kind helps, and that more of it helps more.
What to do: get the full panel, not just the total. Ask for non-HDL cholesterol, which is total minus HDL and is what UK guidelines now use. Then keep training. If LDL stays high despite a good diet and regular exercise, that is genetics, and it is a conversation with your GP rather than another supplement.
5. Testosterone: Read The Time On The Test
For men especially, this marker causes more unnecessary worry than any other, and most of it comes from how the sample was taken. Testosterone peaks in the morning and falls through the day. In one study that drew blood from men at six separate visits, levels in men aged 30 to 40 were 20 to 25 percent lower at 4 pm than at 8 am, and several men who looked low in the afternoon were normal at every morning visit [6]. An afternoon test can give you a problem you do not have.
The second trap is the reference range. When researchers cross-calibrated four large studies to a single reference method, the normal range for healthy, non-obese men aged 19 to 39 came out at 264 to 916 ng/dL, roughly 9 to 32 nmol/L, with a median around 18 [7]. That is a wide band, and the lower half of it is still normal.
What to do: test before 10 am, after a normal night’s sleep, and if it is low, repeat it before drawing any conclusion. Hard training, poor sleep and body fat all pull the number down, and all three are fixable without a prescription.
The Two-Minute Rule For Any Flagged Result
Before you worry about a flag, ask three questions. When was the test taken, morning or afternoon? Was I rested and hydrated, or two days off a long run? Is this one number out of range, or a pattern across several? Most flags in active people fail at least one of those questions. The ones that pass, especially a low ferritin or a consistently low morning testosterone, are worth a proper conversation with your doctor. That is what the numbers are for.
Conclusion
Five numbers, one habit. Ferritin tells you whether your iron store can keep up with your training. Haematocrit tells you how much of what you see is water rather than blood. Vitamin D in Britain is mostly a question of the calendar. Cholesterol rewards any regular activity, and the full panel matters more than the headline figure. Testosterone depends on the clock more than most men realise. None of these should be read in isolation, and none should be read without knowing what you did the week before the test. Learn to ask when, how rested and how many before you ask how bad, and your blood results stop being a source of worry and start being what they should be: the most honest feedback your training will ever get.
Sources
- Sim M et al. Iron considerations for the athlete: a narrative review. Eur J Appl Physiol. 2019;119(7):1463-1478. PMID 31055680.
- Convertino VA. Blood volume: its adaptation to endurance training. Med Sci Sports Exerc. 1991;23(12):1338-1348. PMID 1798375.
- Farrokhyar F et al. Prevalence of vitamin D inadequacy in athletes: a systematic-review and meta-analysis. Sports Med. 2015;45(3):365-378. PMID 25277808.
- Owens DJ, Allison R, Close GL. Vitamin D and the Athlete: Current Perspectives and New Challenges. Sports Med. 2018;48(Suppl 1):3-16. PMID 29368183.
- Mann S, Beedie C, Jimenez A. Differential effects of aerobic exercise, resistance training and combined exercise modalities on cholesterol and the lipid profile: review, synthesis and recommendations. Sports Med. 2014;44(2):211-221. PMID 24174305.
- Brambilla DJ et al. The effect of diurnal variation on clinical measurement of serum testosterone and other sex hormone levels in men. J Clin Endocrinol Metab. 2009;94(3):907-913. PMID 19088162.
- Travison TG et al. Harmonized Reference Ranges for Circulating Testosterone Levels in Men of Four Cohort Studies in the United States and Europe. J Clin Endocrinol Metab. 2017;102(4):1161-1173. PMID 28324103.
Author Bio
Mark Reynolds is a bloodwork and health editor who writes about what lab results mean for people who train. He focuses on reading blood markers in context: what moves them, what the numbers actually say, and when a result is worth a conversation with a doctor.