Your splits are slower than last month, the legs feel heavy on every warm-up and you are sleeping more but waking up flat. Is it the training block, the dark mornings, or something in your blood? Blood tests for athletes cannot answer that on their own, but used well they can take some of the guesswork out of it. This guide explains which markers sports medicine teams commonly look at, what each one means, and how to time your tests so the results are worth comparing.
Why blood tests for athletes are read differently
Training changes the blood. Endurance work expands plasma volume, so haemoglobin can read slightly low in a well-trained athlete without true anaemia. A hard session or a race raises muscle and inflammation markers for a few days. And people differ: a level that is normal for one athlete can be unusual for another.
A review of the blood markers used to manage training load concluded that the conditions of the sample, differences between individuals and each athlete's usual workload all move resting values, and that population reference ranges are of limited use on their own. Its advice is to compare each result with your own baseline, taken in the same conditions (Haller et al., 2023). That is the thread running through this article: one result is a snapshot, a series is a story.
Iron and ferritin: the most common gap
Iron carries oxygen in the blood and is part of the enzymes muscles use to make energy. Low iron is common in athletes, especially women, endurance athletes, vegetarians and anyone who trains at altitude, and it can exist before haemoglobin falls (Sim et al., 2019). Ferritin, the body's iron store, is the number most teams watch. Because inflammation and a hard session can push ferritin up, it is usually read with C-reactive protein (CRP) and the full blood count.
For women who train, periods matter too. A practical review for female athletes recommends looking at iron alongside the menstrual cycle pattern, training load and environment over time, rather than a single ferritin value (Pedlar et al., 2018).
Vitamin D through UK winters
Between October and early March the sun in the UK is not strong enough for the skin to make enough vitamin D, and government advice is that everyone should consider a daily supplement in autumn and winter (NHS). The UK's Scientific Advisory Committee on Nutrition sets a reference intake of 10 micrograms a day for everyone aged 4 and over, all year round, to protect bone and muscle health (SACN, 2016). Athletes who train indoors, cover up or have darker skin are more likely to run low, and late autumn or winter is a sensible time to check.
B12 and folate: diet first
Vitamin B12 and folate are needed to make red blood cells. B12 comes mainly from meat, fish, milk and eggs, so a vegan diet needs a reliable source, usually fortified foods or a supplement (Rogerson, 2017). Cutting out food groups to make weight raises the same question.
Recovery and training load
Creatine kinase (CK) leaks from muscle into the blood after hard or unfamiliar exercise, especially eccentric work such as downhill running or heavy lifting. Urea can rise with endurance load, and CRP with a race or a block of hard sessions. None of these tells you on its own that you are overtrained. They only mean something against your own previous results, taken at the same time of day and at the same point in your training week (Haller et al., 2023).
Energy availability, thyroid and hormones
When food intake does not cover what training burns, the body starts saving energy. The International Olympic Committee (IOC) calls the result relative energy deficiency in sport (REDs), and it can affect hormones, bone, mood, immunity and performance in both women and men. The IOC's 2023 consensus lists blood signs that can go with low energy availability: a low free T3 (the active thyroid hormone), low testosterone in men, low oestradiol with missed or irregular periods in women, and raised total or low-density lipoprotein (LDL) cholesterol (Mountjoy et al., 2023).
These are clues, not a label. If your periods have stopped, you keep picking up stress injuries or your performance falls despite training well, see your GP or a sports doctor. Blood tests are one part of that assessment, alongside your history, diet and training.
