Blood values for strength athletes: what training and supplements change

Your creatinine sits just outside the reference range, and your first reflex is to worry. Understandable, because the word "kidneys" comes up fast. But if you train seriously and have built muscle, that is exactly the value your sport changes in a predictable way. This piece shows which blood values move with your strength training and your supplements, and which ones you do take seriously.

My stance is simple: a reference range is built on an average person who barely sees the gym, not on you. In a trained lifter, a value just outside that range is more often a sign of training status than of disease. You only read your result well once you know what your own biology does to it.

Why strength athletes have different blood values

A reference range is the band that roughly 95% of a test group falls into. That group is a mixed population, not a group carrying a lot of muscle who train hard four times a week. If you carry plenty of muscle or train intensely, you fall just outside the "normal" range more often, while functionally nothing is wrong.

The Dutch Health Council (Gezondheidsraad) stresses that reference values are statistical limits and should always be read in the context of the person and the complaint, not as a hard line between healthy and sick. That is exactly the nuance a lifter needs. In a study of 170 healthy people, creatinine in those who trained moderately to intensely sat clearly higher than in the sedentary group, while their kidney function was the same (Baxmann, 2008). More muscle, higher value, healthy kidneys.

Which blood values strength training affects

Strength training mostly touches your muscle- and kidney-derived values. Creatinine and creatine kinase (CK) rise most clearly, because they come straight from muscle tissue. High-protein food and creatine can amplify that rise. Hormones like testosterone and markers like haematocrit move on a different rhythm. The table below shows per value what your training or supplements do and how often it is concerning.

Creatinine and kidney values: usually your muscles, not your kidneys

In strength athletes, a raised creatinine usually means muscle mass, not kidney damage. Creatinine is a breakdown product from your muscles, so the more muscle, the higher the baseline. The standard eGFR formula calculates from creatinine and does not account for a lot of muscle, so it can underestimate your kidney function.

To check whether it really is your kidneys, cystatin C helps. That marker does not depend on muscle mass and often gives a fairer picture in muscular people (Baxmann, 2008). The Dutch Kidney Foundation (Nierstichting) also points out that a single off result is not a kidney problem and that a trend over time says far more than one isolated reading. You read the details in our piece on high creatinine from muscle mass.

Protein and creatine: what they do to your blood

For healthy athletes, plenty of protein and creatine are usually easy to justify, but they do colour your result. A high-protein diet does not measurably change kidney function in healthy people, and creatine at normal doses raises your creatinine without damaging the kidneys.

A meta-analysis of 28 studies found no difference in kidney function between healthy adults on high or low protein (Devries, 2018). And the sports-nutrition literature describes creatine at doses up to 5 grams a day as well studied and safe in healthy users (Kreider, 2017; Antonio, 2021). The Netherlands Nutrition Centre (Voedingscentrum) uses a guideline of roughly 0.8 grams of protein per kilo of body weight for healthy adults; athletes deliberately sit above that, and for healthy kidneys that is not a problem. Read on: protein intake and your kidneys and creatine and your blood values.

Haematocrit, testosterone and the hormonal picture

Beyond your muscle and kidney values, two markers lifters watch are haematocrit and testosterone. A high haematocrit often simply comes from dehydration right before the draw, but it can also rise on TRT, and then it is worth discussing. Testosterone moves with your sleep, your recovery and how heavy your training block is.

If you want to track these values seriously, look at the trend across several measurements rather than at one number. A testosterone that sits temporarily lower during a heavy training block or after a stretch of poor sleep need not be a problem; it is your body responding to the load. Only when a low value persists and comes with complaints such as low energy, poor recovery or a dropping libido does it become relevant to investigate further. Read more in high haematocrit in athletes and in testosterone and muscle mass.

CK: how you see a hard session in your blood

Creatine kinase (CK) is an enzyme released when muscle fibres take micro-damage, exactly what happens during a heavy or unusual training session. Your CK typically peaks 24 to 72 hours after the effort and then falls again. A high CK shortly after a tough leg session is therefore not a sign of disease but proof that your muscles worked hard.

That has a practical consequence: if you draw right after such a session, you find a CK (and a creatinine) that your own training lifted, not your resting state. Only with an extremely high CK combined with dark urine, severe muscle pain and nausea is there reason to see a doctor quickly, because that can point to muscle tissue breakdown. In practice that rarely happens with ordinary strength training, but it is the exception you should know.

IGF-1: the trade-off of growth

IGF-1 is the marker that helps drive muscle growth and recovery under the influence of growth hormone. Higher is not automatically better: in longevity research, both very low and very high IGF-1 are linked to unfavourable long-term outcomes. For the healthy, naturally training athlete, that is mainly a reason to read the marker with nuance, not to push it up artificially. The full explanation is in IGF-1 explained.

Reading protein, creatine and your hydration together

The values strength training affects are linked, and that is exactly why a single number misleads you so easily. A lifter who eats a lot of protein, uses creatine, trains hard and drinks too little right before the draw stacks four explainable effects on top of each other: more urea from protein breakdown, more creatinine from creatine and muscle mass, a temporary CK peak, and a higher haematocrit from dehydration. On paper that looks like an alarming list of deviations, while each effect on its own is perfectly logical.

The solution is not to change your diet or your training for the blood test, but to standardise the conditions and include the right extra markers. Add cystatin C to judge your kidneys apart from your muscle mass, drink well, and draw on a rest day. Then those "deviations" often fall into place by themselves, and you see what your body really does at rest.

When you do discuss an off value with a doctor

Not every result outside the range calls for action, but some do. The situations below are a reason to consult your GP rather than interpret it yourself.

The three mistakes athletes make when testing

Most skewed results do not arise in the lab but in the preparation. Three mistakes keep coming back. The first is drawing right after a hard session, which leaves creatinine and CK artificially high. The second is showing up dehydrated, which lifts your haematocrit and your kidney values. The third is over-interpreting a single isolated measurement, when a trend across two or three measurements says far more.

The Dutch Kidney Foundation (Nierstichting) and the Dutch Health Council (Gezondheidsraad) both stress the same principle from the medical side: a reference value is a statistical tool, not a verdict, and the context of the person determines what a result means. For you as a lifter, that translates into a simple routine: test in a standardised way, on a rest day, hydrated, and compare against your own previous results rather than against the reference range alone.

How to test your blood values as a strength athlete

Test when you are rested, not right after a hard session. Training in the 48 to 72 hours before your draw can temporarily lift your creatinine and CK, which skews the picture. Drink enough, because dehydration pushes both your haematocrit and your kidney values up. Test testosterone fasted and in the morning, when it is naturally highest.

To judge your kidneys apart from your muscle mass, choose a test that measures cystatin C and urea alongside creatinine. Our kidney health blood test bundles those markers, and with TRT monitoring you keep an eye on haematocrit and testosterone. For the whole hormonal picture, our general hormones test is a logical start.

My advice: do not let one value just outside the range rattle you. Look at the whole, test on a calm day, and discuss a deviation that persists or comes with symptoms with your GP. Data works for you once you know how to read it.

References

1. Baxmann AC, Ahmed MS, Marques NC, et al. Influence of muscle mass and physical activity on serum and urinary creatinine and serum cystatin C. Clinical Journal of the American Society of Nephrology. 2008. PMID: 18235143.
2. Devries MC, Sithamparapillai A, Brimble KS, et al. Changes in kidney function do not differ between healthy adults consuming higher- compared with lower- or normal-protein diets: a systematic review and meta-analysis. The Journal of Nutrition. 2018. PMID: 30383278.
3. Kreider RB, Kalman DS, Antonio J, et al. International Society of Sports Nutrition position stand: safety and efficacy of creatine supplementation in exercise, sport, and medicine. Journal of the International Society of Sports Nutrition. 2017. PMID: 28615996.
4. Antonio J, Candow DG, Forbes SC, et al. Common questions and misconceptions about creatine supplementation: what does the scientific evidence really show? Journal of the International Society of Sports Nutrition. 2021. PMID: 33557850.
5. Dutch Health Council (Gezondheidsraad) and Dutch Kidney Foundation (Nierstichting), public information on reference values and kidney function.
6. Netherlands Nutrition Centre (Voedingscentrum), recommended protein intake for adults.

Disclaimer

Every blood test result includes a professional assessment by a BIG-registered doctor. This article gives general information and is not a substitute for medical advice. A blood test is a tool to walk into the conversation with your GP better informed, not a diagnosis in itself. For treatment decisions, discuss your results with your GP.