Strength training and supplements shift a few blood values, often without anything being wrong. A lot of muscle pushes your creatinine up, a hard session lifts your CK, and creatine changes how much water you hold. Below you read which values move with your training and which ones you do take seriously.
In our experience, athletes get spooked most by exactly the values their training logically changes. That is a shame, because with a bit of context you read your result very differently.
Why athletes have different blood values
Your blood values are compared with reference ranges from a mixed population, not from trained people. If you carry a lot of muscle or train hard, you fall just outside the "normal" range more often. That usually says something about your training status, not about disease.
A reference range is simply the band that 95% of a test group falls into. That group spends a lot less time in the gym than you do.
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 move on a very different rhythm.
| Value | What training or supplements do | Usually concerning? |
|---|---|---|
| Creatinine | Rises with more muscle mass and after hard training | Rarely |
| CK (creatine kinase) | Peaks 24 to 72 hours after unusual or heavy training | Rarely, unless extremely high with symptoms |
| Urea | Can climb with high protein and low fluid intake | Rarely |
| Haematocrit | Can rise from dehydration or on TRT | Sometimes, discuss it |
| Testosterone | Moves with sleep, recovery and training | Depends on symptoms |
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 does not account for that and 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).
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 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).
Read on: protein intake and your kidneys and creatine and your blood values.
How to test your blood values as an 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 several values up.
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.
Also useful for lifters: testosterone and muscle mass.
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.
References
- 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.
- 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.
- 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.
- 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.
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.
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Enhanced Health
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