Blood tests for strength training: CK, creatinine, liver, testosterone

Gym training puts different demands on your body from running. Instead of hours of low-intensity exercise, there are brief, maximal muscle contractions, a high protein intake, often supplements (creatine, pre-workout supplements, fat burners) and periods of ‘bulking’ and ‘cutting’ that alter hormone and lipid metabolism. Blood test results in someone who strength trains look different from those in an inactive person, and a doctor unfamiliar with the context may misinterpret some of them. The result report is issued in Polish. Below: what to test, how often, and how to interpret common ‘strength-training abnormalities’.

Creatine kinase (CK): a marker of muscle breakdown

CK is an enzyme released from muscle fibres when they sustain microscopic damage. A heavy leg session or deadlifting can raise CK to several times the normal range for 2–4 days. This is a physiological response. The concern is CK that remains very high for a week despite rest, especially with muscle pain, swelling and dark urine: this combination requires urgent medical assessment because it may indicate rhabdomyolysis. Checking CK also makes sense after switching to a much heavier training programme and when taking statins.

Creatinine, urea, eGFR: kidneys on a high-protein diet

Creatinine is produced in the muscles, so people with a large muscle mass have higher levels than the general population. Creatine supplementation further raises the result by a few to around a dozen per cent without any actual deterioration in kidney function. A single, slightly raised creatinine result in a bodybuilder is therefore not yet a reason for concern, but this needs to be checked: cystatin C (independent of muscle mass) and urinalysis with assessment of protein determine whether the kidneys are working properly. Urea rises in proportion to the amount of protein consumed; high urea with normal creatinine usually reflects diet, not disease. If your diet provides more than 2 g of protein per kilogram of body weight, checking every six months is reasonable.

ALT, AST, GGT: liver or muscles?

AST is found not only in the liver but also in skeletal muscles. After heavy training, it rises alongside CK, while ALT rises to a lesser extent. If AST and CK are raised but GGT and bilirubin are normal, the source is the muscles. If ALT, GGT and bilirubin rise, it is worth looking for a cause in the liver: fatty liver (common with bulking diets and a high proportion of simple sugars), alcohol, certain supplements and medicines. People using anabolic agents without medical supervision also face a risk of cholestasis and serious liver damage; liver function tests every few months are essential for them.

Lipid profile: bulking, cutting and dietary fats

Strength training itself benefits the lipid profile, but a bulking diet high in saturated fats and providing excess calories can raise LDL and triglycerides within a few months. Conversely, very low-fat cutting diets lower HDL. A lipid profile (total cholesterol, LDL, HDL, triglycerides and, optionally, non-HDL) once a year allows you to adjust your diet before the changes persist for years. Anabolic substances drastically lower HDL and raise LDL, which is one of the main reasons for cardiovascular complications in this group.

Testosterone, SHBG, oestradiol, LH

Total testosterone measured in the morning, on an empty stomach, after a good night’s sleep is a basic marker in men who, despite appropriate training and diet, see no progress, have reduced libido, poorer recovery or an increase in body fat. A prolonged calorie deficit, insufficient sleep and overtraining lower testosterone reversibly. SHBG shows how much hormone is actually available to tissues; oestradiol and LH help the doctor assess whether the problem lies in the testes or the pituitary gland. In women who strength train, oestradiol, FSH, LH, prolactin and menstrual cycle regularity are more important, because loss of periods due to an energy deficit leads to loss of bone mass.

Cortisol and TSH

Morning cortisol at the upper end of the normal range or above it, together with insomnia, irritability and reduced strength, is a typical picture of overtraining. TSH (with FT4) is worth checking if your weight is increasing and your energy is falling despite diet and training: hypothyroidism is common and inexpensive to investigate. Thyroid disorders also alter creatinine and CK metabolism, which can lead to misinterpretation.

Full blood count, iron, vitamin D, magnesium

A full blood count with differential rules out anaemia and infections. Iron and ferritin are important for people cutting body fat and for women. Vitamin D (25-OH-D3) affects muscle strength and bone density; deficiency is common in Poland from autumn to spring. Serum magnesium is not a perfect measure (most magnesium is in bones and cells), but a clearly low result alongside cramps and arrhythmias is clinically significant.

Glucose, insulin, HbA1c

People bulking on a diet rich in simple sugars and those with abdominal obesity should check fasting glucose and glycated haemoglobin (HbA1c) once a year. Fasting insulin allows the HOMA-IR index to be calculated and insulin resistance to be detected, which blocks body fat loss despite an appropriately balanced calorie deficit.

Electrolytes and urinalysis

Sodium, potassium, calcium and phosphorus are important when sweating heavily, using diuretics as supplements (dangerous without supervision) and preparing for competitions. Urinalysis detects proteinuria, haematuria and ketones; on a ketogenic diet, ketones in urine are expected, but protein is not.

How to prepare for testing so that the results are meaningful

• 48–72 hours without heavy training (otherwise CK, AST, creatinine and ferritin will be artificially raised).
• Have blood collected in the morning, on an empty stomach, after 7–8 hours of sleep; have hormone tests between 7:00 and 10:00.
• Creatine: you do not need to stop taking it, but you should mention that you use it when giving your medical history.
• No alcohol for 3 days, and no pre-workout supplement that morning.
• Urine test: collect a midstream sample from your first morning urine.

Suggested monitoring schedule

Once a year: full blood count, lipid profile, glucose, creatinine with eGFR, ALT, AST, GGT, TSH, vitamin D and urinalysis. Every six months on a high-protein diet or when taking supplements: creatinine, urea, liver function tests and CK. If progress stalls, libido drops or sleep problems develop: testosterone, SHBG, cortisol and prolactin. Results outside the reference range, especially creatinine, potassium, ALT and CK, require medical advice; this article is for informational purposes.

Frequently asked questions

Does creatine damage your kidneys?

In healthy people, studies do not show harmful effects of creatine at standard doses of 3–5 g daily. However, it raises serum creatinine, so kidney function in people taking it is better assessed using cystatin C or a urine test.

High AST after training: should you see a doctor?

If it is accompanied by raised CK, while ALT, GGT and bilirubin are normal, the simplest approach is to repeat the test after 3–4 days without training. A persistent abnormality requires a medical appointment.

How often should testosterone be tested?

If the result is normal and there are no symptoms, once every 1–2 years. More often if there are symptoms of deficiency or medication is being used under medical supervision.

Sources

• Riachy R. et al., J Funct Morphol Kinesiol, Various Factors May Modulate the Effect of Exercise on Testosterone Levels in Men, 2020. mdpi.com
• Baxmann AC et al., Clin J Am Soc Nephrol, Influence of Muscle Mass and Physical Activity on Serum and Urinary Creatinine and Serum Cystatin C, 2008. doi.org
• Mann S. et al., Sports Medicine, Differential Effects of Aerobic Exercise, Resistance Training and Combined Exercise Modalities on Cholesterol and the Lipid Profile, 2014. link.springer.com
• Kreider RB et al., J Int Soc Sports Nutr, ISSN position stand: safety and efficacy of creatine supplementation, 2017. link.springer.com