Your blood work comes back. Creatinine a bit above the normal range, eGFR (estimated filtration rate) a bit lower than usual. Your doctor frowns and the word "kidneys" shows up in the conversation pretty fast. If you take creatine, there's a much more boring explanation. And not a lot of people know about it yet.
Creatinine is what's left of your creatine
Every day, a small share of the creatine stored in your muscles turns into creatinine on its own, no enzyme required. It's roughly 1.7% a day, or about 2 g for a 70 kg adult. The fuller your stores, the more of it you make. Your kidneys aren't filtering any worse. They just have a bit more leftover to clear out.
That's exactly what a meta-analysis published in late 2025 in BMC Nephrology found after reviewing 21 studies. Creatine nudges blood creatinine up a little, but the kidneys' actual filtration rate doesn't change in any significant way.
The eGFR trap
Here's the detail that changes everything: on most lab reports, your eGFR isn't measured. It's calculated from your creatinine. So if creatinine goes up for a reason that has nothing to do with your kidneys, the math still spits out a lower filtration number.
A case published in 2010 shows this nicely. A 20-year-old man with only one kidney took creatine for 35 days (20 g a day for 5 days, then 5 g a day). His creatinine went from 1.03 to 1.27 mg/dL, which would worry just about anyone. But his filtration rate, measured directly with a gold-standard method, barely moved: 81.6, then 82.0 mL/min/1.73 m². One case, sure, but a telling one.
What about fasting?
Good news on that front. A cohort study published in 2023 followed 58 overweight men through the month of Ramadan. Their creatinine stayed practically the same (0.847, then 0.834 mg/dL, not a significant difference). Fasting on its own, in people who drink normally, doesn't seem to push that number up.
You may have read that ketones throw off creatinine tests. That's true, but mostly in one specific situation: diabetic ketoacidosis, where ketones climb far higher than they do during a fast. With an older lab method (the Jaffé method), creatinine can then look quite a bit higher than it really is. For an ordinary fast, there's no evidence of an effect that really matters.
What we don't know yet
No study has looked specifically at daily intermittent fasting plus creatine and what it does to lab results. What you're reading here are conclusions drawn from solid research, done separately. Treat it as a good starting point for a conversation with your doctor, not as a diagnosis.
In practice
Before your next blood test, tell your doctor you take creatine. If your creatinine or eGFR come back borderline, ask whether a cystatin C test would help. It's another filtration marker that's barely affected by muscle mass or creatine, and the international nephrology guidelines (KDIGO 2024) recommend combining it with creatinine when creatinine is less reliable.
In healthy adults, the available data don't show kidney harm at the usual 3 to 5 g a day. That said, if you have kidney disease, a single kidney or already reduced filtration, talk to your doctor before you start.
We started Fasted Rebels so you'd understand your own numbers, not so you'd panic over a lab report. Feed the mission. Not the hunger.
This article is for information only and isn't a substitute for advice from a health professional.
Sources: Kabiri Naeini E et al., "Effect of creatine supplementation on kidney function: a systematic review and meta-analysis," BMC Nephrology, 2025; Wyss M, Kaddurah-Daouk R, "Creatine and Creatinine Metabolism," Physiological Reviews, 2000; Gualano B et al., "Effect of short-term high-dose creatine supplementation on measured GFR in a young man with a single kidney," American Journal of Kidney Diseases, 2010; "Effect of Ramadan intermittent fasting on renal and liver function markers among healthy overweight and obese males: a prospective cohort study," Human Nutrition & Metabolism, 2023; "Falsely-elevated serum creatinine values in diabetic ketoacidosis," Clinica Chimica Acta, 1981; KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of CKD; Hajj et al., Cleveland Clinic Journal of Medicine, 2025.
French version: Pourquoi ta créatinine grimpe sur ta prise de sang
Also read, the fasting and electrolytes series: Why You're Cramping Mid-Fast · More Water While Fasted Isn't Always Safer · Why You Get Dizzy Standing Up Mid-Fast · Why Your Heart Pounds Mid-Fast · Why You Twitch and Sleep Like Garbage Mid-Fast · Why You Get a Headache Mid-Fast · Why Your Uric Acid Spikes Mid-Fast · Why You're Always Cold Mid-Fast · Do You Need Sugar to Get Creatine Into Your Muscles?