Endogenous Creatine in Two Paragraphs

Your body makes its own creatine. The liver, pancreas, and kidneys synthesize approximately 1 g/day from arginine, glycine, and methionine. You eat another 1–2 g/day from meat and fish. Ninety-five percent of whole-body creatine sits in skeletal muscle as phosphocreatine — the rapid-regeneration tank that keeps ATP topped off during short bursts of high-intensity work (the first 10 seconds of a sprint, the last rep of a heavy set, the jump off the porch to catch a toddler)^[1].

Creatine turns over. About 1–2% of the total muscle creatine pool degrades every day — non-enzymatically, spontaneously — into a single waste product: creatinine. The kidney filters creatinine and dumps it in urine. That is why serum creatinine is used as a surrogate for GFR. It is also why feeding somebody extra creatine changes their serum creatinine reading.

The Supplement — What Actually Works

More than 90% of the creatine research literature uses one compound: creatine monohydrate. Kre-alkalyn, ethyl ester, creatine HCl, and buffered creatines are marketed as superior. The evidence does not support the claim. The 2017 ISSN position stand explicitly notes that creatine monohydrate is the benchmark and that alternative forms have not demonstrated superiority^[1].

The ISSN 2021 update emphasizes that loading is optional, not required^[2]. For the older adult starting creatine to blunt sarcopenia, skip loading. Go straight to 3–5 g/day. It is gentler on the gut and arrives at the same place a few weeks later.

Common Myths — Debunked in One Scan

The 2021 ISSN expert panel addressed the twelve most-common creatine misconceptions^[2]. The ones PTs field most often:

The Proof — Gold-Standard GFR Measurement

The case to memorize: Gualano and colleagues (2010) took a 20-year-old man with a single kidney and mildly decreased GFR, put him on 35 days of high-dose creatine (20 g/day loading for 5 days, then 5 g/day for 30 days), and measured his GFR by ⁵¹Cr-EDTA clearance — the nuclear-medicine gold standard, independent of creatine metabolism. Pre-supplementation ⁵¹Cr-EDTA clearance: 81.6 mL/min/1.73 m². Post-supplementation: 82.0 mL/min/1.73 m². No meaningful change. At the same time, his serum creatinine rose from 1.03 to 1.27 mg/dL — a number that on a creatinine-based eGFR calculator looks like a kidney getting worse^[3].

Same person. Same kidney. Same GFR. Different creatinine. The rise was the artifact.

Armentano and colleagues (2007) demonstrated the same phenomenon prospectively — 35 active-duty U.S. Army volunteers on 20 g/day creatine for 7 days had "a significant increase in serum creatinine levels… which could be misinterpreted as impairment of renal function" while renal function was unchanged^[12]. The "misinterpretation" framing is exactly what PTs need to block in the primary-care office.

The Meta-Analytic Verdict

The 2023 Nutrients narrative review by Longobardi and colleagues — titled, for emphasis, "Is It Time for a Requiem for Creatine Supplementation-Induced Kidney Failure?" — synthesizes the available clinical trial data and concludes GFR is preserved when measured by reliable methods (⁵¹Cr-EDTA clearance, inulin clearance, cystatin-C-based estimates). Serum creatinine may rise modestly. GFR does not fall^[4].

Naeini and colleagues (2025) extended this base in BMC Nephrology — a systematic review of 21 studies with meta-analysis across 12 (177 participants on creatine, 263 on control). Serum creatinine rose a small, statistically significant amount. GFR did not differ between creatine and control. Their conclusion reads like a one-line summary of this section: "a modest, transient increase in serum creatinine levels, likely due to metabolic turnover rather than renal impairment. No significant changes were observed in GFR, suggesting preserved kidney function"^[10].

Poortmans and Francaux's long-running Sports Medicine review tracked small cohorts on creatine for up to 5 years with clearance-based kidney monitoring and urine protein measurement. No adverse renal effects. Their conclusion: "there is no evidence for deleterious effects in healthy individuals"^[5].

Lab Strategy — How to Handle the "Creatinine Went Up" Call

1. Proactive disclosure. The patient should tell the PCP and any ordering clinician that they supplement creatine before any renal panel. Put it in the medication list.
2. Expect a small creatinine rise (approximately 0.1–0.3 mg/dL) on supplementation. Documented across Armentano 2007, Gualano 2010, and the Naeini 2025 meta^[3][10][12].
3. If the rise triggers concern, three practical options:
   • Hold creatine for 5–7 days and recheck. Serum creatinine should return to baseline.
   • Order a cystatin-C-based eGFR — unaffected by creatine supplementation or muscle mass.
   • Measure urine albumin-to-creatinine ratio (ACR) — a kidney-injury marker independent of the supplementation-related creatinine shift.
4. Do NOT let a single 0.1–0.3 mg/dL rise automatically discontinue creatine in a patient who is benefiting functionally — unless symptoms or other injury markers are present. Coordinate with the PCP; this is where the PT's voice matters most.

This is also why creatine matters in a specific clinical niche: HIV pre-exposure prophylaxis with tenofovir monitors kidney function via serum creatinine. Creatine supplementation can mimic tenofovir nephrotoxicity on the lab panel. Brief interruption of creatine restores accurate monitoring — the kidney was never the problem.

Healthy Adults — The Bar Is High

The safety evidence for healthy adults is substantial. The ISSN 2017 position stand reviews doses up to 30 g/day for 5 years with no evidence of renal dysfunction^[1]. The 2021 ISSN follow-up addresses the kidney question head-on: the evidence does not support kidney damage at recommended doses (3–5 g/day) in healthy adults^[2]. The 2024 ISSN Part II review reinforces the same conclusion across new literature^[6].

The AIN Case Reports — Rare, Real, Respected

Large safety reviews do not make isolated case reports disappear. The most-cited is Koshy, Griswold, and Schneeberger's 1999 letter in the New England Journal of Medicine: a young man developed biopsy-proven acute interstitial nephritis (AIN) approximately four weeks after starting 5 g/day of creatine monohydrate. Kidney function recovered after creatine was stopped^[7].

A handful of similar reports have followed over two decades. The pattern matches classic idiosyncratic hypersensitivity AIN — not dose-related, not class-related, not predictable from baseline labs. The same pattern shows up with penicillins, PPIs, and NSAIDs. The background rate of AIN in the general population already exists; whether creatine raises it above baseline has not been demonstrated.

Special Populations

The Devries–Phillips Meta-Analysis (2014)

Devries and Phillips pooled randomized placebo-controlled trials of creatine plus resistance training versus resistance training alone in adults averaging 63–64 years of age. Across 357 older adults and a mean of 12.6 weeks of resistance training, the creatine groups gained more total body mass (P = 0.004), more fat-free mass (P < 0.0001), more chest-press 1RM (P = 0.004), and more leg-press 1RM (P = 0.02) than resistance training alone. They also performed better on the 30-second chair-stand test (P = 0.03) — a functional measure PTs use every week^[8].

The Chilibeck Meta-Analysis (2017)

A later meta-analysis by Chilibeck and colleagues extended the evidence with 22 RCTs and 721 participants. Creatine supplementation during resistance training produced an additional 1.37 kg of lean tissue mass (95% CI 0.97 to 1.76, P < 0.00001) over and above resistance training alone, plus modest upper- and lower-body strength gains^[11]. For the geriatric and sarcopenic-obesity populations PTs are increasingly asked to program for, that extra 1.37 kg is a clinically meaningful number.

Candow and Colleagues — Bone, Falls, Inflammation

Candow and colleagues (2019) reviewed creatine's effects on aging muscle and bone with specific emphasis on falls prevention and low-grade inflammation. The synthesis: creatine plus resistance training likely increases muscle mass, likely supports bone mineral content, and may modestly attenuate inflammatory markers in older adults — with a safety profile consistent with the broader literature^[9]. The practical PT use: an older patient working to stay off the floor, on the stairs, and out of the walker benefits from the combination more than from either alone.

Integration Into a PT Plan of Care — The Older-Adult Sarcopenia Bundle

Scenario A — The Young Resistance Trainee

Nineteen-year-old college athlete, started creatine 4 weeks ago at 5 g/day, feels great, up 4 lbs. Asks if it is "messing with his kidneys." Healthy, no medications, no symptoms.

PT response: Evidence supports safety at this dose in healthy young adults. Monohydrate with third-party testing is the form that matters. Hydrate as you would any resistance trainee. If he is getting annual labs, a 0.1–0.2 mg/dL creatinine rise is expected and is not kidney injury. Tell the PCP before the next lab draw.

Scenario B — The Cardiac-Rehab Patient

Seventy-one-year-old woman, HFpEF, T2DM, on empagliflozin, losartan, metformin, and a loop diuretic. Asks about creatine because her gym friend said it helps older adults.

PT response: The sarcopenia evidence is genuinely compelling in her age range. But she already has a full cardiology and endocrinology team plus a nephrologist on watch. The 30-second lift for her is a direct message to the cardiologist: "Patient is asking about creatine monohydrate 3–5 g/day to support resistance training. Any concerns?" — then let that answer drive counseling. Do not counsel initiation as a PT in this cardiorenal profile.

Scenario C — The GLP-1 RA Patient Worried About Muscle Loss

Sixty-year-old on semaglutide 2.4 mg for obesity, 8 kg down in 3 months, reports loss of "leg power" on stairs. You've added resistance training per the GLP-1 PT handout.

PT response: Creatine is a reasonable adjunct to the resistance-training prescription during rapid weight loss. Loop the prescribing clinician in. 3–5 g/day monohydrate. Pair with protein at 1.2–1.5 g/kg/day distributed across meals. Benchmarks: 30-second chair stand, TUG.

Scenario D — The Adolescent Athlete

Fifteen-year-old soccer midfielder, parents split on whether to allow creatine.

PT response: Stay out of the decision. ISSN 2021 considers creatine monohydrate safe at recommended doses in adolescent athletes with appropriate supervision; AAP has historically been more conservative. The decision belongs to the pediatrician and the parents. Your lane: if the family proceeds, reinforce third-party certification (see Section 7), monohydrate only, no loading in an adolescent, hydration as for any athlete.

Scenario E — The Patient Whose Creatinine "Went Up"

Sixty-eight-year-old retired teacher, 4 months into sarcopenia-focused resistance training plus creatine 5 g/day. Annual labs show creatinine 1.1 → 1.3 mg/dL. PCP advised stopping creatine. Patient asks if this is "the end of my strength program."

PT response: Textbook artifact pattern — magnitude typical, no symptoms, GFR unlikely actually changed. The right next move is a PCP conversation, not a PT override. Offer the patient language: "Would you be open to rechecking my creatinine after I hold the creatine for a week, or ordering a cystatin-C-based eGFR to confirm the GFR actually moved?" — and let the PCP decide. Cross-link: Creatinine and GFR deep-dive.

The PT Decision Algorithm

Patient considering or taking creatine arrives to PT
        ↓
Any "pause is reasonable" condition?
(AKI; advanced unmonitored CKD; narrow-index drug dosed by eGFR;
 PCP unaware; known intolerance; AIN-pattern symptoms now)
        ├── YES → Counsel hold; coordinate with prescribing clinician.
        │
        └── NO  → Continue below.
        ↓
Is the patient older / frail / losing lean mass (GLP-1 RA / rehab)?
        ├── YES → Creatine + resistance training is the highest-value
        │         use case. 3–5 g/day monohydrate. Pair with protein
        │         (Protein PT handout).
        │
        └── NO  → Reasonable across healthy adult populations at 3–5 g/day.
                  Advise telling PCP before next renal panel.
        ↓
Has a lab come back with a creatinine rise?
        ├── YES → (1) Size up the delta. 0.1–0.3 mg/dL on supplementation
        │         = likely artifact. (2) Any symptoms? No → reassure,
        │         escalate only if needed. Yes → STOP, escalate.
        │         (3) Offer the PCP a cystatin-C eGFR + urine ACR.
        │
        └── NO  → Encourage proactive disclosure to PCP; document.

Patient Teaching Scripts

Red Flags — Stop Session, Escalate

Ten-Second Quick-Reference