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Organ Function

Creatinine & eGFR (Kidney Function)

Creatinine is a waste product generated from the normal turnover of creatine phosphate in skeletal muscle. Your kidneys filter creatinine from the blood and excrete it in urine at a remarkably constant rate — making serum creatinine levels a reliable proxy for how well your kidneys are filtering. eGFR (estimated glomerular filtration rate) is calculated from your creatinine level, age, sex, and ethnicity using the CKD-EPI equation, and estimates how many millilitres of blood your kidneys filter per minute. Together, creatinine and eGFR are the standard screening tests for kidney function used by every GP in the UK.

Optimal Ranges

Clinical (NHS) Range

Creatinine: 59-104 µmol/L, eGFR: >90 mL/min/1.73m²

µmol/L (creatinine), mL/min/1.73m² (eGFR)

Performance-Optimised Range

Creatinine: 70-110 µmol/L, eGFR: >90 mL/min/1.73m²

µmol/L (creatinine), mL/min/1.73m² (eGFR)

The clinical range defines what is considered medically “normal” — broad enough to cover 95% of the population. The performance range reflects where research and clinical experience suggest most people feel and function at their best.

Why It Matters

Why Creatinine / eGFR matters for performance

Chronic kidney disease (CKD) affects around 1 in 10 people in the UK according to the NHS, yet most are undiagnosed because early kidney damage produces no symptoms whatsoever. By the time you notice something is wrong — fatigue, swelling, changes in urination — you may already be at stage 3 or beyond. For active men, creatinine requires careful interpretation: because creatinine is produced by muscle, men with higher muscle mass naturally have higher creatinine levels. A creatinine of 110 µmol/L might flag as 'high' on a standard lab report but be entirely normal for a muscular 85kg man. Conversely, a 'normal' creatinine in someone who has lost significant muscle mass could mask declining kidney function. This is precisely why eGFR exists — it adjusts for body composition variables. High-protein diets (above 2g/kg/day), creatine supplementation, intense training, and dehydration can all transiently elevate creatinine. If your creatinine is consistently elevated across multiple tests despite adequate hydration, that warrants investigation. Kidney function is also tightly linked to cardiovascular health — the British Heart Foundation considers CKD an independent risk factor for heart disease.

Symptoms

Signs your levels may be off

Low / Deficiency

  • Very low creatinine may indicate low muscle mass or liver disease
  • Unexplained weight loss or muscle wasting
  • Malnutrition or prolonged illness

High / Excess

  • Often no symptoms until eGFR drops below 60 (stage 3 CKD)
  • Fatigue and reduced energy
  • Swollen ankles, feet, or hands (fluid retention)
  • Foamy or dark urine
  • Persistent itching
  • Loss of appetite and nausea
  • Shortness of breath

Dietary Sources

Foods that support Creatinine / eGFR levels

Adequate hydration — 2-3 litres of water daily supports kidney filtrationBerries — blueberries, strawberries, cranberries (antioxidant protection)Leafy greens — kale, spinach (potassium and magnesium for kidney health)Oily fish — omega-3 reduces kidney inflammationGarlic and onions — allicin supports renal blood flowEgg whites — high-quality protein with lower phosphorus loadRed peppers — vitamin C and A without excess potassium

Supplementation

Evidence-based supplementation

Kidney function is primarily supported through lifestyle rather than supplementation. Hydration is foundational — even mild chronic dehydration reduces eGFR. If you supplement creatine monohydrate (common among active men at 3-5g/day), expect serum creatinine to rise by 10-20% without any actual kidney damage. Multiple studies, including a 2019 review in the Journal of the International Society of Sports Nutrition, confirm creatine does not harm healthy kidneys, but it will affect your creatinine reading — inform your doctor. For men on high-protein diets (above 2g/kg), kidney function monitoring is advisable not because protein damages healthy kidneys (a myth debunked by the 2018 meta-analysis in the Journal of Nutrition), but because early CKD can be masked when dietary protein keeps creatinine production high. CoQ10 (100-200mg/day) has shown renal-protective effects in some studies. Avoid chronic NSAID use (ibuprofen, naproxen) — these are a leading cause of drug-induced kidney damage. If eGFR drops below 60 on two consecutive tests 90 days apart, your GP should investigate further.

Research

Key study

A new equation to estimate glomerular filtration rate

Levey AS, Stevens LA, Schmid CH, et al.

Annals of Internal Medicine (2009)

DOI: 10.7326/0003-4819-150-9-200905050-00006

Related Biomarkers

Metabolic

Cholesterol (Total, HDL, LDL & ApoB)

Optimal: ApoB: below 1.0 g/L, LDL: below 2.6 mmol/L, HDL: above 1.2 mmol/L

Metabolic

HbA1c (Glycated Haemoglobin)

Optimal: 20-34 mmol/mol (below 5.4%)

Inflammation

hs-CRP (High-Sensitivity C-Reactive Protein)

Optimal: Below 1.0 mg/L

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Complete Guide to Blood Tests

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Test your Creatinine / eGFR levels

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This content is for educational purposes only and does not constitute medical advice. Your data suggests areas for optimisation, but any concerns should be discussed with a qualified healthcare professional. If your results flag values outside safe ranges, we recommend consulting your GP.