Kidney Function for Men Over 40: What eGFR, Creatinine and Cystatin C Actually Measure, Why Creatine and TRT Skew the Numbers, and How to Protect the Filter
A man who lifts, takes creatine and started testosterone therapy last year will often see his eGFR drop from 98 to 82 on the next panel and get a note from the lab that says abnormal. In most cases his kidneys are fine and the equation is confused. The test that would settle it costs about 20 dollars and almost nobody orders it.
Kidney function in men over 40 is estimated, not measured, and the estimate leans on serum creatinine, a waste product of muscle metabolism. That single design choice means a muscular man, a man on creatine, or a man on testosterone therapy will routinely show an eGFR 10 to 20 points below his true filtration rate. Meanwhile the test that catches early kidney damage years before eGFR moves, the urine albumin to creatinine ratio, is missing from most annual panels. A man over 40 needs both numbers, needs to know what distorts them, and needs to know the four habits that do most of the damage.
Kidney function is the rate at which the two kidneys filter blood plasma through roughly two million tiny filtering units called glomeruli, expressed as the glomerular filtration rate, GFR, in millilitres per minute adjusted to a standard body surface area of 1.73 square metres. A healthy 25 year old man filters about 115 to 120 mL/min/1.73m², which works out to around 170 litres of plasma a day, almost all of it reabsorbed. Nobody measures that directly outside a research lab. What the annual blood panel reports is an estimate, the eGFR, calculated from serum creatinine, age and sex using the CKD-EPI 2021 equation. The estimate is good enough for population screening and often misleading for the individual man who trains hard.
Kidney function belongs in the same series as blood pressure, uric acid and coronary calcium for men over 40 because it sits downstream of every risk factor this site covers and upstream of most of the drugs and supplements men in this age group use. High blood pressure and insulin resistance are the two leading causes of kidney damage. Creatine, high protein intake and testosterone therapy all change the number without necessarily changing the organ. GLP-1 medications and SGLT2 inhibitors have produced some of the largest kidney protection results in modern trial history. And the kidneys, unlike the liver, do not regenerate. Filtration lost after 40 is almost always lost for good, which makes this one of the few biomarkers where the whole point is prevention.
~1
mL/min Of GFR Lost Per Year After 40
Population studies including the Baltimore Longitudinal Study of Aging show GFR falling by roughly 0.75 to 1 mL/min/1.73m² per year from the fourth decade onward, so a man at 118 at age 30 is often around 95 at 55 with completely healthy kidneys
60
The eGFR Line For Chronic Kidney Disease
KDIGO defines chronic kidney disease as an eGFR below 60 mL/min/1.73m² or evidence of kidney damage such as albuminuria, present for more than three months. A single reading below 60 is a repeat test, not a diagnosis
10 to 20%
Creatinine Rise On Creatine Supplementation
Creatine loading raises serum creatinine by about 0.1 to 0.3 mg/dL because more creatine is being broken down, not because the kidneys are clearing less. Cystatin C and measured GFR stay flat in every controlled study to date
30 mg/g
Urine Albumin To Creatinine Ratio Threshold
A uACR of 30 mg/g or above is the earliest sign of kidney and vascular damage and predicts cardiovascular events independently of eGFR. Most men over 40 have never had the test
24%
Reduction In Major Kidney Events In The FLOW Trial
Semaglutide 1 mg weekly cut the composite of kidney failure, sustained 50 percent eGFR loss and kidney or cardiovascular death by 24 percent in 3,533 adults with type 2 diabetes and chronic kidney disease over 3.4 years, published in 2024
3 to 6
Point eGFR Drop From Regular NSAID Use
Daily ibuprofen or naproxen constricts the arteriole feeding each glomerulus and lowers filtration acutely, and long term regular use roughly doubles the risk of chronic kidney disease in men with high blood pressure
What is a normal eGFR for a man over 40?
A normal eGFR for a man over 40 is 90 mL/min/1.73m² or above, and a result between 60 and 89 is classed as mildly decreased but is not chronic kidney disease unless there is also albuminuria or another marker of damage. The KDIGO 2024 guideline divides GFR into six categories: G1 at 90 and above, G2 from 60 to 89, G3a from 45 to 59, G3b from 30 to 44, G4 from 15 to 29 and G5 below 15. Roughly a third of healthy men in their 50s sit in G2 with no kidney disease at all, because the age related decline in filtration is built into the biology and the categories do not adjust for it. A 55 year old at 84 is normal for his age. A 35 year old at 84 deserves a second look.
The equation behind the number matters more than most men realise. Since 2021, laboratories in the United States have used the CKD-EPI 2021 creatinine equation, which removed the race coefficient from the earlier version and takes only serum creatinine, age and sex as inputs. The equation was built on populations with average muscle mass. A man carrying 20 pounds more lean tissue than the average produces more creatinine every day, the equation reads that as reduced clearance, and his eGFR comes out lower than his kidneys deserve. The reverse is also true: a sarcopenic 65 year old with very little muscle can show an eGFR of 95 while his true filtration is closer to 70. The number is an estimate of a measurement of a proxy, and it is worth knowing which step has gone wrong before worrying about the kidneys themselves.
Approximate Median eGFR For Healthy Men By Age Band
The bar chart shows the expected slope rather than one published table, and the range inside each band is wide. The point of the chart is that a man of 48 with an eGFR of 88 has not lost kidney function in any meaningful sense, and a man of 48 with an eGFR of 88 who was at 105 two years ago has a trend worth explaining. The absolute number tells a man which category he is in. The change over time, on the same lab, with the same preparation, tells him whether something is happening.
Does creatine raise creatinine and does that mean kidney damage?
Creatine supplementation raises serum creatinine by roughly 0.1 to 0.3 mg/dL, which lowers the calculated eGFR by 10 to 20 points, and this does not indicate kidney damage. Creatinine is the breakdown product of creatine and phosphocreatine in muscle, with about 1.5 to 2 percent of the body's creatine pool converting to creatinine every day. A man who raises his muscle creatine stores by 20 to 40 percent through supplementation produces proportionally more creatinine, the kidneys clear it normally, and the blood level settles at a new, higher steady state. The equation sees higher creatinine and assumes lower clearance. The kidneys have done nothing different.
The evidence on this question is unusually clean. The International Society of Sports Nutrition position stand on creatine, updated in 2017 and reaffirmed since, reviewed studies running up to five years at doses of 5 to 20 grams a day and found no adverse effect on kidney function in healthy adults when measured by cystatin C, iohexol clearance or 24 hour urine creatinine clearance. A 2019 systematic review in the Journal of Renal Nutrition reached the same conclusion. The reports of creatine causing kidney damage are case reports in men who already had kidney disease, were taking NSAIDs, or were severely dehydrated during heavy training. The practical rule for a man on creatine is simple: tell the physician, and if the eGFR comes back under 75, ask for cystatin C before anyone reaches for a diagnosis.
What Moves Serum Creatinine Without Changing True Kidney Function, And What Moves True Kidney Function
| Factor | Effect On Creatinine And eGFR | Effect On True GFR | How To Separate The Two |
|---|---|---|---|
| Creatine monohydrate supplementation | Creatinine up 0.1 to 0.3 mg/dL, eGFR down 10 to 20 points | None | Cystatin C, or stop creatine for four weeks and retest |
| High muscle mass from resistance training | Creatinine chronically higher, eGFR reads low | None | Cystatin C based eGFR |
| Testosterone therapy | Creatinine up 0.05 to 0.15 mg/dL as lean mass rises | Little or none in most men | Cystatin C, check haematocrit and blood pressure alongside |
| Large cooked meat meal in the 12 hours before the draw | Creatinine up 0.1 to 0.3 mg/dL for several hours | None | Fast overnight or eat lightly before the test |
| Hard training in the prior 48 to 72 hours | Creatinine up from muscle breakdown, sometimes markedly | Transient dip if dehydrated | Rest two to three days and hydrate before the draw |
| Dehydration | Creatinine up, eGFR down | Transient real reduction | Normal fluid intake for 24 hours before the test |
| Trimethoprim, cimetidine, some HIV drugs | Block creatinine secretion in the tubule, creatinine up | None | Cystatin C, or note the drug on the requisition |
| Sarcopenia, low muscle mass, amputation | Creatinine low, eGFR reads falsely high | None, but true GFR is hidden | Cystatin C, which does not depend on muscle |
| Regular NSAID use | Creatinine up | Real reduction, reversible early and permanent late | Stop the NSAID for two weeks and retest |
| Uncontrolled blood pressure or blood sugar | Creatinine up slowly over years | Real, progressive loss | Home blood pressure average, HbA1c, uACR |
The Two Questions Before Any Kidney Worry
When an eGFR comes back lower than expected, the first two questions are not about the kidneys. Has anything changed that raises creatinine production: new creatine, a new training block, testosterone therapy, a bigger frame? And has anything changed that blocks creatinine handling: a new antibiotic, an acid blocker, dehydration, a hard session the day before? If either answer is yes, the next step is a cystatin C test, which measures a protein produced at a constant rate by every nucleated cell in the body and is indifferent to how much muscle a man carries or what he ate for dinner. If cystatin C based eGFR is normal, the kidneys are normal.
Does TRT affect kidney function?
Testosterone therapy tends to raise serum creatinine slightly and lower the creatinine based eGFR by a few points, mainly because testosterone increases lean muscle mass and therefore creatinine production, and the available evidence does not show that it damages the kidneys in men with normal baseline function. Studies of men on testosterone for 12 months or more consistently report creatinine rising by around 0.05 to 0.15 mg/dL alongside gains of 2 to 4 kilograms of lean mass, with cystatin C either unchanged or slightly improved. In the TRAVERSE trial, which followed more than 5,000 men on transdermal testosterone or placebo for an average of 33 months, kidney adverse events did not differ between groups.
There are three indirect routes by which testosterone therapy can touch the kidneys, and a man on treatment should know all of them. The first is blood pressure: testosterone causes sodium and water retention and raises systolic pressure by 2 to 5 mmHg on average, and sustained high pressure is the second leading cause of kidney damage. The second is haematocrit: a rising red cell count thickens the blood and increases the workload on the whole vascular tree, kidneys included, which is why haematocrit over 54 percent is a hold point for the dose. The third is the NSAID habit that often accompanies harder training on therapy. A man who starts TRT, trains harder, takes ibuprofen for the aches and never checks his blood pressure has stacked three kidney stressors that the testosterone itself did not cause and that the creatinine number will only report after the fact.
Illustrative eGFR In A Man Over 45 Across The First Year Of Testosterone Therapy And Creatine, Creatinine Based Versus Cystatin C Based
- Creatinine based eGFR (CKD-EPI 2021)
- Cystatin C based eGFR
The line chart is an illustrative pattern rather than a published dataset, but it reproduces what clinicians who work with athletic men on testosterone see routinely: the creatinine based estimate drifts down 10 to 15 points as lean mass and creatine stores rise, while the cystatin C based estimate stays flat because filtration has not changed. A man who only sees the red line will be told he has stage 2 kidney disease. A man who has both lines has a complete picture, and the discrepancy between them is itself a useful measurement of how much muscle he has added.
What is cystatin C and when should a man ask for it?
Cystatin C is a small protein produced by every nucleated cell in the body at a nearly constant rate, filtered freely by the glomerulus and then fully broken down by the tubules, which makes its blood level a cleaner reflection of filtration than creatinine in anyone whose muscle mass is far from average. The CKD-EPI 2021 combined creatinine and cystatin C equation is the most accurate of the estimating equations, and the KDIGO 2024 guideline recommends confirming with cystatin C whenever a creatinine based eGFR of 45 to 59 is not accompanied by albuminuria or another sign of kidney damage, and whenever the creatinine estimate is likely to be wrong because of muscle mass, diet or medication.
Cystatin C has its own distortions and a man should know them before he treats it as the final word. Levels rise with obesity, smoking, thyroid overactivity, corticosteroid use and systemic inflammation, and fall with thyroid underactivity, so a man with a high hs-CRP or an untreated thyroid can see cystatin C move for reasons that have nothing to do with filtration. The strength of using both markers is that they fail in different directions for different reasons. When creatinine and cystatin C agree, the estimate is reliable. When they disagree by more than 15 points, the disagreement is the finding, and it usually points to muscle mass, creatine or inflammation rather than kidney disease. The test costs roughly 15 to 40 dollars from most direct to consumer labs and is available on request from any physician.
The KDIGO GFR Categories And What Each One Means For A Man Over 40
G1
90 and above
Normal filtration. Chronic kidney disease is only diagnosed at this level if there is albuminuria, blood in the urine, structural abnormality or a genetic condition. This is where most men in their 40s sit
G2
60 to 89
Mildly decreased. Normal for many men over 50 and for muscular men on creatine at any age. Not chronic kidney disease without a marker of damage. The correct response is a uACR and, if muscle mass or supplements could explain the result, a cystatin C
G3a
45 to 59
Mild to moderately decreased. This is chronic kidney disease if it persists for three months. KDIGO recommends cystatin C confirmation at this level when there is no albuminuria, because a meaningful share of men here have normal kidneys and high creatinine
G3b
30 to 44
Moderately to severely decreased. Cardiovascular risk roughly doubles, medication doses start to need adjustment, and a nephrology referral is standard. Reversal is rare but the rate of decline can be slowed substantially
G4 to G5
Below 30
Severely decreased to kidney failure. Preparation for dialysis or transplant begins in G4. A man over 40 who tracks the earlier stages and acts on blood pressure, blood sugar, albuminuria and NSAIDs almost never arrives here
What is the urine albumin to creatinine ratio and why does it matter more than eGFR?
The urine albumin to creatinine ratio, uACR, measures how much albumin protein is leaking through the kidney filter into the urine, expressed in milligrams of albumin per gram of urine creatinine, and it detects kidney and blood vessel damage years before eGFR begins to fall. A result below 30 mg/g is normal, 30 to 300 is moderately increased albuminuria, formerly called microalbuminuria, and above 300 is severely increased. Albumin in the urine means the glomerular filter has become leaky, and because the same endothelial damage that leaks albumin in the kidney is happening in the coronary and cerebral arteries at the same time, uACR predicts heart attack, stroke and death independently of eGFR, blood pressure and cholesterol.
The failure of the annual physical on this point is well documented. Fewer than one in five American adults with hypertension, and only about 40 percent of those with diabetes, get a uACR test in a given year even though both guidelines call for it, and screening in adults without those diagnoses is rarer still. The test is a single morning urine sample, costs under 20 dollars, and turns a G2 eGFR result from an ambiguous number into a clear answer: G2 with a uACR under 30 is a healthy man with a slightly lower estimate, and G2 with a uACR of 80 is early chronic kidney disease that will respond to blood pressure control, SGLT2 inhibition and, where relevant, GLP-1 therapy far better now than in five years. A man over 40 who has never had a uACR should ask for one at the next draw, and a man with high blood pressure, insulin resistance, a coronary calcium score above zero or a family history of kidney disease should have one every year.
What damages the kidneys most in men over 40?
The largest drivers of kidney damage in men over 40 are uncontrolled blood pressure, insulin resistance progressing to type 2 diabetes, regular NSAID use, obesity, smoking and high uric acid, and the first two account for roughly two thirds of all chronic kidney disease in the developed world. Each works through the same final pathway: raised pressure inside the glomerular capillaries, endothelial damage, scarring of the filter and gradual loss of nephrons that cannot be replaced. A man addressing blood pressure and blood sugar for his heart and his testosterone is doing most of the kidney work already. What remains is the habit that men in this age group underestimate more than any other, which is the ibuprofen bottle.
What Typically Explains Progressive Kidney Function Loss In A Man Over 40
- Type 2 diabetes and insulin resistance
- High blood pressure
- Regular NSAID use and other drug injury
- Obesity and obstructive sleep apnoea
- Smoking and high uric acid
- Genetic, immune and structural causes
The pie chart is an illustrative weighting drawn from registry data on the causes of chronic kidney disease rather than a single published attributable fraction analysis. Its purpose is to show that the kidney is mainly a casualty of the metabolic and vascular conditions covered elsewhere on this site, and that a man who has his home blood pressure average under 130/80, his HbA1c under 5.7 and his waist under half his height has removed the majority of his kidney risk without doing anything specifically for the kidneys. The NSAID slice is smaller but it is the one a man can eliminate this afternoon.
How much do NSAIDs affect the kidneys?
Non steroidal anti inflammatory drugs such as ibuprofen, naproxen and diclofenac reduce kidney filtration acutely by blocking the prostaglandins that keep the arteriole feeding each glomerulus open, and regular use over years roughly doubles the risk of chronic kidney disease in men who also have high blood pressure. A single dose in a well hydrated man with healthy kidneys does no lasting harm. The problem is the pattern that many men over 40 fall into: 400 to 800 mg of ibuprofen most days for training soreness, a bad back or a knee, often combined with a blood pressure medication and sometimes with dehydration from a long session or a few drinks. That combination, an NSAID with an ACE inhibitor or ARB and a diuretic, is known in nephrology as the triple whammy and is one of the most common causes of acute kidney injury in outpatients.
- Acetaminophen at up to 3 grams a day for pain that does not involve significant inflammation, which does not affect kidney filtration and is the analgesic of choice in people with reduced eGFR
- Topical diclofenac gel for a single joint, which delivers a fraction of the systemic dose and is the form the American College of Rheumatology recommends first for knee and hand osteoarthritis
- A deload week, sleep and a look at total training volume, since soreness that needs daily NSAIDs is usually a programming problem and not a pharmacology problem
- Omega 3 at 2 to 4 grams a day of combined EPA and DHA, which modestly lowers inflammatory markers over 8 to 12 weeks and has no kidney downside
- If an NSAID is genuinely needed, the lowest dose for the fewest days, with a full glass of water, never on a dehydrated day and never alongside a blood pressure medication without the prescribing physician knowing
How do GLP-1 and SGLT2 medications affect the kidneys?
GLP-1 receptor agonists and SGLT2 inhibitors are the two drug classes with the strongest evidence for protecting kidney function in people at risk, and both effects appear to be partly independent of their glucose lowering. The FLOW trial, published in the New England Journal of Medicine in 2024, randomised 3,533 adults with type 2 diabetes and chronic kidney disease to semaglutide 1 mg weekly or placebo and stopped early for efficacy after a median of 3.4 years, with a 24 percent reduction in the composite of kidney failure, a sustained 50 percent fall in eGFR, and death from kidney or cardiovascular causes. The rate of eGFR decline slowed by about 1.2 mL/min per year, and cardiovascular death fell by 29 percent. The mechanism is thought to involve reduced inflammation, lower intraglomerular pressure and weight loss, not just better blood sugar.
SGLT2 inhibitors such as empagliflozin and dapagliflozin work differently, by making the kidney excrete glucose and sodium, which resets the pressure inside the glomerulus and reduces the strain on each filter. The EMPA-KIDNEY trial in 2023 found a 28 percent reduction in kidney disease progression or cardiovascular death across more than 6,600 adults with chronic kidney disease, with or without diabetes, and DAPA-CKD reported a 39 percent reduction in 2020. A man over 40 with a uACR above 30 or an eGFR below 60 is in the population these trials studied, and the question of whether either class is appropriate belongs in a conversation with a physician rather than a supplement order. For a man with normal kidney function on a GLP-1 for weight, the relevant point is simpler: the drug is not a kidney risk in itself, but the nausea and reduced fluid intake in the first weeks can produce dehydration, and a creatinine drawn during that window will look worse than the kidneys are.
How should a man prepare for a kidney function test?
A man should prepare for a kidney blood test by resting from hard training for 48 to 72 hours, drinking normally for the preceding day, avoiding a large meat meal the evening before, holding any NSAID for at least a week, and telling the physician about creatine, testosterone and any antibiotic or acid blocker, so that the creatinine result reflects the kidneys rather than the week. Continuing creatine is fine as long as the lab and the physician know, but a man who wants the cleanest possible number can stop it for four weeks and retest; a man who does not want to stop can order cystatin C and skip the argument. Consistency matters more than any single rule. The same lab, the same time of day, the same preparation each time turns a noisy number into a trend that can be trusted.
A Twelve Month Plan For A Man Over 40 Who Wants To Know And Protect His Kidney Function
Month 1: Get the complete picture
- ✓Order creatinine based eGFR, cystatin C and a morning urine albumin to creatinine ratio together, prepared as described above, and record all three with the date, the lab, current supplements and the training state that week
- ✓Pull every prior creatinine result from old panels into one list. A man who has been at 0.9 to 1.0 mg/dL for a decade and is suddenly at 1.3 has a question to answer. A man who has been at 1.2 since he started lifting at 30 does not
- ✓Run the seven day home blood pressure protocol if it has not been done, and get an HbA1c, fasting insulin and uric acid on the same draw, since these are the inputs that decide the kidneys' future
Months 2 to 3: Remove the avoidable stressors
- ✓Replace daily or near daily NSAIDs with acetaminophen, topical diclofenac or a training adjustment, and track whether the soreness that seemed to need them was really about recovery
- ✓Bring the home blood pressure average under 130/80 through weight, alcohol, sodium and sleep apnoea treatment first, and medication where the physician judges it necessary. This single step does more for the kidneys than everything else on the list combined
- ✓Confirm daily fluid intake is sensible for body size and training load, roughly 35 mL per kilogram plus training losses, without forcing intake beyond thirst, which has no kidney benefit in healthy men
Months 4 to 6: Address the metabolic drivers
- ✓If HbA1c is 5.7 or higher or fasting insulin is above 10 µIU/mL, treat insulin resistance as the kidney priority it is: resistance training three to four times a week, zone 2 cardio, protein at 1.6 grams per kilogram and a reduction in refined carbohydrate. This is the same protocol that raises testosterone and lowers blood pressure
- ✓Discuss with the physician whether a GLP-1 or SGLT2 inhibitor is appropriate if there is diabetes, a uACR above 30 or an eGFR trending down, since the trial evidence for kidney protection in those groups is among the strongest in medicine
- ✓If uric acid is above 7 mg/dL, act on it. High uric acid is both a marker and a probable contributor to kidney decline, and the interventions overlap with everything above
Months 7 to 12: Retest and set the cadence
- ✓Repeat eGFR, cystatin C and uACR at month 6 and month 12 with identical preparation. The goal is a flat or improving cystatin C eGFR and a uACR that stays under 30 or falls toward it
- ✓For a man on testosterone therapy, add these three tests to the routine haematocrit, PSA and estradiol panel every six months, and treat a rising blood pressure or haematocrit as a kidney issue as much as a cardiovascular one
- ✓Set the long term cadence: annual eGFR plus uACR for every man over 40, twice yearly for men with hypertension, insulin resistance, a family history of kidney disease or a uACR that has ever been above 30. Nephrons do not grow back, and a trend caught in G2 is a trend that can be stopped
The Bottom Line
Kidney function in a man over 40 is estimated from a muscle waste product, which means the number is routinely wrong in exactly the men who train, take creatine and use testosterone. Two cheap tests fix that: cystatin C, which does not care about muscle, and the urine albumin to creatinine ratio, which catches damage years before eGFR moves. Order both, prepare the same way every time, and treat blood pressure, blood sugar and the ibuprofen bottle as the kidney interventions they are. The filter does not regenerate. The good news is that almost nothing that protects it is specific to the kidneys, and a man who is already working on his heart, his waistline and his testosterone has most of the job done.
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