Kidney Function for Longevity: The Overlooked Organ

TL;DR: The kidneys are a quiet driver of longevity, and the standard test for them can mislead the very people reading this. That test estimates filtration, called eGFR, from creatinine, a waste product of muscle. Because muscular people produce more creatinine, and because creatine supplements raise it too, an athletic or creatine-using person can show a falsely low eGFR that looks like kidney trouble when the kidneys are fine. Cystatin C, a protein measured by a separate blood test, is barely affected by muscle or creatine and makes an excellent cross-check, and the most accurate estimate combines both. The single most overlooked kidney test is a urine check for albumin, the albumin-to-creatinine ratio, which catches damage years before the blood numbers move and independently flags heart risk. All of this counts because chronic kidney disease is common, usually silent until late, and a powerful multiplier of cardiovascular risk, with most people more likely to die of heart disease than to ever reach dialysis. The practical message is to get the right tests, read them correctly, and remember that one abnormal number is a reason to recheck rather than a diagnosis.

Why the standard kidney test can mislead you

Kidney function is usually reported as eGFR, an estimate of how fast the kidneys filter the blood. On a routine panel, that estimate is calculated from creatinine, a waste product the muscles produce and the kidneys clear. The logic is that if the kidneys are clearing creatinine poorly, it builds up, and a higher creatinine points to lower filtration. That works well on average, but it has a blind spot that matters for anyone who trains.

The blind spot is muscle. Creatinine comes from muscle, so the more muscle you carry, the more creatinine you make at any given level of kidney function. A lean, muscular person can therefore have a creatinine that looks high, and an eGFR that looks low, purely because they have more muscle, with perfectly healthy kidneys. The same thing happens with creatine supplements: creatine is converted to creatinine in the body, so taking it raises blood creatinine by simple chemistry, without harming the kidneys at all.¹ The effect at a normal maintenance dose of a few grams a day is usually modest, but it can be enough to tip a borderline result into scary-looking territory. The key point to hold is that this is a measurement artifact, not kidney damage, and it is not a reason to fear creatine, which is one of the better-studied and safer supplements in healthy people. It is a reason to make sure the test is read in context.

What cystatin C adds

This is where a second marker earns its place. Cystatin C is a small protein made by nearly all the cells in the body at a fairly steady rate, filtered by the kidneys, and, crucially, barely influenced by muscle mass, diet, or creatine. Because it does not depend on how muscular you are, a cystatin-C-based eGFR gives a cleaner read in the very people creatinine misleads, the very muscular and, at the other extreme, the frail with little muscle. When the two disagree, that disagreement is itself informative.

Kidney experts now recommend using cystatin C this way. In 2021, a national task force revised the eGFR equations, removing an old race adjustment and endorsing cystatin C, and the combined creatinine-and-cystatin-C estimate as the most accurate approach.² So the best practice, when a creatinine-based number is surprising or the stakes are high, is not to trust either marker alone but to use the combined equation. That said, cystatin C is a cross-check rather than a truth serum. It has its own quirks: it can be pushed up by thyroid disease, steroids, inflammation, obesity, and smoking, independent of the kidneys. The value is in having two windows onto the same organ, so that when they agree you can trust the number, and when they diverge you know to look closer.

The test almost nobody orders: urine albumin

If the blood tests estimate how well the kidneys filter, a urine test tells you whether they are starting to leak, and that leak often shows up first. Healthy kidneys keep protein in the blood; damaged ones let small amounts of a protein called albumin slip into the urine. Measuring that, as the albumin-to-creatinine ratio in a urine sample, detects kidney damage at an early stage, frequently years before the eGFR falls at all.³ It is one of the earliest warnings the kidney gives.

Two things make this test more valuable than its neglect suggests. First, it is an early marker, so it can catch a problem while there is the most time to act. Second, albumin in the urine is an independent flag for cardiovascular risk, tied to higher rates of heart attack, heart failure, and death even when kidney filtration still looks normal, because it reflects the health of the small blood vessels throughout the body. The kidney-health guidelines stage chronic kidney disease using both numbers together, the filtration estimate and the urine albumin, which is why checking only the blood creatinine leaves half the picture unseen. For anyone serious about prevention, the urine albumin test is cheap, informative, and worth asking for.

Why your kidneys matter for longevity

It is easy to think of the kidneys as a background utility, but they belong near the center of a longevity plan, for one blunt reason: kidney disease is common, it is silent, and it multiplies the risk of the thing most likely to kill you.

Chronic kidney disease affects a large share of adults, often without any symptoms until it is advanced, which is why it goes undetected so often. Its danger is less about the kidneys failing outright and more about what it does to the heart and blood vessels. A person with moderate kidney disease is far more likely to die of cardiovascular disease than to ever end up on dialysis; the kidney problem is, in effect, a cardiovascular risk multiplier that runs silently in the background. This is also why cystatin C draws interest beyond the kidney: higher levels track with higher cardiovascular and mortality risk,⁴ partly because cystatin C catches mild kidney impairment that creatinine misses, though this is an association that reflects overall risk rather than a direct measure of the arteries. The through-line is that the kidneys are an early, sensitive readout of vascular aging, and reading them well gives an earlier warning than waiting for the heart to declare itself.

What to do about it

The actions follow from the tests. The first step is simply to measure the right things and read them correctly: an eGFR that includes cystatin C when creatinine might mislead, meaning in muscular people, creatine users, and anyone whose number is borderline or surprising, plus a urine albumin-to-creatinine ratio to catch early leakage. The second step is to respect the persistence rule: one abnormal eGFR is not a diagnosis of kidney disease, which by definition requires the abnormality to last about three months or to come with albumin in the urine, so a single low reading calls for a repeat and a calmer look, not alarm.

When there is a genuine early signal, the levers are well established and increasingly powerful. Controlling blood pressure and blood sugar, not smoking, and treating the metabolic drivers protect the kidneys and the heart at once. And a group of medications now slows kidney decline meaningfully: SGLT2 inhibitors and, on the strength of the 2026 FIND-CKD trial, finerenone both help in kidney disease with protein leakage whether or not diabetes is present, and, for people with type 2 diabetes and kidney disease specifically, the GLP-1 drug semaglutide has been shown to reduce kidney and cardiovascular events.⁵ The unifying idea is that the kidney is a place where problems can be caught years early and acted on, which is what a longevity strategy should want.

Guidance from the Clinic

"I see this scenario constantly: a fit, muscular patient who lifts and takes creatine comes in frightened because a routine panel said their kidney function was low. Almost always, the kidneys are fine, and the creatinine is high because of muscle and creatine, not disease. So the first thing I do is add a cystatin C and often a urine albumin, and the combined picture usually clears it up. The mistake I want to prevent is a healthy athlete being told they have kidney disease off one misread number, or the opposite, a true early problem being missed because we only ever looked at creatinine. The kidneys are one of the best early-warning systems we have for the heart and for aging, and they are cheap to check properly. My rule is to read them in context, use two markers when it matters, always look at the urine, and never diagnose off a single value."

Related at Fishtown Medicine

• The Advanced Tests Your Doctor Isn't Ordering - where cystatin C and urine albumin fit a fuller workup
• Creatine for the Brain and Body - the supplement behind the creatinine confusion, and why it is safe
• How Much Protein? Muscle, Aging, and Longevity - why high protein raises creatinine without harming healthy kidneys
• Semaglutide (Ozempic) for Kidney Disease - the GLP-1 kidney-protection evidence from FLOW
• SGLT2 Inhibitors: Heart, Kidney, and Longevity - the drug class that slows kidney decline broadly
• Finerenone (Kerendia) - kidney and heart protection, now with evidence beyond diabetes
• How to Prevent Kidney Stones - the evidence-based way to protect the kidneys from stones

Scientific References

1. Kreider RB, Kalman DS, Antonio J, et al. "International Society of Sports Nutrition Position Stand: Safety and Efficacy of Creatine Supplementation in Exercise, Sport, and Medicine." Journal of the International Society of Sports Nutrition. 2017;14:18.
2. Inker LA, Eneanya ND, Coresh J, et al. "New Creatinine- and Cystatin C-Based Equations to Estimate GFR without Race." New England Journal of Medicine. 2021;385(19):1737-1749.
3. Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. "KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease." Kidney International. 2024;105(4S):S117-S314.
4. Svensson-Farbom P, Ohlson Andersson M, Almgren P, et al. "Cystatin C Identifies Cardiovascular Risk Better Than Creatinine-Based Estimates of Glomerular Filtration in Middle-Aged Individuals Without a History of Cardiovascular Disease." Journal of Internal Medicine. 2014;275(5):506-521.
5. Perkovic V, Tuttle KR, Rossing P, et al. "Effects of Semaglutide on Chronic Kidney Disease in Patients with Type 2 Diabetes (FLOW)." New England Journal of Medicine. 2024;391(2):109-121.