Kidney function declines by an average of 0.75 mL/min per year in healthy adults, without any symptom. The glomerular filtration rate (GFR), estimated from creatinine on a simple blood test, puts a number on this filtering capacity. This renal marker also feeds blood-based biological clocks such as PhenoAge or the Bortz model. Your GFR therefore tells part of the story of how your whole body is aging, provided you read it according to your age rather than against a single cutoff.
In brief – Kidney function is the capacity of the kidneys to filter the blood, measured by the glomerular filtration rate (GFR) in mL/min/1.73 m². In healthy adults, GFR falls by an average of 0.75 mL/min per year (Baltimore study, 254 volunteers followed for up to 23 years), and one third of people lose nothing at all. This decline tracks the gradual disappearance of nephrons and nephrosclerosis, seen in 73% of healthy kidney donors aged 70 to 77 versus 2.7% before age 30. A GFR below 60 does not mean the same thing at 35 and at 80: mortality rises from 75 downward in young adults, but only below 45 in older people. Blood pressure control, a DASH-style diet, regular physical activity and caution with anti-inflammatory drugs are the documented levers for slowing this loss.
Definition: what are kidney function and GFR?
GFR is the volume of plasma that the glomeruli filter every minute, adjusted to a standard body surface area of 1.73 m². Normal kidney function in a young adult sits between 90 and 120 mL/min/1.73 m². The laboratory does not measure this flow directly: it estimates it (eGFR) from blood creatinine, age and sex. The reference equation, CKD-EPI, was published in 2009 by Levey and colleagues, then revised in 2021 by Inker and colleagues without the race coefficient. Cystatin C, a second blood marker, refines the calculation in very muscular or very thin people, because creatinine depends on muscle mass.
GFR does not capture everything the kidneys do. Albuminuria, measured by the urinary albumin-to-creatinine ratio, picks up damage to the filtration barrier before GFR even moves. The two parameters are read together: a moderately reduced GFR without albuminuria in an older person most often reflects normal kidney aging, not disease.
Mechanism: why kidney function declines with age
Each kidney starts life with roughly one million nephrons, and that stock is never replenished. Over the years, glomeruli scar, tubules atrophy and fibrous tissue replaces active parenchyma. Biopsies of 1,203 living kidney donors, published by Rule and colleagues in 2010 (Mayo Clinic), show this nephrosclerosis in 2.7% of 18 to 29 year olds, 28% of 40 to 49 year olds, 58% of 60 to 69 year olds and 73% of 70 to 77 year olds. Neither blood pressure nor blood sugar explains this progression. Four processes overlap to erode kidney function:
- Nephron loss: the volume of the renal cortex shrinks and the remaining nephrons do not compensate. GFR per nephron stays stable in healthy aging, according to the review by Hommos, Glassock and Rule (2017), so total GFR follows the fall in nephron numbers.
- Cellular senescence: tubular cells accumulate the p16 protein, stop dividing and secrete pro-inflammatory and pro-fibrotic factors. The review by O’Sullivan and colleagues (2017) describes this mechanism as a central driver of renal decline.
- Low-grade inflammation: the inflammaging described by Franceschi and Campisi (2014) sustains interstitial fibrosis and the loss of capillaries.
- Vascular aging: arterial stiffness and arteriolosclerosis expose the glomeruli to excessive pressure, then to scarring.
The kidney is thus a concrete illustration of one of the aging mechanisms described in our pillar page on cellular senescence: cells that do not die, but poison their surroundings.
What the science says: GFR, aging and mortality
The age-related decline in kidney function has been measured since 1985: in the Baltimore Longitudinal Study, Lindeman, Tobin and Shock followed 254 volunteers with no kidney disease and no medication for up to 23 years. Creatinine clearance fell by 0.75 mL/min per year on average, but one third of participants showed no decline at all. In 2020, the meta-analysis by Eriksen and colleagues, based on 4,209 direct GFR measurements (iohexol clearance) in three European cohorts aged 50 to 97, confirmed the phenomenon. Healthy men lose 0.72 mL/min/1.73 m² per year, versus 1.03 in those with accumulated diseases or risk factors. Women lose 0.92 versus 1.22. Healthy aging slows the loss, it does not abolish it.
A reduced GFR weighs on survival, but in a graded way. The CKD Prognosis Consortium meta-analysis published in The Lancet in 2010 (105,872 participants from 14 cohorts) finds no excess mortality between 75 and 105 mL/min/1.73 m². Compared with a GFR of 95, the risk of death from all causes is multiplied by 1.18 at 60, by 1.57 at 45 and by 3.14 at 15. Albuminuria acts independently and multiplicatively: an albumin-to-creatinine ratio of 3.4 mg/mmol (about 30 mg/g) already raises the risk by 63%.
These figures led 24 European nephrologists, headed by Delanaye (2019), to call for an age-adapted definition of chronic kidney disease. Below age 40, mortality rises as soon as GFR drops under 75; in older people, only under 45. A single cutoff of 60 labels as sick seniors whose kidneys are aging normally, and wrongly reassures people in their thirties who are already losing ground.
Blood-based biological clocks confirm this informative value. Creatinine is one of the 9 biomarkers of PhenoAge, published by Levine and colleagues in 2018. The model by Bortz and colleagues (2023), trained on 306,116 UK Biobank participants with 25 biomarkers, retains cystatin C, a renal marker, among its variables. It predicts mortality with a C-index of 0.778, versus 0.750 for PhenoAge.
| Study | Population | Measurement | Key result |
|---|---|---|---|
| Lindeman 1985 | 254 healthy volunteers, Baltimore | Creatinine clearance, 23 years | Fall of 0.75 mL/min per year; one third with no decline |
| Rule 2010 | 1,203 living kidney donors | Kidney biopsy | Nephrosclerosis: 2.7% before age 30, 73% at 70 to 77 |
| Eriksen 2020 | 4,209 measurements, ages 50 to 97, Europe | Iohexol clearance (measured GFR) | Loss of 0.72 (healthy men) to 1.22 (women with risk factors) mL/min per year |
| CKD-PC 2010 | 105,872 adults, 14 cohorts | Estimated GFR and albuminuria | Mortality x 1.18 at GFR 60, x 1.57 at 45, x 3.14 at 15 |
| Bortz 2023 | 306,116 participants, UK Biobank | 25 blood biomarkers | Cystatin C retained; C-index of 0.778 |
In practice: reading your GFR according to your age
A GFR of 55 at age 78 with no albuminuria and no hypertension most often reflects a kidney that has aged at a normal pace; the same figure at 40 signals an early loss that deserves a workup. The official classification of kidney function, set out by the National Institute of Diabetes and Digestive and Kidney Diseases, distinguishes five stages based on GFR. In France, an estimated 6.5 to 9.9% of people aged 65 to 74 have chronic kidney disease, and diabetic nephropathy accounts for 30 to 40% of new cases of kidney failure.
| Stage | GFR (mL/min/1.73 m²) | Meaning |
|---|---|---|
| Stage 1 | 90 and above | Kidney disease without reduced GFR (albuminuria or imaging abnormality) |
| Stage 2 | 60 to 89 | Mild reduction, common and often physiological after age 60 |
| Stage 3 | 30 to 59 | Moderate kidney failure |
| Stage 4 | 15 to 29 | Severe kidney failure |
| Stage 5 | Below 15 | End-stage kidney failure |
To interpret a GFR result, the alert threshold proposed by Delanaye and colleagues depends on age. This benchmark comes from research and has not yet replaced the official cutoff of 60, but it helps put a slightly low GFR after 65 into perspective, or avoid shrugging off a GFR of 70 at age 35.
| Age group | Current official threshold | Proposed threshold | Reading |
|---|---|---|---|
| Under 40 | 60 | 75 | A GFR of 70 is not reassuring at this age |
| 40 to 65 | 60 | 60 | The classic threshold remains relevant |
| Over 65 | 60 | 45 | Between 45 and 59 without albuminuria, monitoring rather than disease |
Kidney function is then combined with the other blood markers of aging. Our article on the Bortz method and its optimal values details the 25 biomarkers of the model, and the one on phenotypic age and its 9 markers shows where creatinine fits in PhenoAge. To receive this kind of benchmark every week, La lettre santé, the UltraSanté newsletter, covers the key studies on longevity.
Protocol: 5 levers to preserve kidney function
No measure restores lost nephrons. The following levers slow the loss of renal function, with effects measured in cohorts or trials:
- Control blood pressure and blood sugar. Hypertension and diabetes are the two leading causes of chronic kidney failure in France. Annual monitoring of GFR and albuminuria is recommended as soon as one of these factors is present, and after age 60.
- Adopt a DASH-style diet. In the ARIC cohort (14,882 adults followed for 23 years), Rebholz and colleagues (2016) observe 16% more kidney disease in the third of participants furthest from this eating pattern. Red and processed meat weigh negatively, while nuts, legumes and low-fat dairy protect.
- Move more than 150 minutes per week. In 256 patients with chronic kidney disease, Robinson-Cohen and colleagues (2014) measure an annual GFR loss of 6.2% in active patients versus 9.6% in inactive ones. Each additional weekly hour slows the decline by 0.5% per year.
- Drink to thirst, without forcing it. The CKD WIT randomized trial by Clark (JAMA, 2018), conducted in 631 patients, shows that an extra 0.6 L (about 20 fl oz) of water per day for one year does not slow the fall in GFR (2.2 versus 1.9 mL/min). Adequate hydration remains useful, as our article on kidney stones and hydration explains, but drinking more does not rejuvenate the kidneys.
- Limit nephrotoxic medications. Non-steroidal anti-inflammatory drugs taken repeatedly, especially alongside a diuretic or a blood pressure treatment, reduce renal blood flow. Seek medical advice before any prolonged use, and mention a low GFR before any exam with contrast agent.
Tobacco belongs on this list too: it accelerates renal arteriolosclerosis and albuminuria. A GFR that stays stable from one year to the next, with normal albuminuria, is a better indicator of kidneys that are aging well than any single value.
Frequently asked questions about kidney function and GFR
What is the normal decline in kidney function with age?
In healthy adults, GFR falls by 0.72 to 0.92 mL/min/1.73 m² per year after age 50, according to the European meta-analysis by Eriksen (2020). The Baltimore study (1985) found 0.75 mL/min per year in 254 volunteers, with one third of people showing no decline. A GFR of 60 to 75 after age 70 therefore remains within the expected trajectory if albuminuria is normal.
Is a GFR of 55 after age 70 kidney failure?
By the official definition, yes: any GFR below 60 for more than three months is classified as stage 3. But the review by Delanaye (2019) shows that excess mortality only appears below 45 in older people. Seniors without albuminuria or hypertension carry a very modest risk. This finding calls for annual monitoring of GFR and a review of medications, not necessarily a disease label.
Why is GFR part of the biological age calculation?
Creatinine and cystatin C reflect the number of active nephrons, muscle mass and vascular health all at once. These markers predict mortality independently of age, which makes them relevant variables for PhenoAge (Levine, 2018, 9 biomarkers) and for the Bortz model (2023, 25 biomarkers, UK Biobank). Kidney function that is low for your age translates into a higher estimated biological age.
How can you improve kidney function naturally?
Lost nephrons do not grow back, but the decline can be slowed. The documented measures are blood pressure and blood sugar control, a DASH-style diet (16% less kidney disease in the ARIC cohort), more than 150 minutes of physical activity per week, quitting smoking and limiting anti-inflammatory drugs. Drinking beyond thirst showed no benefit on GFR in the CKD WIT trial (2018).
Medical disclaimer. The information provided here is for informational purposes only and does not constitute medical advice. It does not replace a consultation. Ask a healthcare professional before changing your diet, taking dietary supplements or starting a new practice, especially if you have a medical condition, are pregnant or are under treatment. Dietary supplements do not replace a balanced diet or medical follow-up.
