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Renal System
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In one pass Your kidneys do not use pain as an alarm, so the only way to know how they are doing is a lab test.
Educational content, not medical advice — consult a clinician.
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Chapter 1
Two 150-g filters
Your kidneys do not use pain as an alarm, so the only way to know how they are doing is a lab test. The pain nerves sit in the capsule, the thin shell around each kidney; the tissue that actually filters blood cannot feel pain. Only things that stretch or pull on the capsule, such as a stuck stone or an acute infection, hurt. That is why chronic kidney disease (: damage to the kidney's structure or function lasting more than 3 months) gives almost no warning in its early and middle stages. There is no pain, no swelling, and no drop in urine.
The two kidneys sit on either side of your lower back. Their job is not to absorb nutrients but to re-filter all of your blood, over and over, keeping what is useful and discarding waste. The working unit is the nephron: one glomerulus joined to a chain of tubules. The number of nephrons is essentially fixed at birth and they do not regrow. They slowly fall with age, and high blood pressure and diabetes make them fall faster.
Two situations cannot wait. One: you already have kidney disease and a blood test shows clearly high potassium, or you get palpitations, an irregular heartbeat or muscle weakness. Two: you are dehydrated while taking a diuretic, an ACE inhibitor or blood-pressure drug, and a painkiller from the group, and your urine output suddenly drops. Both need medical care right away.
The two kidneys sit on either side of your lower back. Their job is not to absorb nutrients but to re-filter all of your blood, over and over, keeping what is useful and discarding waste. The working unit is the nephron: one glomerulus joined to a chain of tubules. The number of nephrons is essentially fixed at birth and they do not regrow. They slowly fall with age, and high blood pressure and diabetes make them fall faster.
Two situations cannot wait. One: you already have kidney disease and a blood test shows clearly high potassium, or you get palpitations, an irregular heartbeat or muscle weakness. Two: you are dehydrated while taking a diuretic, an ACE inhibitor or blood-pressure drug, and a painkiller from the group, and your urine output suddenly drops. Both need medical care right away.
Numbers · How much blood the kidneys filter a day
Each kidney weighs about 150 g, only 300 g for the pair, which is 0.5% of body weight, yet together they take 20–25% of the heart's output. Gram for gram they receive more blood than the brain does. That blood is not there to feed the kidney. It is there to be filtered again:Blood flow: about 1.2 L/min, so about 1700 L of blood passes through both kidneys each dayFiltration, meaning how much fluid the glomeruli filter out per minute (GFR, glomerular filtration rate): about 125 mL/min, or about 180 L of filtrate a day. That is like filtering the body's whole 3 L of plasma 60 times a dayReabsorption: 99.4%. Nearly all of the water, sodium, glucose and amino acids are taken back, and the final urine is about 1 mL/min, or 1–2 L a dayEnergy: at rest the kidneys use about 8% of the body's oxygen, and about 80% of their goes to the sodium-potassium pump (Na/K-ATPase) moving sodium, which is why the kidney is highly sensitive to a loss of blood supply
The nephron numbers: each kidney has about 1 million nephrons, fixed at birth and never regrown. Each one is a glomerulus followed by 4 tubule segments in series. After age 40 a kidney loses roughly 6000 a year, and aging, high blood pressure and diabetes all speed this up.
Two kidneys are not a simple double backup. Normally each does half the work and each keeps some spare capacity. After someone donates a kidney, the remaining one enlarges and its own GFR rises by 40–50%, but the spare capacity is gone, which is why living donors are carefully assessed for long-term risk. That reserve is invisible day to day and shows its value only in old age or illness.
Besides making urine, the kidneys do all of this every day:
Keep blood pressure up: through a hormone relay called the renin-angiotensin-aldosterone system (), plus the balance of sodium and water going in and outKeep blood volume steady: antidiuretic hormone (ADH) is made in the hypothalamus and released from the posterior pituitary, and it acts on the kidney's collecting ducts to set how concentrated the urine isKeep blood acidity (pH) in range: they excrete H⁺ and recover HCO₃⁻, sharing the work with the lungs. The lungs adjust within minutes; the kidneys take 24–48 h to have a clear effect, but they correct more completelyKeep potassium in range: aldosterone acts on the distal tubule and collecting duct to set how much potassium is excreted. Excess potassium leaves through the kidneys, and high potassium in is a true emergency that can kill within hours if nothing is done. If you already have kidney disease and a blood test shows clearly high potassium, or you get palpitations, an irregular heartbeat or muscle weakness, get medical care immediatelyActivate vitamin D: they add a second hydroxyl group to the sent by the liver, turning it into the hormone form, 1,25(OH)₂DMake EPO (erythropoietin): when they sense low oxygen they release this hormone, which tells the bone marrow to make red blood cellsGluconeogenesis, making new glucose from non-sugar raw materials: during long fasting the kidneys supply a sizable share alongside the liver, which most people do not realize
Chronic kidney disease has no symptoms early on. When swelling, anemia and low urine output finally appear, it is often already at stage G4–G5, the last two stages. That is why the KDIGO 2024 guideline advises people with risk factors such as diabetes or high blood pressure to have two tests together: (estimated glomerular filtration rate, calculated from blood creatinine) and the urine albumin-to-creatinine ratio (UACR, which shows how much albumin is leaking into the urine).
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Chapter 2
How the glomerulus filters blood
Protein leaking into the urine means the glomerulus, the kidney's sieve, has been damaged, and drinking more water will not wash it away.
The glomerulus is a tuft of capillaries wrapped in a small cup called Bowman's capsule. Here plasma passes through a three-layer sieve that sorts by size and electric charge. Tiny windows in the vessel lining hold back blood cells. A negatively charged basement membrane pushes away albumin, which is also negatively charged. Then the slits between podocytes, cells that wrap around the capillaries like octopus arms, add a third cut. If any of the three layers is damaged, protein leaks into the urine.
So proteinuria is chemical evidence that the sieve is damaged. The usual causes are diabetic kidney disease, hypertensive kidney disease, or a primary disease of the glomeruli. If a checkup shows protein in the urine, or a raised urine albumin-to-creatinine ratio (UACR), the right move is to have a doctor find the cause, not to go home and drink more water.
The glomerulus is a tuft of capillaries wrapped in a small cup called Bowman's capsule. Here plasma passes through a three-layer sieve that sorts by size and electric charge. Tiny windows in the vessel lining hold back blood cells. A negatively charged basement membrane pushes away albumin, which is also negatively charged. Then the slits between podocytes, cells that wrap around the capillaries like octopus arms, add a third cut. If any of the three layers is damaged, protein leaks into the urine.
So proteinuria is chemical evidence that the sieve is damaged. The usual causes are diabetic kidney disease, hypertensive kidney disease, or a primary disease of the glomeruli. If a checkup shows protein in the urine, or a raised urine albumin-to-creatinine ratio (UACR), the right move is to have a doctor find the cause, not to go home and drink more water.
Mechanism · How the three-layer sieve stacks
Each nephron has one glomerulus, and here the most important physical event in the urinary system takes place: plasma is sieved by molecular size and electric charge. The three layers, from the blood side to the urine side:1. Fenestrated endothelium: pores of about 70 nm in the capillary lining, which hold back blood cells and large particles
2. Basement membrane (GBM): a negatively charged layer of glycosaminoglycans. Albumin, about 7 nm across and itself negatively charged, is mostly stopped here; when diabetes, high blood pressure, IgA nephropathy and similar diseases damage this layer, proteinuria appears
3. Podocytes: octopus-shaped cells whose foot processes leave slits of about 25 nm, bridged by key proteins called nephrin and podocin. When this layer is destroyed (for example in minimal change disease or focal segmental glomerulosclerosis, FSGS), the proteinuria is often heavy
So proteinuria is not something you "flush out with more water". It means at least one of the three layers is broken. Most of the time it points to diabetic kidney disease, hypertensive kidney disease or a primary glomerular disease, and it is worth a kidney specialist's look.
GFR (glomerular filtration rate, how much fluid is filtered per minute) is the core kidney number:
A normal adult is about 90–120 mL/min/1.73m²Below 60 for 3 months, or signs of kidney damage such as persistent albumin in the urine for 3 months, meets the definition of chronic kidney disease ()Below 15 is kidney failure (formerly called end-stage renal disease, ESRD), and it is usually time to discuss dialysis or a transplant
Clinical · How to read creatinine and eGFR
Serum creatinine (sCr) is the most common kidney marker on a checkup, yet most people do not know what it measures.Where creatinine comes from: every day about 1–2% of the creatine in muscle breaks down on its own, without an enzyme, into creatinine. In someone whose muscle mass is stable, creatinine is produced at an almost constant rate. It is almost entirely filtered by the glomeruli, not reabsorbed, and only slightly secreted by the tubules. So blood creatinine is an indirect measure of how well the kidneys excrete, but it has two traps.
Trap 1: different muscle mass means a different baseline. A muscular man who lifts may have a creatinine of 100–110 μmol/L with perfectly normal kidneys. A frail older woman with a creatinine of 60 μmol/L looks normal, yet her true filtration rate may be far lower than the calculated : chronic kidney disease that is being missed. People who have had an amputation or who are bedridden for a long time make less creatinine, so their blood creatinine is low and their eGFR is overestimated.
Trap 2: creatinine is slow to react. If GFR suddenly falls by half (50%), creatinine usually takes 1–2 days to climb. In the early phase of acute kidney injury (AKI), creatinine can still be in the normal range while the kidney is already being damaged. That is why researchers are looking for earlier markers such as NGAL, KIM-1 and cystatin C.
eGFR (estimated glomerular filtration rate) is the standard way to read it today. The usual formula is the 2021 CKD-EPI equation, which uses creatinine, age and sex and no longer includes race. It is more accurate than creatinine alone but is still affected by muscle mass. For older, very thin, amputee or heavily muscled people, adding cystatin C helps correct it. When a more precise number is needed, as in acute kidney injury, a 24-hour urine collection can measure creatinine clearance.
Reading it by the KDIGO 2024 stages:
G1 (eGFR ≥ 90): normal, as long as there is no albuminuria or other sign of kidney damageG2 (60–89): mildly reduced; without albuminuria it is not chronic kidney diseaseG3a (45–59): early chronic kidney disease, which many people do not know they haveG3b (30–44): moderately reduced; needs closer follow-up and a doctor's managementG4 (15–29): severely reduced; time to start discussing dialysis and transplantG5 (< 15): kidney failure
Judging your kidneys from a single creatinine number is a very common misreading of a checkup report. Read eGFR, UACR and the trend across several results together.
Mechanism · How four tubule segments refine filtrate
The glomerulus only does a coarse filtering: 180 L of filtrate a day, containing everything, including water, sodium, potassium, glucose, amino acids and bicarbonate. The real refining happens in the four tubule segments that follow. They take back 99% of it and adjust each electrolyte and the acid-base balance one by one. Understand these four segments and you understand most of a pharmacology chapter on diuretics.Proximal tubule: the main reabsorption site. About 65% of the sodium and water, nearly all of the glucose and amino acids, and about 80–90% of the bicarbonate are recovered here. It is also where the kidney spends the most (active transport by the sodium-potassium pump), so it is the first part to fail when blood supply drops. Carbonic anhydrase in this segment helps recover HCO₃⁻, and carbonic anhydrase inhibitors such as acetazolamide act here; the same drug is used to prevent altitude sicknessLoop of Henle: it builds a concentration gradient in the kidney's inner zone (the medulla) that gets saltier the deeper you go, which is what lets the body concentrate urine and save water. Loop diuretics such as furosemide block this segment's Na-K-2Cl transporter; they are the strongest diuretics and are used for heart failure and fluid on the lungsDistal tubule: fine-tunes sodium and calcium. Thiazide diuretics act here and are one of the first-line drugs for high blood pressureCollecting duct: the final adjustment point, taking orders from two hormones. Aldosterone decides how much sodium is taken back and how much potassium and H⁺ are excreted. Antidiuretic hormone (ADH) decides whether water channels (aquaporins) are inserted into the duct wall, which sets how concentrated the final urine is
Two things that are often confused become clear here. The first is acid-base balance. The lungs adjust pH within minutes by breathing out CO₂. The kidneys use these segments to excrete H⁺ and to recover and make new HCO₃⁻; they take 24–48 hours to have a clear effect, but they correct more completely. The acid-base chapter of the respiratory story covers the lung half of the same job. The second is potassium. Blood potassium is removed almost entirely by the kidneys, mainly in the collecting duct under aldosterone's control. So when kidney function falls markedly, potassium tends to rise, and high potassium (hyperkalemia) can cause a fatal heart rhythm problem within hours. It is an emergency.
Read the other way, if a checkup shows glucose in the urine while blood glucose is normal, amino acids in the urine, and low blood phosphate, the problem may be in the proximal tubule (for example Fanconi syndrome) rather than in blood sugar. Clues like these only make sense once you know how the four segments divide the work.
Chapter 3
How the kidney restores blood pressure
When blood pressure falls, the kidney starts a hormone relay to bring it back up. The relay is called the renin-angiotensin-aldosterone system ().
The first runner is the juxtaglomerular apparatus, a cluster of cells on the wall of the small artery that carries blood into the glomerulus. As soon as it senses pressure dropping, it releases renin. Renin cuts angiotensinogen, made by the liver, into angiotensin I, and ACE (angiotensin-converting enzyme) in the blood vessels of the lungs converts that into angiotensin II. This runner does three things at once. It tightens small arteries. It tells the adrenal gland to release aldosterone, which makes the kidney excrete less sodium. And it tells the hypothalamus to create thirst and release antidiuretic hormone, which makes the kidney excrete less water. With vessels tighter and blood volume back up, blood pressure returns.
Once pressure has recovered, the juxtaglomerular apparatus senses it and renin release stops: a negative-feedback loop. Two of the most widely used blood-pressure drugs, ACE inhibitors (the "-pril" drugs) and angiotensin receptor blockers (, the "-sartan" drugs), both work by blocking this relay.
The first runner is the juxtaglomerular apparatus, a cluster of cells on the wall of the small artery that carries blood into the glomerulus. As soon as it senses pressure dropping, it releases renin. Renin cuts angiotensinogen, made by the liver, into angiotensin I, and ACE (angiotensin-converting enzyme) in the blood vessels of the lungs converts that into angiotensin II. This runner does three things at once. It tightens small arteries. It tells the adrenal gland to release aldosterone, which makes the kidney excrete less sodium. And it tells the hypothalamus to create thirst and release antidiuretic hormone, which makes the kidney excrete less water. With vessels tighter and blood volume back up, blood pressure returns.
Once pressure has recovered, the juxtaglomerular apparatus senses it and renin release stops: a negative-feedback loop. Two of the most widely used blood-pressure drugs, ACE inhibitors (the "-pril" drugs) and angiotensin receptor blockers (, the "-sartan" drugs), both work by blocking this relay.
Mechanism · How the RAAS relay is passed on
Antihypertensive drugs fall into 5 main classes, and 2 of them (ACEi and ) act on this relay. Follow it runner by runner and you understand much of a cardiovascular pharmacology textbook.The relay starts at the juxtaglomerular apparatus (JGA). It sits on the wall of the afferent arteriole, the small artery that carries blood into the glomerulus, next to a patch of cells called the macula densa. This small cluster of cells watches three things at once, and if any of them goes wrong it releases the first signal, renin:
1. Pressure in the afferent arteriole drops → renin is released
2. The macula densa senses that the fluid flowing past it in the tubule has become low in sodium (Na⁺) → renin is released
3. When you are stressed, sympathetic nerves stimulate its β1 receptors directly → renin is released
Then the baton passes:
Renin is a protease. It cuts angiotensinogen, made by the liver, into angiotensin I (AT-I)ACE (angiotensin-converting enzyme), found mainly on the lining of lung blood vessels, converts AT-I into AT-IIAT-II is the hormone that actually does the work, and it does three things at once: it constricts small arteries directly, raising blood pressure; it stimulates the zona glomerulosa of the adrenal cortex to secrete aldosterone; and it stimulates the hypothalamus to create thirst and release antidiuretic hormone (ADH)Aldosterone is a steroid hormone that acts on the distal tubule and collecting duct, making them take back more Na⁺ and excrete more K⁺
The result: sodium and water are retained → blood volume rises → blood pressure rises → the JGA senses it → renin secretion stops (a negative-feedback loop).
Three drug classes each block one point. ACE inhibitors (ACEi) inhibit ACE, so AT-II cannot be made. Angiotensin receptor blockers (ARBs) block AT-II's receptor. Mineralocorticoid receptor antagonists (MRAs) block aldosterone's receptor.
Clinical · Why ACE inhibitors and ARBs protect kidneys
ACE inhibitors (ACEi, the "-pril" drugs) and angiotensin receptor blockers (, the "-sartan" drugs) are not only blood-pressure drugs. They also protect the kidneys, and that distinction matters.The mechanism: AT-II constricts the small arteries at both ends of the glomerulus, but the efferent arteriole, the one carrying blood out, is more sensitive. With the exit squeezed harder than the entrance, pressure inside the glomerulus rises. In the short term GFR looks normal, but years of high pressure damage the glomerulus. Once an ACEi or ARB blocks AT-II, the efferent arteriole relaxes, pressure inside the glomerulus falls, proteinuria drops, and chronic kidney disease progresses more slowly.
How they are used (guideline-level advice, KDIGO 2024):
Diabetes with proteinuria: an ACEi or ARB is advised even when blood pressure is normalHigh blood pressure with proteinuria: one of the first choicesHeart failure with reduced ejection fraction: unless there is a contraindication, a drug from this family is usually used
Choose one, not both. In a large randomized trial (ONTARGET) in people with cardiovascular disease or high-risk diabetes, telmisartan (an ARB) and ramipril (an ACEi) worked equally well. Taking both together added no benefit and brought more low blood pressure, fainting and worsening kidney function.
The key difference between the two: an ACEi blocks ACE, which lowers AT-II but also slows the breakdown of bradykinin, so some people develop a dry cough (about 10–20%, possibly more often reported in East Asian populations). An ARB blocks the AT-II receptor directly, leaves bradykinin alone, does not cause the cough, and suits people who cannot tolerate the cough from an ACEi.
Contraindications and cautions:
Pregnancy: never used (it causes birth defects, including underdeveloped fetal kidneys)Narrowing of the arteries to both kidneys (bilateral renal artery stenosis): use can trigger acute kidney failure, so it must be avoidedHigh potassium (K > 5.5): use with caution; stacking it with an aldosterone blocker and potassium supplements can be fatalAcute dehydration, heavy diuretic use and an at the same time (the combination is called the "triple whammy"): it can trigger acute kidney injury, a true emergency that needs the drug stopped and medical care immediately
An MRA (mineralocorticoid receptor antagonist, such as spironolactone) blocks aldosterone at the far end of the . It is used for resistant high blood pressure, heart failure with reduced ejection fraction (HFrEF) and primary aldosteronism. Its common side effects are high potassium and breast enlargement in men (gynecomastia); the latter mainly comes from spironolactone, which is non-selective, while the selective drug eplerenone causes fewer side effects.
Chapter 4
Activating vitamin D, making red cells
The kidney does more than filter blood. It also runs two hormone factories: one turns vitamin D into a working hormone, and one tells the bone marrow to make red blood cells.
The vitamin D you make from sunlight or eat is only a half-finished product. The liver processes it first, and then an enzyme called CYP27B1 in the kidney's proximal tubule does the last step, turning it into the active hormone. Elsewhere in the kidney's outer layer, a group of cells acts as an oxygen probe. When tissue oxygen is low, a switch inside the cell called HIF-α (hypoxia-inducible factor) stops being broken down and turns on the gene for EPO (erythropoietin). EPO travels in the blood to the bone marrow and tells it to make more red cells.
When the kidney fails, both lines collapse together. Too little vitamin D is activated, and calcium, phosphate and bone go out of balance. Too little EPO is made, and anemia appears even when iron and folate are not short.
The vitamin D you make from sunlight or eat is only a half-finished product. The liver processes it first, and then an enzyme called CYP27B1 in the kidney's proximal tubule does the last step, turning it into the active hormone. Elsewhere in the kidney's outer layer, a group of cells acts as an oxygen probe. When tissue oxygen is low, a switch inside the cell called HIF-α (hypoxia-inducible factor) stops being broken down and turns on the gene for EPO (erythropoietin). EPO travels in the blood to the bone marrow and tells it to make more red cells.
When the kidney fails, both lines collapse together. Too little vitamin D is activated, and calcium, phosphate and bone go out of balance. Too little EPO is made, and anemia appears even when iron and folate are not short.
Mechanism · Vitamin D activation and EPO
First, vitamin D. The you make from sunlight or eat has to be processed twice before it can work. The liver first turns it into , the storage form, which can stay in the body for several weeks. Then the CYP27B1 enzyme in the kidney's proximal tubule turns it into 1,25(OH)₂D, the form with true hormone activity, also called calcitriol. Only this final form can regulate calcium and phosphate, bone, immunity and muscle, and it is estimated to switch about 1000 genes on or off. How activated vitamin D enters the cell nucleus to switch genes is covered in the kidney chapter of Vitamin D.This explains something easy to miss: people with chronic kidney disease () often still have low 1,25(OH)₂D even when they take plenty of D3, because a failing kidney also loses its capacity for this final step. That is the chemical starting point of chronic kidney disease–mineral and bone disorder (CKD-MBD).
Now the second job: EPO (erythropoietin). A group of interstitial fibroblasts in the kidney's outer layer act as oxygen probes, constantly sensing whether the tissue has enough oxygen. When oxygen runs low, HIF-α (hypoxia-inducible factor) inside these cells becomes stable instead of being broken down, moves into the nucleus and turns on the EPO gene. The EPO travels in the blood to the bone marrow, where it pushes red-cell precursors to mature into red blood cells. In a healthy adult, 90% of EPO comes from the kidney and the other 10% from the liver. So at high altitude, after blood loss or during long-term low oxygen, this pathway raises EPO and red cells rise with it.
The reverse also holds. When the kidney fails, this blood-making command line collapses too. People with CKD are often anemic even when they are short of neither iron, B12 nor folate, because the kidney no longer makes enough EPO. The standard treatment is injected recombinant human EPO (rhEPO, such as epoetin and darbepoetin, which are man-made EPO). Since around 2019 there is also a class of oral drugs called HIF prolyl hydroxylase inhibitors (HIF-PHIs, such as roxadustat and daprodustat). They keep HIF-α stable, making the body act as if it were short of oxygen so that it releases more of its own EPO, and they also improve iron use. They have become a new option for anemia in CKD.
EPO doping in sport abuses the same pathway. Injecting EPO directly raises red cells, so the blood carries more oxygen and endurance rises. The price is blood that is too thick, with a higher risk of clots and cardiac events; the EPO scandals of 1990s professional cycling were about exactly this.
Clinical · How kidney disease hits bone and vessels
Chronic kidney disease–mineral and bone disorder (CKD-MBD) is one of the most underestimated complications of chronic kidney disease. Calcium, phosphate, vitamin D and vitamin all converge here on a single clinical outcome.The chain (it starts at stage G3 and becomes severe at G4–5):
1. The kidney's ability to excrete phosphate falls → blood phosphate rises
2. CYP27B1 activity falls → 1,25(OH)₂D is low → the gut absorbs less calcium → blood calcium falls
3. The parathyroid glands sense low calcium, high phosphate and low vitamin D all at once → parathyroid hormone () stays high, which is called secondary hyperparathyroidism
4. Chronically high PTH → bone loss (especially cortical bone) → fractures
5. High phosphate plus high PTH → vascular smooth muscle cells shift toward a bone-forming, osteoblast-like state → vascular calcification. Low vitamin K may also play a part, but for this link the evidence so far is mainly observational and mechanistic
The results:
Bone: renal osteodystrophy, with a clearly higher fracture riskVessels: calcification of the aorta and coronary arteries, and more cardiovascular eventsHeart: most people with chronic kidney disease eventually die of cardiovascular disease rather than of kidney failure itself; vascular calcification is one of the reasons
KDIGO's CKD-MBD interventions:
A phosphate-limited diet: phosphate additives in processed foods are the easiest to overlook (sodas, processed meats, instant soups). According to the US National Institutes of Health Office of Dietary Supplements (NIH ODS), 40–70% of the phosphorus naturally in food is absorbed (more from animal than from plant sources), and about 70% of additive phosphate; the fuller comparison is in PhosphorusPhosphate binders: calcium carbonate and calcium acetate (the older generation), sevelamer (calcium-free) and lanthanum carbonateActive vitamin D and its analogs (calcitriol, paricalcitol): they bypass CYP27B1 and supply the active hormone directlyCalcimimetics (cinacalcet): they make the parathyroid glands "think" calcium is sufficient, which lowers PTHVitamin K2 (), vitamin D plus magnesium: there is a mechanistic rationale for reducing vascular calcification and some small studies, but the results are inconsistent and KDIGO does not recommend it
Look at any single nutrient and you will not see this chain reaction. Put them together and it becomes clear why KDIGO manages CKD-MBD as one integrated disease.
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Chapter 5
Does high protein harm the kidneys?
If your kidney function is normal, eating a lot of protein will not wear your kidneys out. After a high-protein meal, filtration in the glomeruli rises briefly. That is reserve capacity being called on, not the sieve being torn. The real drivers of kidney damage are poorly controlled high blood pressure and diabetes, long-term use of painkillers from the group, and certain Chinese herbal remedies that contain aristolochic acid, not the grams of protein on your plate.
People who already have chronic kidney disease do need to limit protein, but that is done by stage and managed by a kidney specialist and a dietitian together. Limiting protein never means simply eating less, because not eating enough does its own harm. A patient's prescription cannot be turned around and applied to healthy people.
People who already have chronic kidney disease do need to limit protein, but that is done by stage and managed by a kidney specialist and a dietitian together. Limiting protein never means simply eating less, because not eating enough does its own harm. A patient's prescription cannot be turned around and applied to healthy people.
Evidence · Healthy kidneys and CKD are different
"High protein damages your kidneys" is one of the most widely repeated nutrition claims. It arose from taking the fact that people with chronic kidney disease need to limit protein and applying it to healthy people. Here is the evidence, point by point.How healthy kidneys respond to high protein: Devries 2018 (J Nutr) was a systematic review and of 28 with 1358 adults without kidney disease. It compared higher-protein diets (at least 1.5 g/kg a day, or at least 20% of energy, or at least 100 g a day) with normal- or lower-protein diets in trials that lasted more than 4 days. The result: the change in GFR from before to after did not differ between the groups (standardized mean difference, SMD, 0.11; 95% −0.05 to 0.27, meaning the plausible range crosses 0). Comparing only the end-of-trial values, GFR was slightly higher in the high-protein groups, which the authors judged a trivial effect. Their conclusion was that in healthy adults, high protein did not harm kidney function as measured by GFR. The main limitation was an unclear risk of selection bias in the included trials. For lifters eating 2–3 g/kg for long periods, some small observational studies also saw no decline in kidney function, but these studies are small and short. Mechanistically, a healthy kidney has plenty of filtration reserve, and the 10–30% short-lived rise in GFR after a protein meal is a normal adaptation, not damage.
People with chronic kidney disease do need to limit protein, but by stage and under professional care. For metabolically stable adults with stages 3–5 who are not on dialysis, KDOQI 2020's approach, under close supervision by a physician and a dietitian, is a low-protein diet of 0.55–0.60 g/kg/d with phosphate restriction, or a very-low-protein diet of 0.28–0.43 g/kg/d topped up with keto-acid or amino-acid analogues, to slow progression (it does not single out G4–G5 for a lower figure). People on maintenance dialysis, by contrast, need 1.0–1.2 g/kg/d, because dialysis removes a good deal of protein. This must be managed jointly by a dietitian and a kidney specialist; otherwise protein-energy wasting (PEW, the draining of muscle and energy stores) can do harm sooner than the kidney disease itself.
China's arithmetic, and one landmine specific to the Chinese table. China's national *Dietary Guideline for Adults with Chronic Kidney Disease (2024 edition)* turns the same idea into something you can calculate. For CKD stages 3–5 the low-protein diet is 0.6 g/kg of ideal body weight per day, and ideal body weight (kg) = height (cm) − 105. Someone 165 cm tall has an ideal body weight of 60 kg, so the protein target is 36 g/day. Energy is set at 30–35 kcal/kg of ideal body weight. Note that it uses ideal body weight, not actual body weight; get that step wrong and the whole plan is off.
Part of the grain can be swapped for high-starch, low-protein foods such as sweet potato, potato, lotus root, Chinese yam and Mung Bean vermicelli, saving the protein allowance for high-quality protein.
The Chinese-table landmine: the guideline explicitly says to limit or avoid rich meat broths and long-simmered soups. Many people think of soup as light, but a long-simmered meat broth concentrates purines, phosphorus and sodium into the liquid, while what stays in the meat is the protein.
⚠️ There is also a risk in the opposite direction that is overlooked more often. The guideline puts the prevalence of undernutrition in people with chronic kidney disease at 18%–75%, and people in stages 3–5 often do not eat enough because their appetite falls. So limiting protein never means simply eating less. This has to be managed by a kidney specialist and a dietitian together, not cut back on your own at home.
A rise in creatinine is not the same as kidney damage. High-protein meals, exercise and strength training all raise creatinine production → blood creatinine rises → the calculated appears to fall. Real kidney damage usually also shows protein in the urine, white cells or blood in the urine, a rise in cystatin C, or changes on imaging. Judging your kidneys from one creatinine number on a checkup is a very common misreading; the glomerulus chapter of this story explains why.
The real risk factors for kidney damage:
The two biggest: poorly controlled diabetes and high blood pressureOveruse of (especially dangerous in older people and when dehydrated)Aristolochic acid in certain Chinese herbal remedies (guan-mu-tong, guang-fang-ji, qing-mu-xiang); mainland China and Taiwan saw many cases of chronic kidney disease caused by it in the 1990s–2000sSmokingExcess weight with little activityLong-term exposure to heavy metals (lead, Pb; cadmium, Cd)
Protein intake does not make this list. In healthy people with normal kidney function, trials so far have not seen 1.6–2.0 g/kg/d damage the kidneys.
Myth · Can a detox tea cleanse your kidneys
"Detox and nourish your kidneys" is one of the biggest markets in supplements, but once you lay out how the kidney works, the claim barely holds up.First, the kidney is already one of the body's detox organs. It filters about 180 L of fluid a day, a scale far beyond anything a tea could influence. In someone with normal kidney function, metabolic waste such as urea, creatinine and uric acid is cleared continuously; there is no backlog waiting for outside help. "Detox" assumes a pile of toxins that cannot get out, and for a healthy kidney that assumption does not hold.
Second, what drinking more water really does is dilute the urine, which lowers the chance that certain stones (especially calcium oxalate and uric acid stones) come back. That is backed by evidence, but it works by diluting crystals in the urine, not by rinsing the kidney itself. About 2–2.5 L of urine a day is a reasonable target. Drinking beyond that does not make the kidney "cleaner", and in extreme cases (several liters in a short time) it can cause low blood sodium (hyponatremia), which has caused deaths.
Third, some "kidney tonic teas" actually harm the kidney. The classic example is herbs that contain aristolochic acid (guan-mu-tong, guang-fang-ji, qing-mu-xiang), a confirmed kidney toxin and carcinogen; mainland China and Taiwan saw many people progress to kidney failure (uremia) from it in the 1990s–2000s. Some slimming or diuretic teas with unclear mixed ingredients can, over long use, upset electrolytes or injure the kidney directly.
Fourth, a counterintuitive point. For people who already have chronic kidney disease, drinking large amounts of water offers no protection, and in the middle and late stages (especially once urine output has fallen) it can worsen fluid and sodium retention and heart failure. How much someone with chronic kidney disease should drink has to be set individually by a kidney specialist, not borrowed from the healthy-person logic of "drink more to detox".
The list that genuinely protects the kidney is plain, and it mirrors the risk factors for kidney damage: control blood pressure and blood sugar, do not overuse , stay away from herbs that contain aristolochic acid, do not smoke, keep a healthy weight, and have and UACR checked regularly. None of it is detox by drinking something. All of it reduces the chances of long-term damage to the kidney.
Chapter 6
Salt and blood pressure
Eating less salt lowers blood pressure, but how much it drops varies a great deal from person to person. For most people, keeping salt under 5 g a day is still a reasonable goal.
Almost all of the sodium in salt leaves the body through the kidneys. The saltier you eat, the more sodium the kidneys have to excrete, and water goes out with it. When that balance is poorly tuned, the body holds on to more sodium and water and blood pressure rises; it is the same machinery as the chapter earlier in this story.
First, the numbers different bodies give. The World Health Organization (WHO) advises no more than 5 g of salt a day (equal to 2 g of sodium). The American Heart Association (AHA) in 2017 was stricter: ideally sodium < 1.5 g, with a realistic target of < 2.3 g. China's 2023 dietary reference values also say salt < 5 g a day. For people already eating the DASH way (plenty of fruit and vegetables, whole grains and low-fat dairy), cutting salt further lowers systolic blood pressure (SBP, the higher number in a reading) by about another 3 mmHg.
In reality, most people are far from these numbers. Chinese adults eat 10–12 g of salt a day on average, twice the WHO limit, and the biggest source is the salt added while cooking, followed by high-salt condiments such as soy sauce.
Almost all of the sodium in salt leaves the body through the kidneys. The saltier you eat, the more sodium the kidneys have to excrete, and water goes out with it. When that balance is poorly tuned, the body holds on to more sodium and water and blood pressure rises; it is the same machinery as the chapter earlier in this story.
First, the numbers different bodies give. The World Health Organization (WHO) advises no more than 5 g of salt a day (equal to 2 g of sodium). The American Heart Association (AHA) in 2017 was stricter: ideally sodium < 1.5 g, with a realistic target of < 2.3 g. China's 2023 dietary reference values also say salt < 5 g a day. For people already eating the DASH way (plenty of fruit and vegetables, whole grains and low-fat dairy), cutting salt further lowers systolic blood pressure (SBP, the higher number in a reading) by about another 3 mmHg.
In reality, most people are far from these numbers. Chinese adults eat 10–12 g of salt a day on average, twice the WHO limit, and the biggest source is the salt added while cooking, followed by high-salt condiments such as soy sauce.
Evidence · Who gains from less salt, and how much
Cut salt by the same amount and different people's blood pressure responds very differently, depending on whether they are salt-sensitive. Salt-sensitive people are more common among those with high blood pressure and among older adults. Across the 34 randomized trials pooled by He 2013, eating about 4.4 g less salt a day lowered systolic pressure by about 5.4 mmHg on average in people with high blood pressure, a good return, and by about 2.4 mmHg in people with normal blood pressure, a smaller gain. Unfortunately there is no simple test that tells you which group you are in. In practice you have to try it: cut salt seriously for four weeks and check whether your blood pressure changes.So is less sodium always better? This is an unfinished argument, and it can serve only as disputed observational background, not as a prescription. The PURE community-level analysis published in *The Lancet* in 2018 (Mente 2018) covered 95,767 people in 369 communities across 18 countries, with 82,544 of them in the cardiovascular-outcome analysis, and it measured each community's average sodium intake. The result was not a symmetric U. In the third of communities with the lowest sodium, lower sodium went with more events, and that end was significant. The middle third showed no association. The highest third trended upward but was not statistically significant. Something often overlooked: the lowest third averaged 4.04 g/day and never went below 3 g, so the study itself cannot answer whether less than 3 g a day is harmful. The AHA and WHO also pushed back that PURE estimated a whole day's sodium from a single urine sample, which is inaccurate, and that people eating very little sodium may already have been quite ill (illness causing low sodium, not low sodium causing illness: reverse causation). A 2021 Cochrane review reaffirmed that cutting salt does lower blood pressure, but it did not address the U-shaped mortality curve.
So the targets do not change because of this argument: for healthy adults, WHO's sodium < 2 g (5 g of salt), or the US NASEM 2019 figure of 2.3 g (the intake for reducing chronic-disease risk). Whether pushing sodium very low (< 1.5 g) brings extra benefit is not yet clear; but the blanket claim that less is always better does rest on weak evidence.
Watching the sodium number alone is not enough; potassium matters too. Eating potassium-rich foods (vegetables, fruit, legumes and whole grains) together with eating less salt lowers blood pressure more reliably. A commonly used combined target is potassium ≥ 3.5 g and sodium ≤ 2.3 g a day. But one group must be careful in the opposite direction: people with moderate-to-advanced chronic kidney disease ( G4–G5) can no longer excrete potassium well, and too much potassium can cause fatal hyperkalemia. They need individual advice from a kidney specialist and should not simply follow the eat-more-potassium advice.
In daily life, for healthy adults and people with early high blood pressure, the steps rank by value like this:
First find out where your sodium comes from. In Western diets most sodium hides in processed food and restaurant meals: Americans eat about 8–10 g of salt a day, and Italians and French people are also at 8–10 g, mostly from bread and processed meat. In Chinese households most of it comes from salt added in cooking, then from high-salt condiments, with processed food a growing third sourceHalve the soy sauce, oyster sauce and chicken bouillon: these are the three hidden sodium sources in a Chinese kitchenDouble your vegetables and fruit, which also brings in potassium, magnesium and fiber (this is the DASH approach)Do not smoke, and drink less alcoholExercise regularly and lose excess weight
he-2013-bmj-salt-reduction
References · 11
- Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. (2024). KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney International, 105(4S), S117-S314. Recommendation 3.3.1.1: 'We suggest maintaining a protein intake of 0.8 g/kg body weight/d in adults with CKD G3-G5 (2C)' - a weak suggestion on low-certainty evidence, described as consistent with the WHO allowance for the general population. Practice points: avoid > 1.3 g/kg/d in adults at risk of progression; for those willing, able and at risk of kidney failure, consider a very low-protein diet (0.3-0.4 g/kg/d) supplemented with essential amino acids or ketoacid analogs (up to 0.6 g/kg/d) under close supervision; no low- or very low-protein diets when metabolically unstable; no protein restriction in children. The Work Group states the evidence does NOT support low-protein diets alone (0.4-0.6 g/kg/d) as a strategy to slow CKD progression. Sodium: < 2 g/day (2C) (full text, KDIGO open-access PDF of the supplement, section 3.3). Comparing with kdoqi-2020-nutrition-ckd needs that guideline's own protein statements, which its record does not carry. 10.1016/j.kint.2023.10.018
- Devuyst, O., Olinger, E., & Rampoldi, L. (2017). Uromodulin: from physiology to rare and complex kidney disorders. Nature Reviews Nephrology, 13(9), 525-544. 10.1038/nrneph.2017.101
- Levey, A. S., & Inker, L. A. (2017). Assessment of glomerular filtration rate in health and disease: a state of the art review. Clinical Pharmacology & Therapeutics, 102(3), 405-419. 10.1002/cpt.729
- Patel, S., Rauf, A., Khan, H., & Abu-Izneid, T. (2017). Renin-angiotensin-aldosterone (RAAS): The ubiquitous system for homeostasis and pathologies. Biomedicine & Pharmacotherapy, 94, 317-325. 10.1016/j.biopha.2017.07.091
- ONTARGET Investigators, Yusuf, S., Teo, K. K., Pogue, J., et al. (2008). Telmisartan, ramipril, or both in patients at high risk for vascular events. New England Journal of Medicine, 358(15), 1547-1559. 10.1056/NEJMoa0801317
- Fishbane, S., & Spinowitz, B. (2018). Update on Anemia in ESRD and Earlier Stages of CKD: Core Curriculum 2018. American Journal of Kidney Diseases, 71(3), 423-435. 10.1053/j.ajkd.2017.09.026
- General Office of the National Health Commission of China. (2024). Dietary guidelines for adults with chronic kidney disease (2024 edition). Issued 2024. Ten principles. Verified specifics: at least 12 food varieties a day and 25 a week, with concentrated meat broths and long-boiled soups (老火汤) limited or excluded; CKD stages 1-2 should be plant-food-led with 300-500 g of vegetables daily, dark-coloured over half; CKD stages 3-5 follow a low-protein diet of 0.6 g per kg of IDEAL body weight per day, where ideal body weight in kg is height in cm minus 105 - so a person of 165 cm has an ideal weight of 60 kg and a protein target of 36 g/day - and staples may be partly or wholly replaced with high-starch low-protein foods such as sweet potato, potato, lotus root, Chinese yam or mung-bean noodles; stage 5 dialysis patients remain plant-food-led with adjustment. Energy is 30-35 kcal per kg ideal body weight per day (1800-2100 kcal at 60 kg), reduced by 500-750 kcal for those who are overweight, with BMI generally held at 18.5-23.9 and allowed higher in people aged 65 and over. The guideline states that undernutrition affects 18 to 75 percent of CKD patients, and that appetite loss in stages 3-5 commonly causes weight loss and inadequate intake. www.nhc.gov.cn/cms-search/downFiles/44dfbbd0f46e48cca7d9d20d883d2add.pdf
- Devries, M. C., Sithamparapillai, A., Brimble, K. S., Banfield, L., Morton, R. W., & Phillips, S. M. (2018). Changes in kidney function do not differ between healthy adults consuming higher- compared with lower- or normal-protein diets: a systematic review and meta-analysis. The Journal of Nutrition, 148(11), 1760–1775. 28 trials, 1,358 healthy adults: after the diets GFR was trivially higher on higher-protein diets (SMD 0.19, 0.07-0.31), but the change in GFR did not differ (SMD 0.11, -0.05 to 0.27); the authors conclude higher protein does not adversely affect GFR in healthy adults; selection-bias risk was unclear (abstract, PMID 30383278). 10.1093/jn/nxy197
- Ikizler, T. A., Burrowes, J. D., Byham-Gray, L. D., et al. (2020). KDOQI Clinical Practice Guideline for Nutrition in CKD: 2020 Update. American Journal of Kidney Diseases, 76(3 Suppl 1), S1-S107. 10.1053/j.ajkd.2020.05.006
- Mente, A., O'Donnell, M., Rangarajan, S., et al. (2018). Urinary sodium excretion, blood pressure, cardiovascular disease, and mortality: a community-level prospective epidemiological cohort study. The Lancet, 392(10146), 496-506. 10.1016/S0140-6736(18)31376-X
- National Academies of Sciences, Engineering, and Medicine. (2019). Dietary Reference Intakes for Sodium and Potassium. National Academies Press. The PubMed book abstract (PMID 30844154) gives no reference values; it says the report updates the sodium and potassium DRIs with an expanded model that adds chronic-disease endpoints (the Chronic Disease Risk Reduction Intake). The numeric AIs and CDRRs were not re-read for this note (abstract). www.ncbi.nlm.nih.gov/books/NBK538102