Kidney disease is a condition in which your kidneys lose their ability to filter waste and excess fluid from your blood. It affects roughly 14% of adults worldwide and can range from mild, easily managed damage to complete kidney failure requiring dialysis or a transplant. The condition comes in two main forms: acute kidney injury, which develops suddenly and is sometimes reversible, and chronic kidney disease, which progresses gradually over months or years and is far more common.
How Your Kidneys Work and What Goes Wrong
Each kidney contains roughly one million tiny filtering units called nephrons. Blood flows through each nephron, where waste products, excess salt, and extra water are separated out and sent to the bladder as urine. Clean blood cycles back into circulation. When nephrons are damaged, this filtering process breaks down, and waste builds up in the bloodstream.
The problem compounds itself. As individual nephrons fail, the surviving ones are forced to work harder to compensate. Hormonal signals push these remaining nephrons to increase their workload, which causes them to enlarge and filter at an unsustainable pace. Over time, that extra strain damages them too, creating a cycle: nephron loss leads to overwork, which leads to more nephron loss. The tissue becomes inflamed, scarred, and increasingly starved of oxygen. Once enough nephrons are destroyed, the kidneys can no longer keep up with the body’s demands.
Acute vs. Chronic Kidney Disease
Acute kidney injury happens fast, often within hours or days. It was first recognized during World War II, when doctors observed that crush injuries could cause a sudden, dramatic shutdown of kidney function. Causes today include severe dehydration, infections, blood loss, medication reactions, and urinary blockages. In many cases, kidney function returns partially or fully with treatment.
That recovery, however, isn’t always as complete as it seems. Multiple studies since 2008 have shown that even mild episodes of acute kidney injury carry a significant risk of progressing to chronic kidney disease later on. People who appear to recover can still experience a slow decline in function over the following years.
Chronic kidney disease (CKD) develops gradually, often over decades. It’s the form most people are referring to when they talk about “kidney disease.” Once kidney tissue is scarred, it doesn’t regenerate. The goal with CKD is to slow the progression and manage the complications.
The Two Biggest Causes
Diabetes and high blood pressure together account for the majority of chronic kidney disease cases. They damage the kidneys through different but related mechanisms.
High blood sugar harms the kidneys from the inside out. Persistently elevated glucose alters the tiny blood vessels within the nephrons, increasing pressure inside the filtering units and triggering inflammation. Over time, this causes protein to leak into the urine, a hallmark sign of kidney damage. The ongoing metabolic stress also generates molecules that injure the cells lining blood vessel walls, further restricting blood flow to kidney tissue.
High blood pressure damages the kidneys through sheer mechanical force. Healthy kidneys have a built-in pressure regulation system that protects their delicate filtering structures from swings in blood pressure. When that system is impaired, as it often is in people with diabetes, elevated pressure transmits directly into the kidney’s smallest blood vessels. The result is inflammation, scarring, and progressive loss of filtering capacity. The two conditions frequently occur together, and each one accelerates the damage caused by the other.
Other causes include autoimmune diseases like lupus, inherited conditions like polycystic kidney disease, chronic urinary tract obstructions, and repeated or prolonged use of certain medications that are toxic to the kidneys.
Symptoms by Stage
This is the most important thing to understand about kidney disease: it usually has no symptoms until it’s advanced. Most people in the early stages feel perfectly fine. For many, the only way to catch it is through routine blood and urine tests. That’s why screening matters so much if you have diabetes, high blood pressure, or a family history of kidney problems.
As the disease progresses and filtering capacity drops substantially, symptoms begin to appear:
- Swelling in the legs, feet, ankles, hands, or face, caused by fluid and salt the kidneys can no longer remove
- Fatigue and sleep problems from waste buildup in the blood
- Changes in urination, including more frequent urination, less frequent urination, or foamy urine
- Loss of appetite, nausea, or vomiting
- Itching, dry skin, or darkened skin
- Difficulty concentrating
- Muscle cramps and unexplained weight loss
- Chest pain or shortness of breath in very advanced stages
How Kidney Disease Is Diagnosed
Doctors rely on two main tests. The first is a blood test that estimates your glomerular filtration rate (eGFR), a measure of how efficiently your kidneys are filtering. A normal eGFR is 90 or above. The second is a urine test called the UACR (urine albumin-to-creatinine ratio), which checks for protein leaking into your urine. A UACR above 30 mg/g signals kidney damage.
The eGFR is more reliable than looking at creatinine levels alone, because creatinine is influenced by muscle mass, age, and sex. Together, these two tests can detect kidney disease years before symptoms appear.
The Five Stages of CKD
Chronic kidney disease is classified into stages based on eGFR, with stage 3 split into two substages:
- Stage 1: eGFR 90 or above, but with signs of kidney damage like protein in the urine
- Stage 2: eGFR 60 to 89
- Stage 3a: eGFR 45 to 59
- Stage 3b: eGFR 30 to 44
- Stage 4: eGFR 15 to 29
- Stage 5: eGFR below 15, or on dialysis
Stages 1 and 2 are mild and often manageable with lifestyle changes and blood pressure or blood sugar control. Stages 3 and 4 represent moderate to severe loss, where complications become more likely and dietary changes become more important. Stage 5 is kidney failure, where the kidneys can no longer sustain life without treatment.
Complications Beyond Filtering
Kidneys do far more than remove waste. They produce hormones, regulate blood pressure, and help maintain the balance of minerals in your blood. As kidney function declines, these secondary roles break down too.
Anemia is one of the most common complications. Healthy kidneys produce a hormone that signals your bone marrow to make red blood cells. Damaged kidneys produce less of it, leading to fatigue, weakness, and shortness of breath as red blood cell counts drop.
Bone and mineral problems are another major concern. Failing kidneys can’t remove excess phosphorus or activate vitamin D properly. Phosphorus builds up in the blood, calcium drops, and the body responds by pulling calcium from the bones. Over time, this weakens bones and can cause calcium deposits in blood vessels and soft tissues. The hormonal system that tries to compensate for this imbalance keeps escalating its signals, but eventually the kidneys simply can’t respond.
Dietary Changes for Kidney Disease
What you eat becomes increasingly important as CKD advances, because damaged kidneys struggle to process certain nutrients. The specifics depend on your stage and lab results, but the general pattern involves limiting three things: protein, potassium, and phosphorus.
Protein creates waste products when digested, and kidneys that are already struggling have a harder time clearing that waste. This doesn’t mean eliminating protein, which your body still needs, but rather finding a balance. Shifting some of your protein intake from animal sources to plant-based sources and choosing lean, low-phosphorus meats can help reduce the burden on your kidneys.
Potassium and phosphorus are minerals that healthy kidneys regulate effortlessly, but damaged kidneys cannot. High potassium can cause dangerous heart rhythm problems. High phosphorus weakens bones. Packaged and processed foods are a major hidden source of added phosphorus. Fruits and vegetables, while generally healthy, vary widely in potassium content, so portions may need adjusting depending on your blood levels.
Treatment When Kidneys Fail
Most people start dialysis when their kidney function drops to between 5% and 10% of normal, at which point symptoms of kidney failure are typically severe enough that the treatment provides relief. Dialysis does the filtering work your kidneys can no longer handle, either by cycling blood through a machine (hemodialysis) or by using the lining of your abdomen as a natural filter (peritoneal dialysis). Both approaches require a significant time commitment and lifestyle adjustment, but they sustain life when kidneys cannot.
Kidney transplant is the other option, and it generally produces better outcomes. People who receive a transplant have a greater chance of living longer than those who remain on dialysis. A transplanted kidney filters waste more effectively and restores some of the hormonal functions that dialysis cannot replicate. The tradeoff is availability: you may wait years for a donor kidney, and you’ll need to take medications for life to prevent your immune system from rejecting it. A transplant is not a cure, but it’s the closest thing to restoring normal kidney function.