The kidneys are two bean-shaped organs located in the lower abdomen, to the left and right of the spine. Measuring approximately 11 to 12 centimeters in length and weighing about 150 grams each, they may seem inconspicuous, but their function is impressive: Every day, your kidneys filter about 180 liters of blood and produce about 1.5 to 2 liters of urine. This filtration process is vital, because without functioning kidneys, toxins and metabolic waste products would accumulate in the body and poison it.

More than just filters

The kidneys are far more than simple filter organs. They perform a multitude of vital functions that are often underestimated. First and foremost is detoxification: the kidneys remove urea, creatinine, uric acid, and other metabolic waste products from the blood. They eliminate drug residues and other foreign substances that the body cannot utilize. The kidneys regulate the body's water and electrolyte balance—determining how much sodium, potassium, calcium, and phosphate remain in the body and how much is excreted. This balance is crucial for the proper functioning of the heart, muscles, and nerves. Furthermore, the kidneys control blood pressure through the renin-angiotensin-aldosterone system and by regulating fluid volume. They produce the hormone erythropoietin, which stimulates the production of red blood cells in the bone marrow. And they activate vitamin D into its active form, which is essential for calcium absorption and bone health.

Silent Sufferers

The insidious thing about kidney disease is its often subtle nature. The kidneys possess an enormous reserve capacity – even when a significant portion of their function is already lost, they can still perform their tasks. Symptoms often only appear when kidney function is already severely impaired, sometimes to less than thirty percent of normal capacity. At this point, fatigue, loss of appetite, nausea, fluid retention, or skin changes may occur. However, by this time, much time has already passed during which early treatment could have slowed the progression of the disease. Approximately ten percent of the population has impaired kidney function, but most are unaware of it. Regular checkups are therefore crucial, especially if risk factors such as diabetes, high blood pressure, or a family history of kidney disease are present.

To understand and prevent kidney disease, it helps to know the fascinating structure and function of these organs. Each kidney contains about a million tiny filtering units called nephrons, which together do the main work. These microscopic structures are true marvels of nature and work around the clock to clean your blood.

The nephron – The smallest functional unit

Each nephron consists of two main parts: the renal corpuscle, also called the glomerulus, and the tubular system. The actual filtration takes place in the glomerulus. Here, the blood is forced through a fine network of capillaries, with water and smaller molecules passing through the filter membrane into the tubule. Larger molecules, such as proteins and blood cells, remain in the bloodstream – their presence in the urine would be a warning sign of kidney damage. The filtrate, known as primary urine, initially contains many substances that the body would prefer to retain. Therefore, selective reabsorption occurs in the tubular system: glucose, amino acids, most electrolytes, and about 99 percent of the water are reabsorbed into the bloodstream. What remains is the final urine, containing concentrated waste products.

The glomerular filtration rate (GFR)

The glomerular filtration rate (GFR) is the most important measure of kidney function. It indicates how many milliliters of blood per minute are filtered by all glomeruli combined. In a healthy young adult, the GFR is approximately 90 to 120 ml/min/1.73 m². The GFR naturally declines slightly with age – about one milliliter per year starting around age forty. A GFR below 60 ml/min for more than three months is considered chronic kidney disease. Below 15 ml/min, it is considered kidney failure, which may require dialysis or transplantation. The GFR is calculated from the creatinine level in the blood, taking into account age, sex, and, in some cases, ethnicity. The creatinine level alone is less informative, as it only rises when the GFR is already significantly reduced.

Tracing kidney blood flow

The kidneys receive approximately 20 to 25 percent of the heart's total output—an astonishingly high proportion for organs that together account for only about 0.5 percent of body weight. This intensive blood flow is essential for their filtering function, but it also makes the kidneys susceptible to circulatory disorders. High blood pressure and arteriosclerosis can damage the delicate vessels of the glomeruli. Diabetes leads to changes in the vessel walls through elevated blood sugar levels. These two conditions are the most common causes of chronic kidney disease in Western countries and together account for approximately 50 to 60 percent of all cases requiring dialysis.

Chronic kidney diseases usually develop over years or decades, often without those affected noticing anything. Certain factors significantly increase the risk, and knowing these risk factors is the first step toward prevention. You can't influence some of these factors, but many are within your control.

Diabetes mellitus – The main cause

Diabetes is the leading cause of end-stage renal disease worldwide. Chronically elevated blood sugar levels damage the delicate blood vessels of the glomeruli – a process known as diabetic nephropathy. The filtering membrane becomes more permeable, initially to small amounts of albumin, a blood protein, and later to larger amounts of protein. This microalbuminuria is an early warning sign that can be detected through urine tests long before the glomerular filtration rate (GFR) declines. With good blood sugar control, the progression of kidney damage can be slowed or even halted. An HbA1c below seven percent is the usual target, although individual treatment goals should be discussed with a doctor.

High blood pressure – The silent destroyer

Hypertension is both a cause and a consequence of kidney disease – a vicious cycle that must be broken. High blood pressure damages the delicate blood vessels of the glomeruli due to the increased pressure. Damaged kidneys, in turn, are less able to regulate blood pressure, leading to a further increase. A blood pressure below 130/80 mmHg is recommended for people with kidney disease or at increased risk. ACE inhibitors and AT1 receptor blockers are the preferred blood pressure medications for people with kidney disease because they also have a kidney-protective effect.

Other important risk factors

Family history plays a role – certain kidney diseases, such as polycystic kidney disease, are hereditary, and there is also an increased risk of other kidney problems among affected family members. Age is a factor, as kidney function naturally declines from the age of forty. Being overweight and obese puts a strain on the kidneys due to increased filtration pressure and promotes diabetes and high blood pressure. Smoking directly damages the blood vessels of the kidneys and amplifies the effects of other risk factors. Certain medications can strain the kidneys with prolonged use, especially nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen or diclofenac, but also some antibiotics and contrast agents. Recurrent urinary tract infections, especially if they ascend to the kidneys, can lead to scarring and loss of function. Certain autoimmune diseases, such as lupus erythematosus, can also affect the kidneys.

The challenge with kidney disease is that it often causes no symptoms for a long time. The symptoms are frequently nonspecific and are attributed to other causes. Nevertheless, there are warning signs you should be aware of, and some of these appear earlier than the classic symptoms of advanced kidney disease.

Early signs

Changes in urination patterns can be an early warning sign. More frequent urination at night, medically known as nocturia, can occur because damaged kidneys are no longer able to concentrate urine effectively. Foamy urine indicates protein in the urine and should be investigated by a doctor. Blood in the urine, whether visible or detectable only under a microscope, is always a cause for concern. Fatigue and general exhaustion can occur because the kidneys produce less erythropoietin, leading to anemia. Sometimes, changes are noticed during routine examinations—elevated creatinine levels or protein in the urine—even before any symptoms are present. These incidental findings are valuable because they allow for early intervention.

Late symptoms

In advanced kidney disease, metabolic waste products accumulate in the blood, leading to a complex of symptoms known as uremia. Nausea, loss of appetite, and a metallic taste in the mouth are typical. Itching all over the body results from deposits of phosphate and other substances in the skin. The skin may take on a yellowish-brown discoloration. Fluid retention, especially in the lower legs, ankles, and around the eyes, indicates impaired fluid regulation. Shortness of breath can be caused by fluid accumulation in the lungs as well as by the anemia. Concentration problems and confusion, sometimes progressing to seizures, can occur with significantly elevated urea levels. Muscle cramps and twitching result from electrolyte imbalances. These late-stage symptoms indicate that kidney function is already severely impaired, often below ten to fifteen percent.

When to see a doctor.

You should see a doctor promptly if you experience any of the following symptoms: blood in your urine, even if it only occurs once; persistently foamy urine; significant swelling of the legs or face that cannot be explained by prolonged standing; unexplained, persistent fatigue, especially if risk factors such as diabetes or high blood pressure are present; or frequent urination at night that has recently started. If you have known risk factors, you should also have regular check-ups, even if you are not experiencing any symptoms.

Nutrition plays a central role in kidney health – both preventively and in managing existing kidney disease. A kidney-friendly diet can slow the progression of the disease and alleviate symptoms. It is important to understand that recommendations can vary depending on the stage of kidney disease, and individualized advice is advisable.

Liquid – Finding the right amount

Drinking enough fluids is important for healthy kidneys, as fluids help flush out metabolic waste and support filtration. For people with healthy kidneys, the recommendation is about 1.5 to 2 liters per day, although more is needed during exercise, in hot weather, or when ill. Water is the ideal drink, but unsweetened herbal and fruit teas are also suitable. Sugary drinks and excessive alcohol consumption should be limited. However, in advanced kidney disease, the recommendation may change: If the kidneys can no longer excrete enough fluid, fluid intake may need to be restricted to prevent fluid retention. In this case, individual consultation with a doctor is important.

Salt – Less is more

A low-sodium diet is one of the most important factors for kidney health. Excessive salt consumption raises blood pressure, which in turn damages the kidneys. Salt also leads to fluid retention, which can be problematic for those with impaired kidney function. The WHO recommends a maximum of five grams of salt per day, about one level teaspoon. However, the actual consumption in Germany averages eight to ten grams. Practical tips for reducing salt intake: Cook from scratch instead of using ready-made meals, as the latter are often heavily salted. Season with herbs and spices instead of salt – garlic, lemon, paprika, and fresh herbs offer plenty of flavor. Bread, cheese, and sausages are hidden sources of salt – look for lower-sodium options. Remove the salt shaker from the table and break the habit of adding extra salt at the table.

Protein – quality over quantity

The role of protein in kidney nutrition is viewed in a nuanced way. For people with healthy kidneys, a moderate to high protein intake is generally unproblematic as long as there is no pre-existing damage. In cases of existing kidney disease, a moderate protein restriction can be beneficial to relieve the burden on the kidneys – however, this should be discussed with a nutritionist to avoid malnutrition. Generally speaking, plant-based protein sources such as legumes, tofu, and nuts are often better tolerated than large amounts of animal protein. High-quality protein sources are preferred to meet requirements with a smaller overall quantity. Protein shakes and bars should be consumed with caution in cases of impaired kidney function.

When kidney function is impaired, certain minerals and electrolytes can no longer be adequately regulated. This can lead to dangerous imbalances, which can be influenced by diet. Depending on the stage of the disease, different dietary restrictions are necessary.

Potassium – The underestimated risk

Potassium is a vital mineral for heart function and muscle activity. In healthy kidneys, excess potassium is simply excreted, but in cases of impaired kidney function, potassium can accumulate in the blood. Elevated potassium levels, medically known as hyperkalemia, can lead to dangerous heart rhythm disturbances. Potassium-rich foods that may need to be limited in advanced kidney disease include bananas, oranges, tomatoes, potatoes, spinach, avocados, and dried fruit. However, in the early stages of kidney disease or for individuals with healthy kidneys, potassium restriction is usually unnecessary – fruits and vegetables should continue to be consumed in abundance. Potassium levels are regularly monitored in patients with kidney disease, and dietary recommendations are individually tailored.

Phosphate – protects bones and blood vessels

Phosphate is excreted less efficiently when kidney function is impaired and can accumulate in the blood. Elevated phosphate levels trigger a complex process: The body releases increased amounts of parathyroid hormone, which leaches calcium from the bones, leading to osteoporosis. Simultaneously, phosphate is deposited in the walls of blood vessels, promoting arteriosclerosis. Foods rich in phosphate include dairy products, meat, legumes, and nuts. Phosphate additives in processed foods such as cola, processed cheese, and many convenience foods are particularly problematic – this added phosphate is readily absorbed by the body. In advanced stages of kidney disease, phosphate binders are often prescribed; these bind phosphate from food in the intestines.

The acid-base balance

Healthy kidneys regulate the body's acid-base balance by excreting excess acid. Impaired kidney function can lead to metabolic acidosis, a chronic over-acidification of the blood. This promotes muscle loss and accelerates bone loss. An alkaline-rich diet with plenty of fruits and vegetables can counteract this. Interestingly, despite their often acidic taste, fruits and vegetables have an alkalizing effect on the body, while meat, fish, cheese, and grains have an acidifying effect. Therefore, a higher proportion of plant-based foods in the diet not only supports kidney health in general but also the acid-base balance.

Besides diet, many other lifestyle factors influence kidney health. Some of these factors have a direct effect on the kidneys, while others work indirectly by affecting risk factors such as blood pressure and blood sugar. A kidney-friendly lifestyle is also a heart-healthy lifestyle – both organ systems benefit from similar measures.

Exercise – Indirectly protects the kidneys

Regular physical activity has no direct effect on kidney function, but it offers indirect protection in many ways. Exercise helps maintain a healthy weight and prevent obesity, which is a risk factor for kidney disease. It lowers blood pressure and improves blood sugar regulation in diabetics – both of which protect the kidneys from damage. Endurance training such as walking, cycling, or swimming is particularly recommended, but strength training also has positive effects. The WHO recommends at least 150 minutes of moderate or 75 minutes of vigorous activity per week. If kidney disease is present, the exercise program should be coordinated with a doctor, but generally, exercise is still possible and beneficial.

Quit smoking

Smoking is an underestimated risk factor for kidney disease. Nicotine and other components of tobacco smoke directly damage the blood vessels of the kidneys. Smoking increases blood pressure and exacerbates the harmful effects of diabetes and high blood pressure on the kidneys. The risk of kidney cancer is significantly increased in smokers. In those with existing kidney disease, smoking accelerates its progression. Quitting smoking is worthwhile at any stage – the blood vessels begin to recover after just a few weeks, and the risk of further damage decreases.

alcohol and medication

Moderate alcohol consumption generally does not directly harm the kidneys, but can indirectly strain them through its effects on blood pressure and weight. Heavy alcohol consumption, however, can lead to kidney damage and should be avoided. Certain medications can also strain the kidneys, especially with long-term use. Nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen, diclofenac, or naproxen should not be taken regularly for extended periods, particularly if you have pre-existing kidney problems or risk factors. Certain antibiotics, contrast agents, and herbal remedies can also put a strain on the kidneys. For chronic pain or other conditions requiring regular medication, consult your doctor about kidney-friendly alternatives.

Because kidney disease often progresses without symptoms for a long time, prevention is key. Regular checkups can detect early-stage kidney disease while it is still treatable and its progression can be slowed. Early detection is especially important for people with risk factors, but occasional checkups can be beneficial for everyone.

Who should be tested regularly?

People with diabetes should be tested for kidney disease at least once a year, as the risk of diabetic nephropathy is high. Annual checkups are also recommended for people with high blood pressure. Those with family members who have kidney disease should have regular examinations, especially if hereditary kidney diseases such as polycystic kidney disease (PCKD) are known to run in the family. After the age of fifty, an occasional kidney check is advisable even without known risk factors. People who regularly take painkillers such as ibuprofen should have their kidney function checked. For those with existing cardiovascular disease, kidney monitoring is part of routine care.

The most important studies

The creatinine level in the blood is the starting point for calculating the glomerular filtration rate (GFR), a measure of kidney function. However, a single elevated value is not necessarily cause for alarm – various factors such as muscle mass, meat consumption the previous day, or dehydration can influence the value. Therefore, further testing is recommended if the result is abnormal. Urinalysis provides information about protein and blood in the urine. Microalbuminuria, meaning small amounts of albumin in the urine, is a very early warning sign of kidney damage, especially in diabetics, and can be detected before the creatinine level rises. Cystatin C is an alternative marker for calculating the GFR that is less dependent on muscle mass. The electrolytes sodium, potassium, calcium, and phosphate provide information about how well the kidneys regulate mineral balance.

Ultrasound and further diagnostics

If abnormal lab results or symptoms are present, an ultrasound of the kidneys is often performed. This shows the size, shape, and structure of the kidneys and can detect cysts, stones, or tumors. The examination is painless, quick, and involves no radiation exposure. In some cases, further imaging procedures such as CT or MRI scans, or even a kidney biopsy, are necessary for an accurate diagnosis.

When you have your kidney function checked, you'll receive a series of numbers that can initially seem confusing. Understanding these values ​​will help you assess your kidney health and have informed conversations with your doctor. Here, we explain the most important parameters.

Creatinine and GFR

Creatinine is a waste product of muscle metabolism that is excreted almost exclusively by the kidneys. The reference range is approximately 0.7 to 1.3 mg/dL for men and 0.5 to 1.1 mg/dL for women. People with a lot of muscle mass tend to have higher values ​​without any impairment of kidney function. The glomerular filtration rate (GFR), calculated from creatinine levels taking age and sex into account, provides a more accurate picture of kidney function. A value above 90 ml/min is normal, 60 to 89 ml/min indicates slightly impaired function, below 60 ml/min is indicative of chronic kidney disease, and below 15 ml/min signifies kidney failure.

Urea and uric acid

Urea is another waste product excreted by the kidneys. It is produced during protein breakdown and is therefore also dependent on diet – a protein-rich meal the day before can increase its level. The reference range is approximately 17 to 43 mg/dl. Significantly elevated levels can indicate kidney disease, but dehydration or high protein consumption can also be the cause. Uric acid is the end product of purine metabolism. Elevated levels can lead to gout and are also associated with an increased risk of kidney disease. The reference range is approximately 3.4 to 7.0 mg/dl for men and 2.4 to 5.7 mg/dl for women.

Protein and albumin in urine

The detection of protein in urine, especially albumin, is an important marker for kidney damage. Normally, there should be very little protein in urine – the filtering membrane of the glomeruli retains it. If albumin appears in the urine, it indicates that this filtering barrier is damaged. Microalbuminuria, a small but elevated albumin excretion between 30 and 300 mg per day, is often the first measurable sign of incipient kidney damage, particularly in diabetics. Albumin excretion above 300 mg per day is called macroalbuminuria and indicates more advanced damage. The albumin-to-creatinine ratio in urine is a convenient method for assessment, as it can be determined from a single urine sample.

The kidneys work quietly and unobtrusively, but their function is essential for life. They filter the blood, regulate fluid balance, control blood pressure, and perform important hormonal functions. When they become diseased, this usually happens gradually and without early warning signs – all the more reason to take preventative measures.

Key findings summarized

Chronic kidney disease is common, but often preventable. Diabetes and high blood pressure are the main causes – and managing them well is the most important protective factor for the kidneys. Other risk factors, such as being overweight, smoking, and certain medications, can be influenced. A kidney-friendly diet primarily means less salt, sufficient fluid intake, and good blood pressure control. In advanced kidney disease, additional restrictions on potassium, phosphate, and protein may be necessary, but these should be individually tailored. Lifestyle plays a major role: Regular exercise, not smoking, and responsible medication use indirectly protect the kidneys through their effects on blood pressure, weight, and metabolism.

Prevention saves kidneys

Because kidney disease often remains asymptomatic for a long time, early detection is crucial. A simple blood test measuring creatinine and calculating the glomerular filtration rate (GFR) can provide valuable information. A urine test for microalbuminuria can detect early damage before the GFR declines. People with diabetes, high blood pressure, or a family history of kidney disease should have their kidney function checked regularly. However, even without known risk factors, an occasional check-up is worthwhile, especially from the age of fifty.

Your next step

If you have risk factors or simply want to know about your kidney health, a kidney check is a good first step. If you have abnormal results or symptoms, you should seek medical advice. The good news is that even if early-stage kidney disease is detected, its progression can often be significantly slowed with the right measures. Early action makes the difference between stable kidney function and kidney failure requiring dialysis. Your kidneys deserve your care – after all, they work for you around the clock.