The 2024 Levels guide to kidneys and metabolic health

Kidney function plays a more prominent role in metabolic health and glucose regulation than you might realize. Here’s what you need to know to keep them healthy.

WRITTEN BY

Leslie Goldman

REVIEWED BY

Dr. Ami Kapadia

UPDATED: 01/01/2025

PUBLISHED: 01/01/2025

🕗 10 MINUTE READ

Most of us likely don't think of our kidneys as metabolic multitaskers---if we think of them at all. When they work well, it's easy to overlook these vital organs, which serve as the body's built-in blood filtration system. (Healthy kidneys filter all the blood in your body twice an hour.)

The kidneys are also principal players in multiple metabolic processes, including glucose regulation, blood pressure control, and mineral (electrolyte) balance in the blood. The authors of a 2024 Annual Review of Physiology paper dedicated to the bean-shaped organs go so far as to call them "a powerhouse of human metabolism," and some scientists now view chronic kidney disease (CKD)---which affects more than one in seven U.S. adults---as a metabolic condition.

Here's what you should know about kidneys and metabolic health. 

What are kidneys, and what do they do?

The fist-sized reddish-brown organs flanking your spine in your lower back are among the most metabolically active organs in the body. Metabolic activity refers to the amount of energy required to maintain normal processes, and they're about as metabolically active as their equivalent weight in the heart, twice as metabolically active as the brain, and 34 times as active as muscle.

The kidneys' primary function is filtering blood, and the volume they handle is impressive: about a large bathtub's worth daily. Here's how it works:

• Step 1: The heart pumps blood into the kidneys via the renal artery, which branches into smaller blood vessels in the kidneys. These smaller vessels ferry the blood into about a million minuscule filtering units (per kidney) called nephrons.
• Step 2: Inside the nephrons, structures called glomeruli filter out waste products like urea (nitrogen waste), creatinine (muscle waste), and uric acid. 
• Step 3: The blood passes to tiny tubules in the kidney that continue the filtering journey, removing excess fluids and waste while returning necessary substances, including sodium, potassium, and other vital minerals.
• Step 4: The purified blood returns to the heart via the renal vein. Waste products pass from the kidneys to the bladder before being eliminated in urine. (Like the bladder, the kidneys are part of the urinary tract.)

Besides filtration, kidneys ensure an appropriate balance of sodium, potassium, calcium, phosphorus, and other minerals and electrolytes in the blood, which is crucial for keeping bones, muscles, nerves, and other cells and tissues working.

For these reasons and more, Katherine Tuttle, MD, a professor of medicine in the Division of Nephrology and Kidney Research Institute at the University of Washington in Spokane and co-principal investigator for the Institute of Translational Health Sciences, calls kidneys "the master chemists of the body," responsible for regulating blood volume, blood pressure, and blood composition, including blood glucose levels [more on that below]. 

While most people are born with two kidneys, you can live with just one, making living kidney donation possible. Additionally, about one in 1,000 Americans are born with a single kidney, and others need to have a kidney surgically removed (nephrectomy) due to an illness like cancer or injury. It's possible to thrive with one kidney, but only if that kidney is healthy. 

The kidneys as metabolic health organs

The kidneys play an essential role in multiple metabolic health processes. Tuttle calls them "perhaps one of the most central organs to the regulation of metabolic health."

Although the pancreas and liver get most of the attention for blood glucose (blood sugar) regulation, the kidneys help maintain this balance, too. Dietary glucose serves as cellular fuel, and the body uses an elegant system to ensure blood glucose levels remain at a Goldilocks level--not too high, and not too low--to maintain health. When things function correctly, the pancreas releases insulin in response to elevated blood sugar levels, like after a high-carb meal. In a metabolically healthy body, this insulin helps shuttle glucose from the blood into cells, which helps blood sugar levels return to a standard, pre-meal range within about two hours of eating.

The kidneys also participate in this regulation as they filter and reabsorb glucose. If blood sugar levels exceed around 180 mg/dL, the kidneys can remove excess glucose and excrete it through urine rather than return it to the bloodstream. This is why glucose in the urine can be a sign of diabetes. 

In addition to deriving glucose from our food, the body can produce glucose on its own (a process called gluconeogenesis) to help maintain blood sugar levels through fasting states, such as between meals and while sleeping overnight. Credit for this job typically goes to the liver, but the kidneys are responsible for about 40-50% of all gluconeogenesis during stress, acidosis, or fasting.

The kidneys also remove excess insulin from the blood, as well as other metabolic waste (including that created by the metabolism of carbohydrates, proteins, fats, and other nutrients).

What are the links between compromised metabolic health and kidney health?

If the kidneys are struggling, the consequences can be felt throughout the body, including increased risk of heart disease and stroke. The three main concerns about kidneys and metabolic health are chronic kidney disease (CKD), cardiovascular-kidney-metabolic (CKM) syndrome, and kidney stones (a.k.a. nephrolithiasis).

Chronic kidney disease

Chronic kidney disease is the fastest-growing noncommunicable disease in the U.S., affecting 1 in 7 adults, and it's the third fastest-growing cause of death globally. A CKD diagnosis means your kidneys have sustained enough damage over time that they can't properly filter waste, maintain blood pressure, or perform their other duties. 

There are five stages of CKD; as it progresses, kidney function diminishes. (In stages 1, 2, and 3, the kidneys can still filter waste from the blood, but filtration may stop altogether by stages 4 and 5.) The stages are determined using a measure called the estimated glomerular filtration rate (eGFR) and are as follows:

• Stage 1: eGFR of 90 or greater + mild kidney damage
• Stage 2: eGFR between 60 and 89 + mild kidney damage
• Stage 3: eGFR between 30 and 59 + mild to severe kidney damage (Stage 3 is split into two substages: Stage 3a = eFGR between 45 and 59 + mild to moderate kidney damage, and Stage 3b = eGFR between 30 and 44 + moderate to severe kidney damage.)\
• Stage 4: eGFR between 15 and 29 + severe kidney damage\
• Stage 5: eGFR less than 15 + severe kidney damage, including possible kidney failure. This stage is also called end-stage kidney disease (ESKD) or end-stage renal disease (ESRD).

High blood sugar levels, insulin resistance, Type 2 diabetes, and metabolic syndrome are key risk factors for CKD. When obesity, poor nutrition, low levels of physical activity, stress, and other lifestyle or environmental factors conspire to keep blood sugar levels chronically elevated (a condition called hyperglycemia), it triggers repeated insulin surges that eventually cause cells to become "numb," or resistant, to insulin's effects. Insulin resistance affects 40% of U.S. adults ages 18 to 44 and is associated with many chronic health conditions, including Type 2 diabetes (an extreme form of insulin resistance).

Chronically high blood sugar also narrows blood vessels throughout the body, including in the kidney, and overworks the nephrons. These changes hamper filtration, and the accumulated waste products---including excess glucose---can damage the kidneys and other organs. (Kidney damage resulting from diabetes is called diabetic nephropathy, not to be confused with diabetic neuropathy, which is nerve damage caused by diabetes.) 

Diabetes is the leading cause of CKD, accounting for 44% of all new CKD cases. However, high blood pressure, heart disease, obesity, smoking, and a family history of CKD can also cause or worsen CKD.

Untreated CKD can progress to kidney failure, a life-threatening condition potentially requiring dialysis (assisted filtration) or a kidney transplant. Most people with CKD show no symptoms until the condition has progressed to a more advanced stage, at which point symptoms can include:

• Urinating more or less frequently than usual
• Foamy urine
• Dry or itchy skin
• Fatigue
• Nausea and loss of appetite
• Numbness in the limbs
• Breath that smells like fish, ammonia, or urine

Cardiovascular-kidney-metabolic (CKM) syndrome

CKD rarely travels alone. Far more often, kidney disease coexists with cardiovascular disease, Type 2 diabetes, and metabolic disorders like metabolic syndrome (MetS), a constellation of risk factors that significantly increase disease risk, including abdominal obesity, low HDL cholesterol, and high blood pressure, blood sugar, and triglycerides.

The interconnectedness between heart and kidney physiology, and the overlap of metabolic risk factors between heart and kidney disease, diabetes, and metabolic dysfunction, inspired a 2023 American Heart Association presidential advisory proposing a new diagnosis: cardiovascular-kidney-metabolic (CKM) syndrome. This designation is meant to guide doctors in screening better and treating people with CKM and further scientific research on the condition.

One in three American adults has three or more risk factors for CKM syndrome, including elevated weight, blood pressure, cholesterol, blood glucose, and triglycerides. Perhaps even more alarming, in a 2024 research letter to JAMA, physicians from Harvard Medical School's Brigham and Women's Hospital Heart and Vascular Center warned that nearly 90 percent of U.S. adults meet the criteria for CKM syndrome, per findings from a recent National Health and Nutrition Examination Survey (NHANES) analysis.

The AHA presidential advisory, authored by a team of cardiologists, nephrologists, and other physicians, including Tuttle, calls kidney dysfunction "a key mediator of the relationship between metabolic risk factors and CVD [cardiovascular disease]," especially heart failure (HF), a serious condition affecting more than six million U.S. adults and characterized by the heart's inability to pump sufficient quantities of blood to meet the body's needs. 

CKM can lead to cardiac issues and other serious health conditions like:

• Kidney failure
• Early cognitive decline
• Obstructive sleep apnea
• Metabolic dysfunction--associated steatotic liver disease (formerly non-alcoholic fatty liver disease)
• Cancer 

Kidney stones

Kidney stones are also connected to metabolic health. Composed of crystalized minerals or salts and typically ranging in size from a grain of sand to a pebble, these painful stones can form in the kidneys if a person consumes a diet high in salt, meat, and oxalate, a substance found in foods such as leafy greens, peanuts, seeds, grains, legumes, tea, and chocolate. Kidney stones result from the relationship between sodium and calcium excretion in the urine: When the kidneys have to excrete excess sodium in your urine, it also increases the level of calcium they excrete, and this hypercalciuria contributes to stone formation.

Other kidney stone risk factors include:

• Diabetes: People with Type 2 diabetes who use insulin have more than triple the odds of developing kidney stones. Insulin resistance is thought to make urine more acidic. Ph imbalances in the kidneys, either acidic or alkaline, prompt the crystallization of different types of kidney stones. 
• Consuming too little liquid: Chronic dehydration increases the concentration of minerals in the kidneys.
• Obesity: Carrying excess weight may intensify the stone-forming effects of oxalate and other minerals. Increased body mass index (BMI) has been linked with increased sodium and other stone-forming substances in the urine. Additionally, people with obesity are more likely to have conditions associated with kidney stones, such as high blood pressure, diabetes, and MetS.
• Weight loss surgery: Gastric bypass surgery can alter urine composition to promote stone formation. It also often leads to decreased urinary output (either because people tend to reduce their liquid intake or as a direct effect of the surgery itself).

About one in 10 people will develop kidney stones at some point, although diabetes increases the lifetime risk.

How can I check my kidney health?

Kidney trouble is often silent in the beginning stages. More than one in seven U.S. adults (35.5 million people) have kidney disease, but 90 percent of them don't know it. So, it's wise to know your risk factors and how to check your kidney health. Some of the more common tests are listed below. 

Blood tests

• Serum creatinine test: Creatinine forms as your body digests the protein in your food, as well as during normal muscular wear and tear, like intense exercise. The kidneys are charged with filtering creatinine from the body. Generally, a healthy serum creatinine level is 0.7 to 1.3 mg/dL for males and 0.6 to 1.1 mg/dL for females. Measurements higher than these may indicate kidney disease, including poor blood flow to the kidneys, infection, or kidney failure. It can also suggest underlying diabetes or heart failure, both of which can impair kidney functioning.
• Estimated glomerular filtration rate (eGFR): Another measure of kidney filtration, an eGFR value lower than 60 suggests kidney distress; an eGFR of 90 or more is desirable.
• Blood urea nitrogen (BUN): Measuring urea nitrogen, another waste byproduct created during protein breakdown, doesn't offer much insight on its own, but when considered alongside creatinine and eGFR levels, it can help paint a more holistic view of whether your kidneys are filtering correctly. Generally, a BUN level between 7 and 20 mg/dL is considered healthy.

Urine tests

• Urinalysis: Giving a small urine sample (as you'd do at a typical physical) can let your doctor check for abnormalities, including elevated levels of acid, bacteria, protein (albumin), sugar, or blood.
• 24-hour urine collection: For this test, you'll collect urine at home for 24 hours. The larger sample allows for assessing multiple kidney health parameters, including overall functioning and indicators of kidney disease (including kidney stones).
• Urine albumin-creatinine ratio (uACR): Measuring albumin (protein) and creatinine in your urine helps assess the kidneys' filtering ability. Small amounts of albumin in the urine may be an early sign of kidney disease. Dividing the amount of urine albumin by the amount of urine creatinine results in your uACR ratio, ideally lower than 30 mg/g. 

If you've been diagnosed with diabetes or high blood pressure, Tuttle says it's wise to be screened at least annually. If any results remain abnormal for three months, a CKD diagnosis is likely, and it's time to discuss treatment options with your physician. 

Some of these tests are part of routine doctor's appointments. For example, eGFR is often included in the basic metabolic blood panel. If you've ever been pregnant, you've likely provided a urine sample at every doctor's appointment for urinalysis. However, uACR testing is typically reserved for those at high risk for kidney disease.

You may also get imaging tests, including ultrasounds, computerized tomography (CT) scans, and magnetic resonance imaging (MRI). 

Increased screening may also be recommended for those with a family history of kidney or cardiovascular disease, though Tuttle says more data is needed before doctors can define a standard schedule. She adds that people with two copies of an APOL1 genetic mutation (more common in people with Western or Central African Ancestry, which may include those who identify as African American, Afro Caribbean, and Latina/Latino) are at increased risk of kidney disease. Genetic testing can determine if you're a carrier.

How to support healthy kidneys

With kidney health, a smart rule of thumb is "What's good for your heart is good for your kidneys."

• Exercise regularly and follow a nutrient-dense, plant-forward diet
• Don't smoke
• Limit alcohol intake
• Stay hydrated
• Monitor and maintain healthy cholesterol and blood sugar levels and blood pressure
• Curb sodium intake (less than 2,300 milligrams a day) 

These strategies can help you maintain a healthy weight, minimize excess abdominal fat, and avoid prediabetes and impaired glucose tolerance (which are CKM stage 1 indicators). 

If you've been diagnosed with Type 2 diabetes, kidney disease, or another metabolic condition, the following steps may help avoid or slow the progression of diabetes-related kidney disease, though everyone should talk to their doctor about individual recommendations.

1. Take prescriptions as instructed and prioritize healthy lifestyle habits. This can help you reach your blood sugar and blood pressure goals. 
2. Avoid or minimize use of nonsteroidal anti-inflammatory drugs (NSAIDs). Over-the-counter pain relievers like ibuprofen and naproxen can damage the kidneys.
3. Prioritize plant-based protein and avoid excess amounts of red meat. Plant proteins like beans, lentils, nuts, seeds, and whole grains are easier for the kidneys to handle (when it comes to filtering protein waste from the blood) compared with red meat. Plant-based foods are also thought to foster kidney health in general, partly due to the anti-inflammatory effects of the fiber they contain. The more red meat you eat, the harder your kidneys must work to filter protein waste from your blood. However, the exact amount of protein an individual should eat depends on the stage of kidney disease and how it is treated. For example, patients with CKD who are not on dialysis will be advised to limit protein to create less stress on the kidneys. In contrast, those on dialysis are typically told to eat more protein to maintain general health. (The dialysis machine handles the filtering.
   The National Kidney Foundation has a wealth of information on eating with CKD, including suggestions for limiting sodium, potassium, calcium, and other nutrients and how to determine the optimal number of calories needed to keep energy up, maintain or build muscle, avoid infections, and prevent kidney disease from progressing.
4. Talk to your healthcare provider for personalized advice. It's imperative to address any kidney problems sooner rather than later. Medication, such as the below, can help preserve kidney function and prevent heart complications (usually by lowering blood pressure and blood glucose levels):
   • Sodium-glucose cotransporter-2 (SGLT2) inhibitors 
   • Renin-angiotensin-aldosterone system (RAAS) blockers, including ACE (angiotensin-converting enzyme) inhibitors, ARBs (angiotensin receptor blockers)
   • Mineralocorticoid receptor antagonists 

The glucagon-like peptide 1 receptor agonists (GLP-1RAs) making headlines for facilitating weight loss may soon be considered a fourth pillar of CKD medication. In the recent landmark FLOW kidney outcomes trial, 3,533 participants with CKD and Type 2 diabetes were randomly assigned to weekly injections of semaglutide (a GLP-1RA and the active ingredient in Ozempic and Wegovy) or placebo. After around three years of use, subjects in the semaglutide group experienced a 24 percent reduction in kidney events, an 18 percent reduction in heart attack and stroke or death from cardiovascular causes, and a 20 percent drop in all-cause death compared to those in the placebo group.

Tuttle, a FLOW trial investigator, says GLP-1RAs appear to protect kidneys from inflammation and oxidative damage while simultaneously protecting the heart. Although weight loss occurred, it was small and not the main driver of the protection, she adds. "There's something else about the agents that's restoring the metabolism."

Don't overlook your kidneys' role in metabolic health

Rather than waiting until you have symptoms of kidney disease, proactively talk to your healthcare provider about your kidneys. If you already get metabolic panels and eGFR tests as part of routine care, discuss the results and any concerns. Also, discuss how to manage abdominal fat, blood sugar, blood pressure, and other metabolic factors if needed. All of your organs, including your kidneys, will thank you.

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