What you need to know about a low-carb diet and your kidneys

By Dr. Keith Runyan, MD, medical review by Dr. Michael Tamber, MD Evidence based

First, we’ll look at what our kidneys do and how they can get damaged.

Then, we will examine the research on low-carb diets for the average low-carb eater with healthy kidneys. We will evaluate the two biggest concerns most potential low-carbers have:

1. Will a high-protein, low-carb diet stress kidney function?
2. Do low-carb diets lead to kidney stones?

Our last section will evaluate the evidence about the safety of low-carb diets for those with a confirmed diagnosis of either mild or advanced kidney disease.

Feel free to use the linked outline above to skip to the section most relevant to your health history.

What your kidneys do

The kidneys are a pair of bean-shaped, fist-sized organs at the back of your abdomen. Each kidney produces urine that drains to the bladder 24 hours a day. Your kidneys filter large volumes of blood on a continuous basis in order to:

• Remove excess fluid and acids from the body
• Optimally balance fluids, minerals, and electrolytes
• Regulate blood pressure
• Remove waste products, toxins, and drugs

The kidneys also make hormones that help keep your blood and bones healthy, including making calcitriol, the active form of vitamin D.

In short, your kidneys are like the waterworks department of a big city. We cannot survive without functioning kidneys, so we must support them by following a healthy diet and lifestyle.

How kidneys are damaged

Many different diseases, drugs, toxins, and inherited disorders can lead to kidney damage.

However, by far the two most common causes of kidney disease are diabetes and high blood pressure, both of which can damage the kidney’s delicate blood vessels and tissues.¹

When the root causes of these two diseases are not addressed, chronic kidney disease and ultimately kidney failure can be the result. In the US, diabetes causes 44%, and high blood pressure causes 29% of all cases of end-stage kidney failure requiring dialysis or a kidney transplant.²

Why does this happen?

In the case of diabetes, high blood sugars over long periods of time can cause glucose to bind to vital proteins in the bloodstream. This process, known as glycation, results in the formation of advanced glycation end-products (called AGEs). AGEs can cause abnormal changes to proteins and to receptors that ultimately injure the filtering segments of the kidney. This glycation creates a vicious cycle of additional injuries to tissues that results in progressive kidney damage, called diabetic nephropathy.³

With high blood pressure, the problem is simply too much pressure. Exposing the delicate filtering portion of kidneys to blood that is moving too hard and fast causes scarring, which damages the kidneys. This results in a loss of kidney function, leading to chronic kidney disease or eventually kidney failure.⁴

The bottom line is that the best way to prevent kidney damage and failure is to treat and prevent diabetes and high blood pressure.

Fortunately, there’s good news: diabetes and high blood pressure can be greatly improved and – to some extent – reversed with a low-carb lifestyle.

How to reverse type 2 diabetes

Guide Do you have type 2 diabetes, or are you at risk for diabetes? Do you worry about your blood sugar? Then this guide is for you.

How to normalize your blood pressure

Guide Elevated blood pressure is a common health issue today. The good news is that you can improve your blood pressure by way of simple lifestyle changes.

A 2019 study in people with type 2 diabetes by Virta Health showed those following a ketogenic diet were able to improve their blood glucose control, with most patients reducing blood sugar to much safer levels.⁵ Those on the keto diet also had significant improvements in blood pressure, body weight, and other markers of metabolic syndrome. The majority were able to discontinue oral diabetes medications and reduce or eliminate injectable insulin.

In addition, a review of multiple randomized controlled trials concluded that low-carb diets were more effective than low-fat diets for reducing blood pressure and other cardiovascular risk factors, at least over the course of 6-12 months.⁶

When diabetes and high blood pressure are controlled or even reversed, the long-term damage those conditions cause to the kidneys can be slowed or prevented completely.

Low-carb diets in those with healthy kidneys

We have learned that low-carb diets can improve diabetes and high blood pressure, which may prevent kidney damage caused by those diseases. But is there any concern that the diet itself might have its own impact on the functioning of healthy kidneys?

Over the years, two concerns about low-carb, ketogenic eating and kidney health have surfaced:

1. Do higher levels of protein intake potentially stress the kidneys?
2. Do low-carb diets increase the risk of kidney stone formation, especially when the diet is first started?

We will take a closer look at both.

Protein and normally functioning kidneys

Why is dietary protein intake sometimes raised as a concern for kidney health? Well, when kidneys are damaged one of the first signs is protein leaking out of the kidney and appearing in the urine. This condition is called proteinuria and it shows that the kidney’s filtering system is malfunctioning.

Because of this, some hypothesize that eating too much protein might stress the kidneys and cause proteinuria and kidney damage. And, because people associate low-carb diets with high-protein, this leads some to wonder if low-carb diets lead to proteinuria.

Of course, a well-formulated ketogenic diet is typically not high in protein.  Even low-carb diets that aren’t ketogenic do not necessarily equate with eating large amounts of protein. At Diet Doctor, we recommend most people eat 1.2 – 2.0 grams of protein for each kilogram of desired body weight — which is “moderate” or “adequate” protein consumption.

This guide about protein gives lots of examples of what that moderate level of protein could look like in a typical day of eating a low-carb diet:

Protein on a low-carb or keto diet

GuideAlong with fat and carbohydrates, protein is one of the three macronutrients (“macros”) found in food, and it plays unique and important roles in the body. Here’s a guide to everything you need to know about protein on a low-carb or keto lifestyle.

For the average person on a low-carb diet, there is probably no reason to worry; the most credible research available shows that low-carb diets are not associated with kidney damage.⁷

Keeping the drawbacks of these studies in mind, a 2016 meta-analysis of nine RCTs in overweight and obese individuals with healthy kidneys showed a greater improvement in kidney function in those who ate a low-carbohydrate diet compared to those who consumed a control diet, though the improvement was likely too small to be clinically meaningful.⁸

A more recent meta-analysis of 12 RCTs found no evidence that low-carbohydrate diets are harmful for kidney health, even in people with type 2 diabetes.⁹

Even for those not on a low-carb diet, evidence suggests that people who eat more protein don’t increase their risk of kidney disease. The following observational study reported those who ate higher amounts of protein had a significantly reduced risk of developing kidney disease.¹⁰

We are not aware of any randomized clinical trials showing that consumption of moderate amounts of protein can negatively impact kidney function in people with healthy kidneys. There are observational studies, however, which have suggested that low-carb diets with moderate or high protein intake predispose people to kidney damage.¹¹

As we responded to these trials when they were first published, there were numerous variables other than protein which could have contributed to the findings of harm. Therefore, we are inclined to rely more heavily on the RCT data – flawed as it is – and conclude that low-carb diets with moderate protein do not appear to damage the kidneys of healthy people.

Higher levels of protein consumption

Some people on a low-carb diet, especially body builders or athletes doing a lot of resistance training to build muscle, as well as people following a carnivore diet, consume higher levels of protein. They may consume more than double the amount of protein that is found in a typical Diet Doctor low-carb recipe.

Is eating a lot of protein — in the form of red meat, poultry, seafood, and eggs — potentially harmful for their kidneys?

No. Again, research shows even at this higher level, it is not a concern if their kidneys are healthy.

In 2005, kidney experts reviewed all the available scientific literature and concluded “while protein restriction may be appropriate for treatment of existing kidney disease, we find no significant evidence for a detrimental effect of high protein intakes on kidney function in healthy persons.”¹²

A 2016 randomized cross-over study followed 14 male bodybuilders for a year. The men ate their normal diet for a total of six months and a high protein diet for a total of six months. The study found no harmful effects on kidney function on the high protein diet.¹³

Another study looked at five healthy bodybuilders who continued to consume a high-protein diet (> 2.2 grams/kg/day) for a total of two years without any change in their normal kidney function measurements or other negative effects.¹⁴

While caution should be exercised regarding extrapolation of the bodybuilder data to sedentary people with less muscle mass, these studies plus the review article cited above are generally reassuring.

Low-carb diets and the risk of kidney stones

Can a low-carb diet increase the occurrence of kidney stones? This painful concern requires careful evaluation.

Kidney stones are hard deposits of mineral salts. They occur when supersaturated chemicals in the urine collect to form a crystal. At first the crystals can be as small as grains of sand, but they can grow to become the size of pebbles or even in some people the size of a golf ball.

The majority of kidney stones are either made of calcium oxalate or calcium phosphate (80%). Stones formed by uric acid and by struvite make up the other 9% and 10%, respectively. Very rarely, and usually only in families with a genetic risk, stones are made of cystine (~1%).¹⁵

If diagnosed and treated early, a single attack of kidney stones is unlikely to cause permanent damage to the kidney, but the episode can be incredibly painful and may require treatment with drugs, sonic waves (called lithotripsy) or even surgery.

Risk factors for the development of kidney stones include obesity, type 2 diabetes, high blood pressure and metabolic syndrome, which themselves are risk factors for the development of future chronic kidney disease. As well, individuals who regularly form kidney stones have been found to have higher risks of future high blood pressure, chronic kidney disease, and end-stage renal failure.¹⁶

Individuals who have had an episode of kidney stones are 50% more likely to have another attack within five years. Preventing kidney stone formation, therefore, is important to general health, future kidney health, and individual well-being.

What is the evidence for a low-carb, ketogenic diet increasing the risk of kidney stone formation?

Kidney stones have been reported in children with epilepsy who use special, highly-restrictive versions of ketogenic diets, but supplementing with potassium citrate may reduce the risk of kidney stones five-fold.¹⁷

Further, a meta-analysis of clinical trials and observational studies reported a pooled incidence of kidney stones at 5.6% after 4 years on a ketogenic diet. Compared to the kidney stone incidence in the general population of 0.3% per year for men and 0.25% per year for women, it would seem that ketogenic diets may increase the risk. However, the trials included in this meta-analysis included mostly studies using a highly-restrictive version of keto, so we should be careful about extrapolating these data to the general population.¹⁸

We must also note that very-high animal protein diets (more than 2 grams/kg/day) have been associated with an increased risk of uric acid stones.¹⁹

Many anecdotal reports exist — primarily on internet forums — from adults who claim they developed a kidney stone soon after starting a low-carb diet.²⁰ However, since kidney stones are very common in the US, occurring in 10% of all men and 7% of women — the majority of whom are on a standard American diet — the timing of a stone forming might have had nothing to do with beginning the low-carb diet.

Certain contributing factors could increase the risk of kidney stones while on a keto diet, especially if you have already had an attack of kidney stones. These factors include not drinking enough water, eating too many vegetables that are high in a chemical called oxalate, taking high doses of vitamin C supplements, not consuming enough calcium or magnesium, and eating a very high protein diet. Read more about these potential contributing factors in our Learn More section.

Low-carb diets in established kidney disease

We have learned a low-carb diet doesn’t jeopardize kidney function in people with healthy kidneys, but what about individuals who already have chronic kidney disease? Is a low-carb way of eating safe for them?

Briefly, in early-stage kidney disease, some data show that low-carb diets are safe and, by controlling diabetes and high blood pressure, they may even prevent the worsening of kidney function. Other studies show that low or very-low protein diets may be beneficial for diabetic patients with early-stage kidney disease. We will explore the conflicting research later in this section.

In more severe, end-stage kidney disease, there aren’t as many studies of low-carb diets, given that significant protein restriction is a hallmark of therapy for very poor kidney function. It is essential if you have advanced kidney disease that you work closely with a qualified medical professional to establish an individualized diet that is best for you and your condition.

Stages of chronic kidney disease

It is important to understand that kidney disease exists on a spectrum; chronic kidney disease is divided into five stages by estimated glomerular filtration rate (eGFR), a measure of kidney function based on how efficiently the kidneys filter toxins from the blood.

1. Stage 1: eGFR greater than 90 ml/min (healthy filtration rate but protein is abnormally present in urine).
2. Stage 2: eGFR of 60 to 89 ml/min.
3. Stage 3a: eGFR of 45 to 59 ml/min.
4. Stage 3b: eGFR of 30 to 44 ml/min.
5. Stage 4: eGFR of 15 to 29 ml/min.
6. Stage 5 eGFR of less than 15 ml/min.

This is a little technical, but it is important to know where you stand. The first three stages, Stage 1 to Stage 3a represent early kidney disease and mild kidney dysfunction. The last three stages represent worsening or end-stage kidney disease when kidney function is significantly impaired. In Stage 5, when eGFR is less than 15 ml/min, your physician can determine when regular dialysis is necessary to take over the essential job of the kidneys.

Low-carb diets in early kidney disease

There is no research clearly showing that low-carb diets worsen early-stage kidney disease. Also, we learned earlier in the guide that there is evidence that low-carb diets can help control diabetes and improve high blood pressure. Therefore, a low-carb diet may mitigate the main risk factors that lead to kidney disease.

Published research that actually demonstrates improvement in kidney function is sparse. However, in a case report, an obese man with type 2 diabetes who switched to a low-carb diet providing 80 to 90 grams of carbs per day experienced a stabilization of his kidney function, which had been steadily declining during the previous six years. In addition, he was able to discontinue insulin after two weeks on the diet, reduce his body weight by 46 pounds (21 kg), decrease his blood sugar levels, and lower his HbA1c from 9.4% to 6.5%. The authors concluded that the man’s stabilized kidney function was likely due to both his better blood sugar control and the resolution of his obesity.²⁹

Earlier in this section, we alluded to conflicting research about the effect of reducing protein intake on early-stage kidney disease. A 2021 meta-analysis of RCTs examining stage 1-3 diabetic kidney disease showed that restricting protein intake to < 0.8 g/kg/day did confer protective benefits on kidney function.³⁰

It should be noted that the strength of the authors’ conclusions are somewhat diminished by the trials’ low quality, small sizes, and heterogeneity of study populations. Nonetheless, this does serve as a reminder that the data regarding the safety of low-carb diets in early-stage kidney disease should not be considered to be conclusive.

Low carb diets and late stage chronic kidney disease

When the kidneys’ function is already severely damaged, can eating a low-carb diet help?

The short answer is probably not.

In later stages of chronic kidney disease, the extra demand placed on the remaining functional portions of the kidneys actually results in a continued decline in kidney function. Improvement in advanced disease would be rare even if the underlying causes were improved or reversed. Use of a low-carbohydrate diet for these advanced stages should only be contemplated under the direction of a qualified physician or a nephrologist, a specialist in kidney disease.

Further, a moderate to high protein intake in the setting of more advanced kidney disease can accelerate the deterioration of kidney function; restricting protein is strongly recommended when your kidneys are significantly damaged. Cutting protein intake to the range of 0.6 – 0.8 grams per kilogram of body weight per day (about 40-50 grams per day) – or less than half of what most people typically eat – can slow the progression to end-stage kidney failure, reduce protein in the urine, delay the onset of the symptoms of kidney failure, and delay the need for dialysis or transplantation.³¹

However, an individual assessment of protein requirements by a dietitian is recommended in those with advanced CKD to prevent sarcopenia (severe muscle loss) and frailty, which are both associated with early mortality.³²

Another issue is that in advanced kidney disease, the kidneys’ impaired ability to excrete sodium, potassium, magnesium, acid loads, and fluids can result in severe health consequences. Starting a low-carb diet — which typically encourages increased salt, fluids, potassium, and magnesium intake — could precipitate serious and sometimes life-threatening fluid, electrolyte, and mineral disturbances. For these reasons, anyone with advanced kidney disease should not attempt a low-carb diet without direct supervision by a nephrologist or qualified physician.

Summary

That was a lot of information. Let’s break it down one more time.

A well-formulated low-carb diet is completely safe for people with normal kidney function, and it may help treat diabetes and high blood pressure. Because these diseases are the biggest causes of kidney damage, a low-carb diet may actually help prevent kidney damage in those with normal kidney function or early-stage kidney disease.

But note: there are three major exceptions.

1. First, people with early-stage diabetic kidney disease should be aware that some studies show a benefit to eating low-carb, while others show that protein restriction can prevent worsening of kidney function. Talk to your doctor about whether the potential benefits of a low-carb diet for glucose levels, blood pressure, and weight (which may ultimately preserve kidney function) outweigh the potential benefit to the kidneys of protein restriction.
2. Next, those with a history of kidney stones, or who experience an episode of kidney stones after starting a low-carb diet, should ensure their diets minimize consumption of oxalates, optimize fluid and mineral intake, and include moderate (rather than high) amounts of protein. These steps will help prevent further kidney stones.
3. The third and most important caveat is for individuals with advanced kidney disease: a low-carb diet might be dangerous, so consultation with a nephrologist is necessary prior to making any changes to your current diet. Talk to your doctor.