When it comes to chronic diseases, the big names are cardiovascular disease and type 2 diabetes. Chronic kidney disease doesn’t really get much press – which is weird, because about 10% of the population has it. Kidney disease is painful and exhausting to live with, and most people eventually need dialysis or a kidney transplant to survive.
Like most chronic diseases, kidney disease is affected by diet. There’s an old warning, born in the 1980s, that eating protein damages the kidneys, but the evidence actually proves otherwise: healthy people don’t need to worry about protein hurting their kidneys. Sugar, on the other hand, is really not your kidneys’ best friend.
Meet your Kidneys
The kidneys don’t have a particularly glamorous job. They filter out waste from your blood and direct it into your urine to get it out of your body. If this job didn’t get done, you wouldn’t be able to regulate the balance of salt and other electrolytes in your body, keep your blood at the right pH, or maintain a normal blood pressure.
It’s pretty important stuff, even if it’s not something most people spend a lot of time thinking about it. The kidney is kind of like the highway repair crew of your body. If it’s working right, you barely notice it and everything just hums along smoothly and feels automatic. But if it’s not working right, you’re in trouble. So how does diet play into that?
Protein and the Kidneys
Before handling anything else, let’s tackle the old myth that protein is bad for your kidneys. The idea that protein causes kidney damage sounds logical on the surface. When you break down protein, your body produces certain waste products in the process. If those waste products stay in the blood, they’re very dangerous, so the kidneys filter them out into the urine instead. The more protein you eat, the harder your kidneys have to work to filter out the by-products. The symptoms of chronic kidney disease and kidney failure are mostly caused by failure to excrete protein byproducts.
All of that might make it sound like protein is somehow stressful to the kidneys, and you can give your kidneys a break by eating a low-protein diet. But in healthy people who don’t have kidney disease, the adaptations to a higher-protein diet don’t damage the kidneys. You eat more protein; your kidney adapts to the increased workload; nothing happens and everyone moves on. A Cochrane review concurred: the changes to kidney function on a high-protein diet just aren’t terribly concerning.
There is some evidence that people who already have kidney disease might do better with a lower-protein diet. For those people, kidney function is already impaired, and the kidneys might not be up to the kind of adaptations necessary to a higher protein load. But that doesn’t mean that eating a low-protein diet will prevent kidney disease in healthy people. There’s also plenty of evidence that people with egg allergies do better avoiding eggs, but that doesn’t mean everyone should avoid eggs for fear of developing an egg allergy.
If you already have kidney disease, go see a doctor. The doctor might very well put you on a low-protein diet, or some other kind of diet (there are all kinds of kidney diets, depending on specifically what is wrong with you). But if you don’t have kidney disease, there’s no evidence that eating protein will do you any harm.
The Real Danger: Sugar
So eating protein doesn’t cause kidney disease or damage the kidneys in healthy people. But there’s another potential villain to look at here: sugar and refined carbs. Sugar and refined carbs damage the kidney indirectly, through diabetes and liver damage.
Diabetes and Kidney Health
We’ll start with diabetes. If you know anything about diabetes, the term “diabetic nephropathy” might be familiar – that’s the fancy word for kidney failure caused by diabetes. Diabetic nephropathy is the most common cause of kidney failure in the US, and as rates of diabetes keep increasing, it’s only going to get more and more common.
How does this work? Very briefly, high blood sugar levels in diabetes damage blood vessels in the kidneys. This allows too much protein to pass into the kidneys and overworks them. Ultimately, it prevents the kidneys from effectively filtering waste out of the bloodstream.
Eating a diet high in sugar and refined carbs is one of the fastest ways to cause high blood sugar and eventually type 2 diabetes. And that metabolic damage is really hard on kidney health.
Liver Health and Kidney Health
This one is less commonly known, but still important. Scientists have recently been tracing the connection between liver health and kidney health. If your liver isn’t happy, your kidneys won’t be happy.
Patients with type 2 diabetes and nonalcoholic fatty liver disease are even more likely to develop chronic kidney disease than patients with “just” the diabetes. The same goes for patients with type 1 diabetes (the genetic/autoimmune kind, aka juvenile diabetes) and prediabetes. And it’s not just diabetes: nondiabetic patients who have liver disease also have higher rates of chronic kidney disease.
This article offers some possible explanations for how that works:
- Liver disease can affect various enzymes that also affect kidney health
- Liver disease can affect the gut microbiome in ways that also hurt the kidney.
- Liver disease can dysregulate various hormonal cascades that affect the kidney.
- Liver disease can provoke inflammatory problems that also hurt the kidney.
Basically, if you want your kidneys to be happy, you want your liver to be happy. And what makes your liver unhappy? Fructose! That might be why fructose is particularly dangerous to the kidneys, or at least it is in rats – and even in healthy rats. It might also be why sugary soft drinks in particular are associated with chronic kidney disease in humans (they’re a big source of fructose). It’s not like protein, which is harmless unless the kidneys are already struggling. Fructose is dangerous even in the absence of pre-existing kidney disease.
Fructose might also affect the kidney directly (here’s another study showing the same thing). The takeaway is basically that fructose, and anything else that contributes to liver disease, is an important threat to kidney health.
Diet and Kidney Health: The To-Do List
Here’s a 1-minute summary of everything above:
- For healthy people without pre-existing kidney disease, protein doesn’t hurt the kidneys. There’s no reason to avoid protein if you don’t have kidney problems.
- Sugar and refined carbs are dangerous to the kidneys. High blood sugar stresses the kidneys and ultimately contributes to kidney failure. Sugar is also bad for the liver, and liver disease can contribute to kidney disease.
So far, this suggests that if you’re healthy and have no kidney problems, protein isn’t an issue but sugar and refined carbs are potentially dangerous. If you do have kidney problems, your doctor can tell you about the particular type of diet that will be helpful for you.