Do low-carb diets harm kidney function?
At Diet Doctor, we recently published our evidence-based guide on low-carb diets and kidney health. Based on the available evidence, we concluded that moderate protein intakes commonly consumed on low-carb diets (1.2-1.7 grams per kg of reference body weight) did not pose a risk to kidney health. Imagine our surprise when two publications in Nephrology Dialysis and Transplantation claimed that protein and low-carb diets harm kidney function. How could we have gotten it wrong? Do we need to completely reverse our policy?
To make sure we provide the most trustworthy and up-to-date health information, we need to take a deeper look at these studies and place them into the context of existing research.
The first observational study followed 2,255 patients aged 60-80 with a history of myocardial infarction (heart attack). They filled out a standard food frequency questionnaire (which as we have reported before is frequently inaccurate and unreliable) and followed the GFR (a laboratory measurement of kidney function) after 41 months.
Observational trials like this provide weak data compromised by uncontrolled confounding variables. For instance, in this study those who ate more than 1.2 grams per kg of body weight per day (g/kg/d) of protein averaged 2,250 calories per day. Those who ate less than 0.8g/kg/d averaged 1,346 calories per day. That is an almost 1,000 calories per day difference!
But wait, there’s more. The higher-protein consumers also ate 268 grams of carbohydrates per day, compared to 173 grams per day for the lower-protein consumers. Finally, the high-protein group also ate 1,300 mg more sodium than the lower-protein group. While sodium may not be all that important on a low-carb diet, on a higher-carb diet, sodium intake likely does correlate with poor health outcomes.
Interestingly, the authors concluded that the higher-protein group had a more rapid decline in kidney function. But here’s the best part. We can also conclude that the group that ate more carbohydrates or sodium had the more rapid decline in kidney function. They just happened to all be the same group.
What was the true culprit? Protein? Carbs? Sodium? Or could it have been something completely different that wasn’t even measured? This study doesn’t help us with any of those questions. Any attempt to say with certainty that protein was the culprit is an unfortunate misrepresentation of the study.The second study was also observational, this time following 9,226 Koreans over 13 years. Again, the data came from food frequency questionnaires, and again the populations show completely different food intakes. Those who ate the lowest amount of protein averaged 0.6 g/kg/d of protein and 4.3 g/kg/d of carbohydrates. Those who ate more than 1.7 g/kg/d of protein also ate 7.3 g/kg/day of carbohydrates. That’s almost 60% more carbohydrates! Once again, the sodium also differed significantly, as did the frequency of smoking, alcohol consumption and fasting glucose, all of which were worse in the higher protein-group.
That doesn’t mean eating more protein causes you to smoke or drink. It means that those who smoke and drink happen to be more likely to eat more protein, carbohydrates and sodium.
What other poor health habits or other factors could have contributed to their worsening renal function? Again, this study does not tell us that.
At the risk of being repetitive, what truly caused the worsening kidney function? Was it the protein? The carbs? The alcohol? The smoking? Or other poor lifestyle choices?
We believe the quality of evidence matters when it comes to making decisions for our health. Whether it is red meat, protein or carbohydrate intake, the best way to identify a beneficial or harmful effect is with a randomized, controlled trial (RCT). Not a data-mining journey into an uncontrolled population dataset. Our guide to low-carb diets and kidney function cites meta-analyses of RCTs (the highest level of scientific evidence) and individual RCTs (moderate level of evidence) showing that protein intake does not harm kidney function.
The new evidence, based on old observational cohorts, is not sufficient to change our position on protein intake, low-carb diets and kidney health.
Please continue to check in with us to get trustworthy and evidence-based updates on the latest trials and scientific reports.
Thanks for reading,
Bret Scher, MD FACC
More
Guide: What you need to know about a low-carb diet and your kidneys
I pointed out that the study was likely only an observation of past data, and cautioned people of such studies that only show correlation, not cause.
It was not received well, with typical responses of “are you a doctor” and “mansplaining.”
I even used the example of a study showing people who own more than two pairs of running shoes are thinner, but buying shoes won’t make you thinner.
Our gullibility and refusal to read more than the headline is really allowing us to be easily manipulated.
Does anyone have any tips on how to help people understand they are being mislead by things like this?
Epidemiological studies are great to derive questions, but not to deliver any causation or concrete evidence.
We found that eating a little less carb and having protein is ok so far .
I make milk kefir using full cream milk , we have that every morning with 2 tbsps of groung linseed , 1 banana , (1 tesp of cinnamon , 1teasp of beetroot powder or spinach powder in my drink ) i wiz it up in my Nutri Bullet and its delicious.
Hopfully this is helping us keep healthy .
I told them about my keto diet and they surmised it was because I had a couple glasses of wine that it shut down my kidneys
I recovered in the hospital, but since then, very scared of this diet. How can I almost die just because I had a couple glasses of wine while on keto and fasting?
Anna
The first candidate, a mathematician, comes in and immediately answers, "obviously the answer is 4."
The second candidate, an engineer is also asked for the answer to 2 + 2. "It's difficult to say for sure," he says. "I'll need to make a few calculations and draw up some graphs. I can get back to you in a week or so."
The final candidate, a statistician, comes in, and is asked the question. The statistician stands, then deliberately and quickly closes all the blinds in the room, shuts the door, and unplugs the phone line.
He sits down and with a sly smile replies, "What do you want it to be?"
I'm not a doctor, but I would guess it was a dehydration related issue. Not enough electrolytes, and then alcohol on top. I've had a horrible muscle cramping reaction to alcohol when I wasn't watching my electrolytes while on low carb. Everyone's ability to hang on to electrolytes is different, I need lots of electrolyte supplementation to stay hydrated. Just my 2 cents.
Both parents had many health issues including diabetes, stroke, dementia and heart conditions, yet they both had longer than average lives.
Can I attribute my good health to my excellent DNA (as per 23andMe) or my chosen lifestyle? There is no way to know.
Testing people old enough to have a lifetime of bad choices reflected in their health will never be equal. Even if we started testing babies at birth, it would still not be equal. There's simply no way to compare apples and oranges except preference.
I am sorry you had that experience. If you are on any medication, please make sure and work with your doctor as different medications may need to be reduced when starting keto. It could also have been dehydration, and a lack of electrolytes not to mention food in the stomach coupled with the alcohol. Fasting and alcohol don't mix very well. Again, please follow up with your doctor for more specific advice.
This article is a brief outline on the basics of Street Epistemology: https://streetepistemology.com/publications/street_epistemology_the_b...
For more detail, see the 40-page "Complete Street Epistemology Guide: How to Talk About Beliefs": https://streetepistemology.com/publications/the_complete_se_guide