January portends a wave of diet and exercise resolutions. As I said in my last post, I am not joining in this year, at least not in terms of trying to lose weight, but the sheer volume of health advice on websites has led me to looking into whether there is a healthy diet recommended for polycystic kidneys. The ADPKD population is so heterogenous that one size certainly does not fit all, it very much depends on where you are for renal function and whether you have other comorbidities.
Protein seemed a good place to start.
I began with the PKD websites (the more reputable ones are listed to the right under links).
It felt like I had jumped into a muddy puddle.
Take “pkdcure.org”, under general advice you will find:
As a general guideline, moderate-protein diets are typically recommended for patients pre-dialysis …….. low-protein diets are usually no longer the recommendation.
But following the link “protein” and you will read:
consuming less protein may slow the progression of kidney disease, including PKD
Others contain a similar message :
endpkd.ca advises “a low protein diet helps slow the progression of PKD”
pkdclinic.org tells you to “limit the amount of protein”
I have previously mentioned the multiple dubious sites advising on PKD, it is sometimes hard to know who is behind the organisation. There are a raft of websites that link back to a Chinese hospital and without exception these all tell you to eat a low protein diet.
So where does the “low protein diet” advice come from?
In 1869 a scientist, Lionel Beale, speculated that a low protein diet (low meat content) might be therapeutic in renal disease. His argument was that it would reduce the workload of the kidneys. Attempts to put this into practice were associated with malnutrition and poor outcomes, so in the early 20th century a high protein diet was advised generally in the belief that high protein equated with strength.
In the 1960s a lower protein diet was used to treat uraemia (Giordano, 1963) but the patients were in end stage renal failure and the benefit was short term only. Dialysis was developed around the same time and the dietary management took a back seat.
One hundred years on from Beale’s experimentation papers still claim that low protein diets help renal disease, one even stating “PKD is one of the kidney conditions that benefit most from a low protein diet” (Gretz, 1989).
The MDRD Study (Klahr et al, 1994)
Then in 1989 a study was approved to test the hypothesis that restricting dietary protein and lowering the blood pressure would slow the progression of renal disease. This is the MDRD (modification of diet in renal disease) study, results published in 1994.
Unexpectedly the results showed that low protein diets had only a minimal effect on the progression of renal disease.
The study included patients recruited from renal clinics and by advertisement but it excluded any with diabetic nephropathy – that’s a large subgroup to exclude so seriously limits the applicability of results.
Within the MDRD cohort of over 800 patients were a subgroup of 200 patients with ADPKD.
There were two arms in the MDRD study, the second comparing low protein with very low protein diets. At the time neither group demonstrated significant outcome differences. However these patients were reviewed again some years later (Menon et al, 2009) and the very low protein diet patients had a higher risk of dying compared with the low protein diet group even though the diets had only be advocated for 2-3 years (the average duration of follow-up in the study). This is not indicative of cause and effect but it does seriously raise concerns about dramatic protein restriction.
Despite that warning about the long term effects, a more recent study (Terukuni, 2012) has concluded that “Severe low protein diet in ADPKD patients will delay the progression to renal failure”. Before throwing out the steak you should know that they looked at just 10 patients with very low renal function and there were no control patients in the study.
There are problems when it comes to studying a low protein diet in humans.
When rats or mice are used to investigate the effect of diet on their disease, they begin their allocated diet from birth and other factors are strictly controlled. But humans with established kidney disease are a mixed bunch, with varied age, pathology and previous dietary history. Compliance with an unpalatable diet is also hard to enforce.
What, exactly, is a low protein diet?
Comparison of trials falls down when it comes to how each defines “low protein diet”, with ranges from 0.28-0.8g/kg/day.
This measure needs some explanation. The kg part is not your actual weight, but your ideal weight. So, at 100kg I might have hoped for an allowance of 80g protein each day, but it is more likely to be around 64g.
Then you need yet more maths as 100g steak does not contain 100g of protein, but approximately 31g protein.
Meat protein content 31-32g per 100g
Kidney beans 6.9g
Baked beans 5.2g
Crab sticks 10g
Suddenly working out the protein content of your diet has become like a nightmare maths exam. Furthermore, food preparation methods can also alter the type and amount of protein available for digestion.
What about the source of the protein?
Proteins consist of chains of amino acids. Different sources of protein will have varying mixes of these amino acids. Human physiology is pretty good at pulling these building blocks apart and reconstructing new proteins for what ever the body needs – think of a giant box of Lego bricks. But if you only have grey bricks all you can build is a Millenium Falcon (Star Wars). Variety in amino acids is important.
There have been several studies comparing casein-based diets with plant proteins in mouse models of polycystic kidney disease. I will look into this in greater depth in a later post, though my reading so far suggests that soy protein diets may be linked with slower cyst expansion. But not all plant proteins are renoprotective and there isn’t sufficient evidence yet to make any definitive statements about meat vs plant proteins.
Where have we got to in the question of protein?
Despite numerous studies on dietary protein in renal disease, the results are inconsistent and even the meta analyses have disagreed. For ADPKD in particular there is not enough information to come off the fence either one side or another. So the advice to consume a “moderate amount of protein” is the current guideline. Unfortunately for us even the term “moderate” is open to interpretation.
It seems to cover from 0.6g/kg/day to 1g/kg/day.
The bottom line is that while protein restriction in laboratory animals with renal disease may protect renal function, humans may not get the same benefits. Any extreme dietary manipulation risks malnutrition and what suits one patient may be dangerous for another, especially for the range of patients with ADPKD. Personally I shall aim for the “moderate” and eat somewhere between 0.6 and 1mg/kg/day, though I shall need to buy a calculator for the kitchen and grow accustomed to doing sums.
Gretz N, Does a low protein diet really slow down the rate of progression of chronic renal failure? Blood Purif. 1989;7(1)33-8
Giordano C. Treatment of uraemia using essential amino acid and low protein diets. 1963 Proceedings of the 2nd International Congress of Nephrology 752-7
Klahr S et al. The effects of dietary protein restriction and blood-pressure control on the progression of chronic renal disease. New Engl J Med 1994 330:877-84
Menon V et al. Effect of a very low protein diet on outcomes: long-term follow-up of the MDRD study. Am J Kidney Dis 2009 53:208-17.
Shaw AB, FJ Bazzard, EM Booth, S Nilwarangkur, GM Berlyne. The treatment of chronic renal failure by a modified Giovannetti diet. Quart J Med 1965 34:237-53
Terukuni et al, Effect of low protein diet on chronic renal failure due to ADPKD. Kidney Research and Clinical Practice. June 2012, Vol.31(2):A37, doi:10.1016/j.krcp.2012.04.409