Last night, I came across the following Tweet:
Put down that protein shake! High protein diet increases risk of kidney disease http://t.co/rTeQaqsYgN@EurekAlertAAAS
— OtherSide of Science (@ososcience) January 23, 2014
My sister is a body-builder who maintains a diet of nearly 200 grams of protein every day - so naturally I was curious to see if her diet might have negative consequences for her kidneys.
The link in this Tweet is to a EurekAlert! press release from the University of Granada. The press release starts out:
"High-protein diets, like the popular Dr. Dukan diet, increase the long-term risk of developing kidney disease and have a negative effect on renal urinary and morphological markers. What's more, they may promote serious pathologies like nephrolithiasis (calcium kidney stones) because they drastically reduce urinary citrate (an inhibitor of calcium salt crystallization) and urinary pH, and increase urinary calcium (to compensate for the metabolic acidity caused by excess protein)."
Sounds serious, right? But the next line of the press release worried me:
"University of Granada scientists have proved this in an experiment in rats that examined the effects of a high-protein diet on renal urinary, plasma and morphological parameters."
First of all, "proved" is a VERY strong word in science. You would almost never say it this way - responsible scientists would say that their data support the hypothesis that high-protein diets increase the risk of kidney stones.
This press release describes the results of a single experiment with only 20 rats. High-protein diet rats received supplements with greater amounts of soy protein, and lesser amounts of wheat starch. High-protein diet rats had 10% lower body weight, 88% lower urinary citrate levels and 15% more acidic urinary pH than normal-protein diet rats, which "could constitute a favorable environment for kidney stones formation in high-risk patients" (Aparicio et al., 2013). Plasma urea concentrations in high-protein diet rats did not increase significantly (the authors claim a "close to significant" increase at a p-value of 0.08 - NOT a significant level). High-protein diet rats did have enlarged kidneys, but renal interstitial connective tissue was not affected, nor did the authors observe renal fibrosis.
According to the paper:
"other authors have observed that in long interventional studies performed in humans, including overweight or obese healthy subjects, without preexisting renal dysfunction, the HP diet did not adversely affect renal function. [...] Something to consider is that the effect of proteins also depends on the presence of other nutrients in the diet. High intakes of fruit and vegetables are associated with a reduced risk for stone formation in high-risk patients."
So, bottom line? The high-protein diet (that is, high in soy protein supplements, according to this experiment) could lead to a favorable environment for kidney stones/disease in high-risk patients who don't also have high intakes of fruits and vegetables... however, this single experiment only shows this effect for high-protein diets in rats (NOT humans). The experiment does not "prove" anything, but instead is only one piece of evidence in a stream of studies on high-protein diets. While the results are interesting and may warrant future research in the area, I think the researchers and the public relations writers who penned the press release should be careful in making human diet recommendations based on this experiment alone.
So should you jump to boycotting those high-protein shakes you love? Not necessarily. Just keep an eye on future research, and a stock of fruits and vegetables in your refrigerator - just in case.
— Paige Brown (@FromTheLabBench) January 23, 2014
Reference: High-protein diets and renal status in rats V. A. Aparicio, E. Nebot, R. García-del Moral, M. Machado-Vílchez, J. M. Porres, C. Sánchez and P. Aranda Nutrición Hospitalaria. 2013;28 (1):232-237 ISSN 0212-1611 • CODEN NUHOEQ S.V.R. 318