Replacing animal protein with vegetable protein not shown to improve renal function
HIGH-PROTEIN diets have been shown to accelerate renal deterioration in a number of studies of populations with even mildly reduced kidney function.
Most Western-style diets are high in animal protein and relatively low in plant protein sources such as legumes. Consequently, researchers have suggested limiting animal protein as a way to preserve renal function. Animal proteins have higher dietary quality protein because they have a better balance of the two required sulfur amino acids—cysteine and methionine—than plant protein. A common misunderstanding is that the total sulfur amino acid content of animal protein is higher than that of plant protein, but they are similar in total sulfur content per gram.
Excessive sulfur-containing amino acid consumption is detrimental whatever the source because their catabolism increases the amount of sulfuric acid generated. Another concern is that because vegetable proteins have poorer dietary quality than animal proteins, a higher intake may be required to maintain adequate protein nutrition. In spite of these basic facts of nutritional biochemistry, vegetarian diets are being promoted as better for preserving kidney function than omnivore diets. What is the current evidence available for advising patients?
Adam M. Bernstein, MD, of the Harvard School of Public Health in Boston, and his colleagues recently reviewed the literature on vegetable-based diets and kidney function (J Am Diet Assoc. 2007;107:644-650). “Although data on persons with chronic kidney disease are limited,” the authors wrote, “it appears that high intake of animal and vegetable proteins accelerates the underlying disease process not only in physiologic studies but also in short-term interventional trials. The long-term effects of high protein intake on chronic kidney disease are still poorly understood.”
Only a few studies comparing animal and vegetable protein diets have been published. James W.
Anderson, MD, and colleagues (Am J Clin Nutr. 1998;68[suppl]:1347S-1353S) reported findings of an eight-week randomized cross-over study of eight diabetics with proteinuria and moderate renal insufficiency. A decrease in glomerular filtration rate (GFR) was observed both in patients receiving a 1 g/kg animal protein diet versus a 1 g/kg protein diet with one half of the protein as isolated soy protein, suggesting there is no difference in diabetic CKD. Giuliano Barsotti, MD, and colleagues (Am J Nephrol. 1991;11:380-385) followed patients with non-diabetic nephropathy and moderate-to-severe renal insufficiency on a vegan protein diet at 0.7 g/kg/day for three months. Compared with a conventional animal-protein based diet at 0.6 g/kg, no difference in creatinine clearance was observed.
In a later study, Dr. Barsotti and his collaborators (Nephron. 1996;74:390-394) followed 22 patients with nondiabetic nephrosis transitioned from a mixed animal-vegetable (1.0 to 1.3 g/kg/d) to either a conventional low-protein diet at 0.6 g/kg or a vegan diet at 0.7 g/kg/day. After four to six months, both groups showed a significant decrease in GFR and no effect on proteinuria.
In an analysis of data from the Nurses' Health Study, a group led by Eric L. Knight, MD, MPH, found that women with mild renal insufficiency had a correlation of higher protein intake with decrease in GFR (Ann Intern Med. 2003;138:460-467). Subanalysis revealed that the consumption of non-dairy animal protein led to greater decreases in GFR than either dairy or vegetable protein; however, high intakes of any protein had deleterious effects.
The differences in protein adequacy of low-animal versus vegetable protein for preventing protein malnutrition is even less studied. Only the 1996 study by Dr. Barsotti's group examined this possibility; in that study, the researchers observed no change in body weight, serum total protein, serum albumin or serum transferrin over three months in 22 patients placed on the vegan diet.
Since the various high-protein foods differ greatly in amino acids as well as accompanying nutrients, such as iron, zinc, omega-3 fatty acids, and isoflavones, more study is needed to determine if any of these secondary nutrients may affect renal function with longer consumption of animal versus vegetable diets. For example, researchers have reported that consumption of 0.5 g/kg isolated soy protein compared with 0.5 g/kg purified casein for four weeks not only decreased urinary albumin by 9% but also improved the lipid profile of men with type 2 diabetes mellitus and nephropathy (J Nutr. 2004;134:1874-1880). In another study (Nephrol Dial Tranplant. 2007;22:229-234), researchers substituted 25 grams soy protein for 25 grams animal protein for five weeks in 20 renal transplant patients. Total protein intake was about 1.0 g/kg. The investigators observed no difference in creatinine clearance, urinary protein excretion, and serum albumin, but LDL cholesterol decreased from 136 to 123 mg/dL. Lipid peroxides also decreased.
Overall, the limited research available suggests no advantage of low- and moderate-protein diets with vegetable versus animal protein, but high-protein diets from any food source can be deleterious.