Historically, vegetarian diets or diets heavy in legumes, such as those from Latin and Asian cultures, have met some resistance in the United States for patients with chronic kidney disease (CKD).
The rationale was that the renal diet needed to be low in phosphorus and potassium; legumes and nuts tend to be high in both. Therefore, for many years, dietitians would tell their patients to restrict plant-protein foods such as beans, soy, nuts, and whole grains. This paradigm may be changing, however.
Vegetarian diet and CKD
Data on the safety and efficacy of vegetarian or plant-based diets in patients with CKD is sparse. Back in 1988, Chan et al (J Clin Invest. 1988;81:245-254) conducted a clinical trial and concluded that animal protein, but perhaps not fish protein, differs from plant protein in its effect on glomerular filtration rate (GFR), and patients with CKD and diabetes may be impacted differently than patients with CKD alone.
This issue has been examined in other studies, many using a single meal or short duration diet (e.g., one to four weeks) to test the effect of vegetarian diets on GFR. A review of many of these studies by Bernstein et al concluded that high-protein animal diets, but not high-protein plant protein diets, impact renal function as measured by GFR (J Am Diet Assoc. 2007;107:644-650).
An initial consideration for placing CKD patients on vegetarian diets is the high prevalence of protein energy wasting. A study in China, however, examined the rate of malnutrition and found there was no significant difference in subjective global assessment scores (a tool to assess nutrition status) or physical functioning in patients who were vegetarian compared with those who were not (Nephrology 2011; published online ahead of print). The study found differences in body mass index (BMI) and doses of erythropoietin-stimulating agents (ESAs), with vegetarian patients having a lower BMI but requiring higher ESA doses to maintain optimal hemoglobin levels.
Another important consideration with respect to vegetarian diets is phosphorus control. Clinically evident hyperphosphatemia typically begins in stage 5 CKD or end-stage renal disease. Early rises in serum phosphorus may be mediated by fibroblast growth factor-23 (FGF-23), which has been shown to cause an increase in phosphorus excretion. It is unclear whether a diet low in phosphorus or use of phosphate binders prior to actual rises in serum phosphorus would be beneficial in retaining normal serum phosphorus throughout stage 5 CKD.
Vegetarian diets include foods such as legumes, nuts, and whole grains as protein sources. Both animal and plant proteins contain organic phosphorus. Organic phosphorus is hydrolyzed to inorganic phosphorus in the gastrointestinal (GI) tract, with approximately 30%-60% of the overall phosphorus absorbed into the circulatory system, according to a study by Noori et al (Iran J Kidney Dis. 2010;4:89-100).
Absorption of phosphorus is affected by many factors. However, a major difference between animal and plant proteins is that, with plant proteins, phosphorus is bound by phytate, which reduces organic phosphorus availability to less than 50%. In fact, Noori et al suggest that “it is likely that prescribing patients with CKD a higher proportion of protein from plants may not only meet their required protein but also lead to better management of their body phosphorus burden.”
It is important to note that an extremely high percentage of processed foods manufactured in the United States use inorganic phosphorus as an additive. In contrast to organic phosphorus, approximately 90% of inorganic phosphorus is absorbed through the GI tract. Therefore, a CKD patient following a vegetarian diet high in processed foods may see no benefit from the vegetarian organic phosphorus based concept.
Testing in pre-dialysis patients
To test whether a plant-based diet would have positive effects on serum phosphorus in pre-dialysis CKD patients, Moe et al (Clin J Am Soc Nephrol. 2011;6:257-264) conducted a one-week cross-over design study involving nine patients with a mean GFR of 32 mL/min/1.73 m2. The two diets consisted of the following macronutrient distributions: approximately 2,100 kcals/day, 28%-30% fat, 57%-58% carbohydrates, and 14% protein.
The meat-based diet had 16 g of plant protein and 62 g of animal protein, resulting in 847 mg of phosphorus whereas the plant-based diet included 4.1 g of animal protein and 74.8 g of plant protein, resulting in 818 mg of phosphorus. The results showed that the plant-based diet was associated with a significant decrease in serum phosphorus (from 3.5 to 3.2 mg/dL) and plasma FGF-23 (from 84 to 61 pg/mL).
The authors concluded, “Therefore, dietary counseling of patients with CKD must include information on not only the amount of phosphate but also the source of protein from which the phosphate derives.”
In conclusion, we may need a new paradigm whereby vegetarian or plant-based diets are promoted to preserve GFR and maintain phosphorus homeostasis in our CKD population. However, before this can be established as optimal clinical care, more long-term randomized clinical trials need to be done.
Dr. Steiber is Coordinator of the Dietetic Internship/Master’s Degree Program at Case Western Reserve University in Cleveland.