Dietary Assessment of Lithogenic Factors in Plant-Based Meat Products

Introduction

Dietary modification is an instrumental component of management and prevention of kidney stones. The medical management of kidney stones guidelines include recommendations to limit intake of nondairy animal protein, oxalate-rich foods, and sodium. ¹ Patients should aim for a normal dietary calcium level at the Recommended Daily Allowance of 1000 to 1200 mg. ¹ Animal protein decreases urinary citrate (an inhibitor of kidney stone formation), increases urine calcium, and promotes urine acidity. ¹ Increasing dietary oxalate increases urinary excretion as well. ¹ Sodium intake increases urinary calcium and decreases urinary citrate. ¹ Based on these biochemical pathways, these nutritional components have been targeted to decrease the risk of kidney stones. It has been shown that diet influences the urine chemistry, which affects kidney stone risk. ²

Nondairy animal protein is an interesting topic in light of the recent advent of plant-based meat products (PBMPs) as a substitute protein source. PBMPs are processed food products designed to emulate the taste and texture of a certain category of animal protein but using a different protein source. Substitute protein sources include protein extract from soy and legumes. For the past few years, PBMPs have seen a rise in popularity because of health concerns, ^{3 –5} increased retail availability, decreased environmental impact, ⁵ and meat supply shortages during the COVID-19 pandemic. ⁶ There may be similarities in public perception regarding PBMPs and whole food plant-based diets.

Although PBMPs contain plant-based ingredients, the products undergo processing and combination with other food components, which may render them, and thus their downstream metabolic impact, as distinct from the “whole” or unprocessed plant content of whole food diets. Lithogenic patients who are counseled to decrease their nondairy animal protein intake may turn to PBMPs. However, we know that plants can be oxalate-rich, whereas animal protein contains no oxalate. In addition, PBMPs are commercial products that have added salt. Both these points may mitigate the potential benefit of switching from animal protein to PBMPs.

Despite the perceived benefits of PBMPs, the potential lithogenic risks associated with PBMPs are not well characterized. We sought to analyze the nutritional profile of prominent PBMPs and compare them with traditional animal protein.

Methods

IRB review was not required for this analysis of public, non-patient data. A list of widely available PBMPs was compiled from multiple databases, including government (USDA) and industry (Plant-Based Food Association). The FoodData Central Database ⁷ was queried with a sample of PBMPs widely available to U.S. consumers. Assembled products and meals were excluded. The respective animal protein was queried; animal protein items included raw and cooked options. Nutrient profile data were compiled and compared with animal protein data using standardized serving sizes. For the sodium comparison specifically, the PBMP was compared against the cooked item for that respective animal protein. Primary protein sources were identified using verified ingredient lists. Oxalate content was extrapolated from the oxalate nutrition profile list from the University of Chicago. ⁸

Results

We identified a total of 47 PBMPs (16 beef, 11 pork, 10 chicken, and 10 seafood items). For each of these four categories, we included two or three respective animal proteins with at least one raw item and one cooked item to address the topic of added salt during cooking. Compared with their respective animal protein sources, most PBMPs contained on average fewer calories (plant-based beef has 77% of respective animal protein beef, pork 94%, chicken 86%, and seafood 83%) and less protein (plant-based beef 68%, pork 96%, chicken 53%, and seafood 54%). PBMPs contained less fat (plant-based beef 58%, pork 109%, chicken 76%, and seafood 63%) (Table 1).

In terms of oxalate content, most PBMP products used soy protein as the primary protein source (26/47, 55%). Soy-based beef contained the highest average oxalate content (18 mg per serving), whereas soy-based seafood had the lowest (7 mg) (Table 2). The most common non-soy protein source was pea protein (14/47, 30%), which contains trace amounts of oxalate. Other plant protein sources included wheat, corn, rice, and various beans. In contrast, animal protein contained negligible oxalate.

Sodium content was higher in the majority of PBMPs overall (34/47, 72%) and in each category (plant-based beef 109%, pork 128%, chicken 100%, and seafood 148%) compared with animal protein (Table 1). Individual PBMPs contained as much as 216% of the sodium in the respective animal protein. Calcium content was markedly higher in PBMPs (plant-based beef 317%, pork 144%, chicken 291%, and seafood 295%) compared with animal protein (Table 1). All PBMPs had low cholesterol content compared with animal protein (0.4 mg vs 79 mg).

Discussion

There are limited data on how plant-based whole-food diets affect health and specifically kidney stones. Plant-based diets have been shown to have a protective effect on a myriad of medical conditions, including ischemic heart disease and cancers. ^9,10 As for kidney stones, there certainly are possible avenues of antilithiasis properties of specific phytonutrients found in dietary plants. ⁹ Yet there is a paucity of literature discussing specifically these processed PBMPs and their impact on lithogenic patients and their risk of urolithiasis.

Although there are articles discussing the relationship between plant protein and kidney stones, none discuss plant-based protein specifically. We sought to supplement the existing, sparse literature by characterizing the nutritional profile of PBMPs. Compared with respective animal proteins, the PBMPs contained fewer calories, less protein, and fat. PBMPs contained much more oxalate, sodium, and calcium than respective animal proteins. The nutritional profiles of PBMPs seem quite different from that of animal proteins.

As PBMPs become more widespread, it is important for urologists to understand the nutritional profiles as compared with traditional animal protein. Shu et al utilized food frequency questionnaires correlated with self-reported stone events to assess associations between dietary patterns and urolithiasis risk. Plant protein, compared with nondairy animal protein, was associated with lower stone risk. ¹¹ Since a diet high in fruits and vegetables is associated with decreased stone risk, the switch to plant protein may be favorable. In addition, there is an association between obesity and risk of kidney stones and so the decreased calories, fat, and cholesterol in PBMPs may be beneficial. ¹² As follows from these studies, one may at first glance extrapolate from the promising literature about plant-based or plant-forward (high in fruit and vegetable) diets that PBMPs are better for the lithogenic patient than animal protein.

Counter to the potential beneficial points, our study found higher sodium levels in PBMPs despite including animal protein items that were cooked and, therefore, had added salt as well. Disparate from raw plant protein foods (i.e., legumes, nuts, and seeds) and raw animal protein that start with negligible sodium content, PBMPs are commercial products that have added salt. This is an important point to recognize as many PBMPs are meant to substitute raw animal protein. It may be prudent to educate patients about this fact so that they can be mindful of it when cooking and preparing meals. Massey and Kynast-Gales conducted a randomized crossover trial of beef and plant proteins (legumes, nuts, and grains) and analyzed urinary composition. To the aforementioned, they found higher urinary sodium levels in the plant protein diet. ¹³ Ultimately, they concluded that restricting excess protein is important, but that protein source is not.

Moreover, we found that PBMPs have higher oxalate content. Although animal products have negligible amounts of oxalate, dietary recommendations categorize different plant foods into tiers of oxalate content; 10 to 15 mg oxalate per serving is considered high content, and >15 mg per serving is considered high. ¹⁴ The average plant-based pork and chicken are high oxalate content; the average plant-based beef is high oxalate content. There is increasing literature that plant-forward diets increase the risk of developing kidney stones. ¹⁵ Unique among the plant-based proteins, pea protein contains negligible oxalate. Borin et al conducted a similar study with plant-based milks; the milk alternatives had different compositions of the urolithiasis-related nutrients. They concluded that some milk alternatives have a favorable nutritional profile compared with dairy milk. ¹⁶ This is a potential dietary modification point much like our findings about pea protein PBMPs.

We have highlighted some literature pertaining to plant protein, but there is not much reported on PBMPs. Our study indicates that different facets of PBMPs are theoretically beneficial or detrimental to lithogenic patients when compared with traditional animal protein; the answer of whether they are actually beneficial is not straightforward. Our findings on these processed products do not apply to people on whole food plant-based diets nor those who mix PMBPs into whole food plant-based diets. A future avenue of research is analyzing a cohort of patients on PBMP diets vs traditional animal-protein diets and vs whole food plant-based diets. Primary outcomes should include changes in 24-hour urine chemistry and kidney stone events over time.

Conclusions

The nutritional profile of PBMPs is quite different from respective animal proteins and should not be thought of as straight substitutes. The majority of these processed products consist of high-oxalate protein sources and have higher sodium and calcium content compared with animal protein. Stone-forming patients should be counseled about the potential lithogenic risk of higher dietary oxalate, sodium, and calcium in these products. Patients and providers should take caution when deciding to switch from animal protein to PBMPs.