Potential renal acid load of non-dairy plant-based milk alternatives

ABSTRACT The market for plant-based milk alternatives has been growing continuously within the last years, and numerous new food items including almond, oat, and soy milk are nowadays commercially available. The impact of said foods on the acid-base balance in humans has been rarely assessed but is of great importance for individuals who adopt an alkaline diet for health reasons. Using the Potential Renal Acid Load (PRAL) score as a measurement of the amount of acid or base a certain food produces in the human body, we analyzed n = 39 plant-based milk items available on the German market. For this cross-sectional study, we used manufacturer-derived data and aimed to compare the PRAL values of different plant-based milks. The median PRAL score for all plant-based milk alternatives was −0.23 (2.14) mEq/100 ml, and indicated a slightly alkalizing potential. Soy milks yielded, on average, the lowest PRAL score (−1.11 ± 1.14 mEq/100 ml), whereas mixed milks were slightly acidifying (mean PRAL score: 0.19 ± 0.39 mEq/100 ml). When compared to the PRAL value of cow’s milk, plant-based milk alternatives were slightly more alkaline. Our results allow for a better characterization of plant-based milk alternatives and are useful for individuals and researchers that aim to estimate the potential renal acid load from a growing group of food items.


Introduction
Dairy milk is often cited as a good source of protein and a traditional part of the American diet. [1,2][5] Due to these unfavorable nutritional characteristics and other health concerns associated with dairy products, more and more individuals choose plant-based milk alternatives over cow's milk. [6,7]This trend is also driven by environmental concerns, animal welfare aspects and the widespread adoption of vegetarian and vegan diets [8,9] The increasing demand for plant-based milks was well received by the food industries, and the market for plant-based milk alternatives has been growing continuously within the last years. [10]umerous plant-based milk alternatives, including almond, oat, rice, soy and coconut milk are nowadays commercially available. [11,12]Multiple reviews and studies have investigated the nutritional value of these new food items. [11,13]13] To the best of our knowledge, however, there is no study available that systematically analyzed the potential renal acid load (PRAL) of these relatively new foods. [14]The PRAL-score is a measurement of the amount of acid or base a certain food produces in the human body. [15,16]Higher PRAL values indicate a higher amount of acid produced from a specific food, whereas negative PRAL values indicate that base is produced. [17] frequent and regular intake of high PRAL-foods may translate into a high Dietary Acid Load (DAL), [14] which has been associated with numerous unfavorable health outcomes including type-2-diabetes, impaired bone health and cardiovascular disease.[18,19] Assessing the PRAL value of foods is thus of great importance, and central to patients with chronic kidney disease who benefit from an alkaline diet.[15] Specific PRAL tables are available for many common foods to facilitate this process, however, these tables usually base on the work of Remer and Manz from the early 1990s.[20] Updated tables that include new "plant-based food items," such as plant-based dairy alternatives are scarce, [14,21] and rarely include many items.[22] Assessing the PRAL-value of plant-based milk alternatives is particularly difficult, because nutrition labels usually do not provide values for the potassium and phosphorus content of these specific foods.
Using manufacturer-derived data, we sought to determine PRAL values for many plant-based milk alternatives available on the German food market.Our major hypothesis was that the PRAL-scores would differ by milk type (i.e.oat milk vs. soy milk), depending on the unique nutritional values of each plant-based milk alternative.We also hypothesized that plant-based milk alternatives would differ in their PRAL when compared to cow's milk.

Data acquisition
The study used a convenience sampling technique.In April and May 2023, we sent written inquiries to n = 16 manufacturers of plant-based milk alternatives available on the German market.Manufacturers were identified using internet search engines and a dietary assessment software that included lists of available products on the German market (NutriGuide® plus software (Version 4.9, Nutri-Science GmbH, Hausach, Germany).After presenting some background information on the aims and scope of our study, we inquired whether manufacturers would be willing to share detailed nutritional information for their plant-based milk portfolio.
Data requested included macronutrient content, energy content per 100 ml of each food, and the content of the following micronutrients: potassium, phosphorus, magnesium and calcium.Finally, we inquired whether products were fortified, and, if yes, in what way.The initial contact via e-mail was made in the first week of April.In case of a positive reply, phone contact was sought.If successful, data was then transferred and extrapolated to a pre-defined Microsoft Excel File.We sent a second follow-up e-mail 10 work days later in case we received no reply.A final contact attempt was made via phone an additional week later.Reasons for declined requests were recorded for analyses purposes only.Inquiries were sent by two independent authors (AM and MAS).
Previously correlated with urinary pH values, this validated formula considers ionic dissociation, intestinal absorption rates for the included micro-and macronutrients and sulfur metabolism. [20]We refrained from estimating the PRAL value for a specific amount of energy intake from a particular food.Instead, PRAL was estimated for portions of 100 ml of edible food.

Statistical analysis
PRAL values in mEq/100 ml were calculated for each food item analyzed.In addition to that, we grouped plant-based milks according to their main ingredient (i.e.oat milks, soy milks, etc.) and calculated group specific PRAL means and medians depending on the data distribution and group size.To check for normality of the data, we used histograms, box plots and the Shapiro -Wilk test.Normally distributed variables were described with their mean and standard deviation.For nonnormally distributed variables we presented medians and their corresponding interquartile ranges.Finally, we used scatterplots and Spearman's correlations coefficients to examine potential associations between the estimated PRAL scores and the nutrient content of the examined plant-based milk alternatives.
We used STATA 14 statistical software (StataCorp.2015.Stata Statistical Software: Release 14. College Station, TX: StataCorp LP) for the entire analysis.Due to the exploratory character of this analysis and the unclear response rate by the manufacturers, we performed no power analysis.Nevertheless, we used one-way analyses of variance to determine whether there were any statistically significant differences between the PRAL means of the different milk groups.Statistical significance was determined at α = 0.05.

Results
From n = 16 contacted manufacturers, 3 provided us with the requested data.From the remaining 13 manufacturers, n = 3 did not reply after all and n = 10 either did not have the requested data available (e.g.due to a lack of measurements) or were unwilling to share the requested data with us.A total of n = 39 food items were analyzed for the present analysis.The following milk types were included: n = 12 soy milk items; n = 10 oat milk items; 8 = almond milk items; n = 3 coconut milk items; n = 1 rice milk items; n = 1 hazelnut milk items (Table 1).Moreover, we included n = 4 mixed milks, thereof n = 3 oat/almond milks and n = 1 coconut/rice milk.
The median PRAL score for all plant-based milk alternatives was −0.23 (2.14) mEq/100 ml, indicating a slightly alkaline potential.When comparing the different milk items to each other, food item #1, an almond milk, yielded the lowest PRAL score (−3.57mEq/100 ml).This was followed by items #16 (a soy milk with chocolate flavor) and #34 (a calcium enriched soy milk) with PRAL values of −3.05 and −2.75 mEq/100 ml, respectively.
The highest mean PRAL score was found in the mixed milk group (0.19 ± 0.39 mEq/100 ml) when comparing milk groups with more than 3 items.Mean PRAL scores for the remaining milk groups were as follows: −0.50 ± 1.67 mEq/100 ml in the almond milk group, −1.11 ± 1.14 mEq/100 ml in the soy milk group, and −0.79 ± 1.16 mEq/100 ml in the oat milk group.The median PRAL score in the coconut milk group was 0.57 (2.83) mEq/100 ml.Using one-way analysis of variance, no statistically significant differences were found for the PRAL values of oat, soy, almond and mixed-milks (p = .328).
In a next step, Spearman's correlations were run to assess the relationship between nutrient content and PRAL in n = 39 plant-based milk items.Negative and statistically significant correlations between PRAL and potassium content (as well as magnesium content) were observed.Figure 1 displays the obtained scatter plots, visualizing this interrelationship in the analyzed sample of plant based milks.
PRAL was inversely correlated with potassium content (Spearman's rho = −0.74,p < .001)and the magnesium content (Spearman's rho = −0.39,p = .015) of the examined plant-based milks.It is noteworthy that the examined food items showed a large variance when it comes to their potassium and magnesium content.In this context, Figure 2 shows that glancing at a specific milk group does not allow for reliable conclusion with regard to its magnesium and phosphorus content.Rather, a foodspecific approach is warranted due to the large heterogeneity.

Discussion
For the very first time, we sought to systematically analyze the PRAL value of plant-based milk alternatives using manufacturer-derived data.This is important for several reasons: PRAL tables for many emerging plant-based food categories (e.g.plant-based milks and meat analogues) are still unavailable but urgently required for those individuals who wish to critically assess the PRAL of their overall diet.This may involve health-conscious individuals but also persons suffering from chronic kidney disease and other health repercussion who benefit from an alkaline diet. [15,16]oreover, plant-based milk alternatives enjoy growing popularity among specific groups, [24,25] particularly among vegetarians and vegans who prefer to avoid dairy products for health and ethical reasons. [8]While Deriemaeker et al. reported PRAL values for some popular new food items among vegetarians and vegans, [22] their list was limited to only a handful of products. [14]Plant-based milk alternatives were not covered in detail (Table 2).
We addressed this gap in the literature with our current submission and estimated PRAL values for n = 39 plant-based milk alternatives available on the German food market.Our results demonstrated a high heterogeneity in the PRAL values of plant-based milks.Plant-based milks were, on average, slightly alkalizing.The potassium and magnesium content of the analyzed food items was inversely associated with their PRAL value.This does not come as a surprise, as both are important variables in the PRAL formula. [20]Among the plant-based milk alternatives, soymilk is a particularly good source of magnesium and its potassium content is comparable to cow's milk. [13]While the magnesium content of soy milk is twice as high as in cow's milk (21.5 mg/100 g vs 12 mg/100 g), other plantbased milk alternatives fall rather short here. [13]Almond and oatmilk have only about 6 mg/100 g of milk, whereas coconut milk has none.Unsweetened almond milk has about 31 mg of potassium per 100 g, whereas soy milk has more than five times as much (158 mg/100 ml).When compared to the PRAL value of cow's milk, which we calculated using data from the U.S. Department of Agriculture (Table 3), [26] plant-based milk alternatives were slightly more alkalizing.The overall difference, however, appears marginal.While cow's milk and plant-based milk alternatives have been frequently compared in the literature with regard to their differing nutritional profile and sensory properties, [27][28][29][30] we are the first group to add to this discussion from an acid-base perspective.While the differences appear minimal, our findings reiterate that plant-based milks are a potential alternative to cow's milk when properly considering contextual factors and the remaining diet quality.From an acid-base perspective, plant-based milk alternatives differ substantially in their PRAL value, a fact that should be taken into account depending on the daily intake quantity.
The present analysis has several strengths and limitations that warrant further discussion.We present exploratory results from an innovative study and were able to draw conclusions from a moderate sample size (n = 39).The intergroup comparison between the different plant-based milk types is of particular interest.A reservation must be made, however, that our sample may not be representative for the entire plant-based milk alternative market but is confined to the Germanspeaking countries.Data was obtained using convenience sampling strategies and the results are thus subject to a potential selection bias. [31]Several manufacturers did not respond to our written enquires or rejected our requests -thus the overall sample size is limited.Some milk types (e.g.hazelnut milk or pea/barley milk) are underrepresented or not-existent in our sample, which also limits our conclusions.Nevertheless, we believe our data to be important, particularly in light of the lack of comparable studies and the growing demand for plant-based milk alternatives.

Conclusion
The median PRAL score for plant-based milk alternatives (regardless of the milk type) was −0.23 (2.14) mEq, and indicates slightly alkalizing properties.When comparing the different milk types to each other, soy milk yielded, on average, the lowest PRAL score (−1.11 ± 1.14 mEq/100 ml).In comparison to cow's milk, plant-based milk alternatives were slightly more alkalizing.Our results allow for a better characterization of plant-based milk alternatives, and are useful for individuals and researchers that aim to estimate the potential renal acid load from an emerging group of food items.Our results might also be of great importance for the field of nutritional epidemiology, where the role of a high PRAL is often investigated with regard to chronic kidney disease and other health repercussions.The table is based on Deriemaeker et al. [22]  based on data from the U.S. Department of Agriculture: Food Data Central [26]

Figure 1 .
Figure 1.Scatterplots displaying the interrelationship between PRAL and magnesium (top) as well as potassium content (bottom) of plant-based milks.

Figure 2 .
Figure 2. Scatterplots displaying the interrelationship between PRAL, milk type and magnesium (top) as well as potassium content (bottom) of plant-based milks.

Table 1 .
Nutritional profile and potential renal acid load of plant-based milk alternatives: an overview.

Table 2 .
PRAL-values of typical foods consumed by vegetarians.