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Phosphorus-based food additives increase the total phosphorus content of processed foods. However, the extent to which these additives augment total phosphorus intake per day is unclear.
Design and Methods
To examine the contribution of phosphorus-based food additives to the total phosphorus content of processed foods, separate 4-day menus for a low-additive and additive-enhanced diet were developed using Nutrition Data System for Research (NDSR) software. The low-additive diet was designed to conform to U.S. Department of Agriculture guidelines for energy and phosphorus intake (∼2,000 kcal/day and 900 mg of phosphorus per day), and it contained minimally processed foods. The additive-enhanced diet contained the same food items as the low-additive diet except that highly processed foods were substituted for minimally processed foods. Food items from both diets were collected, blended, and sent for measurement of energy and nutrient intake.
The low-additive and additive-enhanced diet provided approximately 2,200 kcal, 700 mg of calcium, and 3,000 mg of potassium per day on average. Measured sodium and phosphorus content standardized per 100 mg of food was higher each day of the additive-enhanced diet as compared with the low-additive diet. When averaged over the 4 menu days, the measured phosphorus and sodium contents of the additive-enhanced diet were 606 ± 125 and 1,329 ± 642 mg higher than the low-additive diet, respectively, representing a 60% increase in total phosphorus and sodium content on average. When comparing the measured values of the additive-enhanced diet to NDSR-estimated values, there were no statistically significant differences in measured versus estimated phosphorus contents.
Phosphorus and sodium additives in processed foods can substantially augment phosphorus and sodium intake, even in relatively healthy diets. Current dietary software may provide reasonable estimates of the phosphorus content in processed foods.
Disturbances in phosphorus homeostasis play a central role in the pathogenesis of disordered bone and mineral metabolism in persons with chronic kidney disease (CKD).
Because dietary phosphorus intake makes up most of the obligate phosphorus load that the kidneys must eliminate on a daily basis to maintain phosphorus balance, diet intake has been the primary target of interventions aimed at mitigating the development of phosphorus overload in CKD patients.
Phosphorus consumption in developed countries far exceeds recommendations for daily intake.
In Europe and North America, an important reason for this is the nearly ubiquitous distribution of phosphorus-based food additives in commercially processed foods. Phosphorus additives are heavily used by the food manufacturing industry to enhance the appearance, taste, and shelf-life of processed foods such as baked goods, cheese products, and meats.
Despite the increasing recognition that phosphorus-based food additives are a significant source of phosphorus intake in Westernized diets, relatively little is known about the extent to which additives augment daily phosphorus intake in the present era. This is because most of the available estimates in the literature come from studies conducted over 20 years ago,
As a result, the actual contribution of phosphorus-based food additives to total daily phosphorus intake in contemporary Western diets remains poorly understood. To address this issue, we compared the measured phosphorus content of 2 different diets: a low-additive diet made up of fresh and minimally processed foods and an additive-enhanced diet consisting of the same food items and meant to provide identical caloric and nutrient contents but with the major difference being that the food items were known to be enhanced with phosphorus-based food additives.
Separate menus for a low-additive diet and an additive-enhanced diet were developed by the Bionutrition Core of the Clinical Research Unit at the University of Alabama at Birmingham using the Nutrition Data System for Research (NDSR) software version 2011, developed by the Nutrition Coordinating Center at the University of Minnesota.
Each menu consisted of 4 separate days of food (breakfast, lunch, and dinner, Supplemental Table 1). The low-additive menu was designed to provide 1,800 to 2,000 kcal (15% from protein, 55-60% from carbohydrates, and 25-30% from fat) and 900 mg of phosphorus per day using fresh, nonprocessed or minimally processed foods. These targets were meant to conform to current U.S. Department of Agriculture (USDA) recommendations for daily intake.
The additive-enhanced menu was designed to provide identical energy and nutrient content per day to the low-additive menu, but substituting highly processed for minimally processed foods (e.g., canned pineapple vs. fresh pineapple, deli chicken vs. fresh chicken breast, etc.). All study foods were purchased from local supermarkets in the greater Birmingham area. Low-additive foods were chosen by a certified research nutritionist (S.S.C.) on the basis of food label notices such as “no preservatives added,” “organic,” “baked fresh,” or “no processing.” Additive-enhanced foods were chosen if they were identified as “enhanced” or if phosphorus-based additives were listed on the packaging (e.g., dicalcium phosphate, pyrophosphate, sodium acid pyrophosphate, sodium aluminum phosphate, sodium phosphate, sodium tripolyphosphate, etc.).
To measure the energy and nutrient contents of the 2 menus, all foods from each individual day of the 2 menus were prepared in the metabolic kitchen of the Clinical Research Unit according to standard local practices and then weighed. The foods for each day were then placed into a Waring Commercial CB15 1-gal blender and weighed again for a preblend weight. Foods were thoroughly blended for 20 to 30 minutes, after which a postblend weight was taken. The 3 weights—total of the individual food items, preblend weight, and postblend weight—were checked to ensure that no loss of food occurred during the blending process. Aliquot samples for each day (half a cup) were taken from the food mixture at 3 different heights in the blender: the top, middle, and bottom portions. These 3 aliquots were frozen and sent to Covance Laboratories Inc. (Madison, WI), along with the total food mixture volume and weight, to determine the energy and nutrient contents for each day of the 2 menus. The fat content was determined as follows: Samples were hydrolyzed with hydrochloric acid, after which the fat was extracted using ether and hexane, and the extract was filtered through a sodium sulfate column.
Nutrient elements (calcium, phosphorus, potassium, and sodium) were determined as follows: Food samples were ashed at high temperature, digested in acid, and then nutrient elements were measured by inductively coupled plasma emission spectrometry.
Total energy and nutrient contents of the foods for each menu day were based on the mean value of the 3 aliquot measurements expressed as kilocalories per 100 g or milligrams per 100 g food. Total energy and nutrient intake per day were then derived by multiplying these values by the total grams of food prepared for each day. Total energy intake from protein, carbohydrates, and fat was estimated using established conversion factors: protein, 4 kcal/g; carbohydrates, 4 kcal/g; and fat, 9 kcal/g. The percentage of total kilocalories provided from protein, carbohydrates, and fat was calculated as follows: (kcal from nutrient/ total kcal) × 100. This study was reviewed and approved by the Institutional Review Board at the University of Alabama at Birmingham.
Differences in the nutrient contents of each diet were compared using Student's t test. Generalized linear models were used to compare the energy-adjusted mean nutrient contents of the 2 diets. A 2-tailed P value less than .05 was considered statistically significant. All analyses were done using SAS version 9.2 (SAS Institute, Cary, NC).
Table 1 depicts food items for each menu from a representative day (a full listing of the menus for each day is provided in Supplemental Table 1). As per study design, the food items of the 2 menus were virtually identical, with the only major differences being the use of highly processed foods in the additive-enhanced diet (e.g., canned, frozen foods) as compared with fresh or minimally processed foods in the low-additive diet.
Table 1Representative Example of Food Item Selections From 1 Menu Day of the Low-Additive Diet As Compared With the Additive-Enhanced Diet
Corn flakes (Kellogg's brand), 1.75 cups
Special K Low-Carb Lifestyle (Kellogg's Brand), 1.5 cup
Low-fat soy milk (Horizon Organic), 8 fl oz
Skim milk (Dairy Fresh), 8 fl oz
Fresh pineapple, 1.5 cups
Canned pineapple (Dole), 12 oz
Pita bread (Toufayan Bakeries), 1 medium
Pita Bread, wheat (Kangaroo), 1 medium
Chicken breast, fresh (Publix Greenwise), 45 g
Chicken, deli (Oscar Meyer), 45 g
Mayonnaise, regular (Kraft), 12.4 g
Mayonnaise, regular (Kraft), 12.4 g
Nectarine, fresh, 1 medium
Canned peaches (Del Monte), 4 oz
Buttered, light popcorn (Orville Redenbacher), 1.6 oz
Natural, light popcorn (Orville Redenbacher), 1.6 oz
Jolly Rancher candy (Hershey), 5 pieces
Jolly Rancher candy (Hershey), 5 pieces
Tilapia (Publix brand, frozen), 1 filet (4 oz)
Canned salmon (Chicken of the Sea), 1 can (2.8 oz)
Table 2 compares the measured energy and nutrient contents of the low-additive diet and the additive-enhanced diet, standardized per 100 g of food, for each of the 4 menu days. Although individual days displayed some variability in total energy, protein, carbohydrate, and fat content per 100 g of food, overall these values were similar for each day. More consistent differences were noted in the measured calcium, phosphorus, sodium, and potassium contents between the 2 diets. In general, the additive-enhanced menus had lower amounts of calcium and potassium per 100 g of food than the low-additive menus. In contrast, the phosphorus and sodium contents of additive-enhanced foods were substantially higher than low-additive foods, with the differences ranging from 30% to 40% on average.
Table 2Energy and Nutrient Contents (Standardized Per 100 g of Food) of the Low-Additive Vs. Additive-Enhanced Menu Per Day
The total sodium, phosphorus, calcium, and potassium contents of the 2 diets per day are depicted in Figure 1. Calcium and potassium contents varied in some, but not all, of the menu days (Fig. 1, A and B), with the direction of the variation being inconsistent. In contrast, total sodium content was consistently higher each day of the additive-enhanced menu as compared with the low-additive menu (Fig. 1C), with the magnitude of the difference ranging from as low as 703 mg to as high as 1,750 mg of sodium. Likewise, total phosphorus content was significantly higher each day of the additive-enhanced menu as compared with the low-additive menu (Fig. 1D), with the magnitude of the difference ranging from 483 to 790 mg of phosphorus. When the phosphorus and sodium contents of the 2 diets were averaged over the 4 menu days, the additive-enhanced diet contained on average 606 ± 125 mg more phosphorus (P < .001 for difference) and 1,329 ± 642 mg more sodium (P = .02 for difference) than the low-additive diet (Table 3). No statistically significant differences were observed in the 4-day averaged total caloric, calcium, or potassium contents of the 2 diets.
Table 3Estimated and Measured Energy and Nutrient Content of the Low-Additive and Additive-Enhanced Diets Averaged Over the 4 Menu Days
Table 3 also depicts the contents of the diets (averaged over the 4 menu days) estimated by the NDSR software as compared to the actual measured values. When comparing the estimated content of the low-additive diet to the measured content of the low-additive diet, there were no significant differences in any of the components except for the phosphorus content, which was slightly higher in the measured as compared with the estimated value (1,070 ± 58 vs. 924 ± 82 mg, P = .03). This latter difference was attenuated after accounting for differences in the measured versus estimated calories of the low-additive diet (energy-adjusted phosphorus content of additive-enhanced menu 1,041 ± 78 mg; energy-adjusted phosphorus content of low-additive menu 954 ± 80 mg, P = .12). Likewise, when comparing the estimated content of the additive-enhanced diet to the measured content, there were no statistically significant differences in total energy, calcium, phosphorus, sodium, or potassium content, even after accounting for differences in the total caloric intake.
Dietary restriction of sodium and phosphorus are key components of nutritional counseling in individuals with CKD. However, as underscored by the results of the current study, achieving these goals is immensely complicated by the presence of sodium- and phosphorus-based additives in processed foods. Even in the relatively healthy diet created for this study, additives in processed foods augmented total phosphorus and sodium content by approximately 60% per day. These findings highlight the magnitude of the barrier that food additives present in lowering sodium and/or phosphorus intake in CKD patients.
Several studies have previously documented that phosphorus-based food additives can substantially augment the total phosphorus content of staple items of Westernized diets, particularly processed meats.
However, these prior studies almost exclusively focused on the phosphorus content of individual food items and not full-day menus, making it difficult to determine the extent to which additive-enhanced foods contribute to total phosphorus intake per day. This is important in that studies from our group have shown that additive-rich foods such as processed meats and dark colas are consumed relatively infrequently in comparison to other more energy-dense items in the general population.
Therefore, it is possible that, although rich in additives, these foods may not be consumed in high enough quantities to substantially augment total phosphorus intake per day. The results of the current study argue against this possibility by demonstrating that phosphorus additives in commonly consumed foods can substantially increase daily phosphorus intake—even in a balanced 2,200-kcal/day diet such as that in this study—with important implications for the management of bone and mineral metabolism in CKD patients. For example, similar to what has been shown by other investigators,
a hypothetical patient consuming the additive-enhanced diet in this study would need to take 18 extra tablets of calcium acetate, 23 extra tablets of sevelamer hydrochloride, or 5 extra tablets of lanthanum carbonate over the course of each day to bind just the excess phosphorus found in the additive-rich foods. Likewise, using published phosphorus clearance data for a standard hemodialysis session lasting 4 hours using a high-flux, high-efficiency dialyzer, a blood flow of ∼400 mL/minute, and a dialysate flow of approximately 500 mL/minute,
and assuming 100% intestinal absorption of inorganic phosphorus from additives, 12 extra hours of hemodialysis a week would be required to remove just the extra phosphorus found in the additive-enhanced diet prepared for this study.
One other recent study examined differences in the total phosphorus content of an additive-free versus an additive-containing diet.
Using a commercially available data set of grocery sales in northeast Ohio from 2009 to 2010, León et al. selected the top 5 best-selling food products containing phosphorus additives within 15 general food categories (prepared frozen foods, dry food mixes, packaged meats, etc.) and matched them 1-to-1 to similar products without phosphorus additives. Additive-containing and nonadditive-containing products were then purchased from local food stores and sent for measurement of phosphorus content after preparation of the food items according to packaging labeling. Sample meals were then developed using analyzed matched foods to approximate the mean calorie, protein, carbohydrate, and total fat intake of U.S. adults as estimated by the 2007 to 2008 National Health and Nutrition Survey What We Eat in America study. The main finding of their study was that, as compared with additive-free foods, additive-containing foods were estimated to contribute 736 ± 91 mg (or 70%) more phosphorus per day. This is in line with our results, suggesting that our findings do not merely reflect the phosphorus additive content of foods local to our region, but instead are likely reflective of the upper extreme of phosphorus additive content in the broader U.S. food supply.
Given that sodium is commonly complexed with phosphorus in food additives, it is not surprising that along with total phosphorus intake, additives in the processed foods chosen for this study also substantially increased total sodium content. Sodium additives almost completely account for the excess intake of sodium in many populations, particularly in Western countries where 75% of sodium intake is estimated to come from sodium added to processed foods.
it is important to note that the diet created for the purposes of this study was relatively healthy, meeting USDA guidelines for total energy, fat, and carbohydrate intake per day. Nonetheless, with just the substitution of highly for minimally processed foods, total daily sodium content averaged nearly 3.5 g, far above the current USDA tolerable upper limit of 2.3 g per day. This highlights the reality that, in addition to phosphorus, CKD patients may be consuming substantially higher quantities of sodium than appreciated, even in “healthy” appearing diets, and that any serious effort to reduce sodium intake in these patients must include a discussion of where and what kinds of foods are being purchased for consumption at home.
An unexpected finding of this study was that the NDSR-estimated phosphorus content was nearly identical to the measured phosphorus content of the foods in the low-additive diet and the additive-enhanced diet. Prior investigators have reported that current dietary software programs such as NDSR—which utilizes the USDA Nutrient Data Laboratory as its primary source of nutrient values and nutrient composition—do not adequately account for phosphorus additives in processed and fast foods; thus, they may underestimate the true phosphorus content of these foods.
In contrast, we found that the estimated phosphorus contents of the processed foods chosen for this study were nearly identical to the actual measured values, suggesting that NDSR-derived estimates of phosphorus content may provide reasonably accurate assessments of phosphorus intake. Because most food frequency questionnaires and/or 24-hour dietary recalls used in population-based research rely on USDA Nutrient Data Laboratory estimates, further studies will need to determine whether phosphorus intake estimates derived from these instruments demonstrate similar degrees of accuracy in capturing the phosphorus content of processed foods.
It should be noted that the diets were specifically designed to accentuate the differences between a low-additive and an additive-rich diet by preferentially choosing minimally processed foods in the former and highly processed foods in the latter. Because of this, the results may not be representative of more real-world diets in which individuals are consuming a mixture of fresh and processed foods, potentially overestimating the contribution of processed foods to phosphorus intake in typical Western diets. At the same time, given that we designed the diet to conform to USDA recommended guidelines for fat, protein, carbohydrate, and phosphorus intake, it is also possible that we underestimated the true effect of processed foods on phosphorus content in more typical “convenience” diets consumed by increasing numbers of Americans, which may not meet the USDA guidelines. Other limitations of our study include the fact that we sampled a relatively limited number of unique food groups between the 2 diets, and we measured nutrient contents during only 1 time of year (given the potential for changing phosphorus content of food throughout the year).
In conclusion, food additives substantially augment sodium and phosphorus contents of modern American diets. Future studies will need to examine the physiological effect of this excess additive intake on individuals with normal and decreased kidney function to understand the public health implications of these findings.
Sodium- and phosphorus-based food additives in processed foods represent large barriers in restricting dietary intake of sodium and phosphorus, even in relatively healthy diets. Any serious attempt to restrict sodium and/or phosphorus intake in patients with CKD will require a detailed inventory of where and what kind of foods are purchased by patients.
Financial Disclosure: O.M.G. has served as a consultant to Vifor Pharma. The other authors declare that they have no relevant financial interests.
Support: This work was supported by grant UL1TR000165 (University of Alabama at Birmingham Center for Clinical and Translational Science) from the National Institutes of Health . O.M.G. was supported by grants K23DK081673 and R03DK095005 from the National Institutes of Health .