Lowering intake of both nutrients is complicated; foods low in one may be high in the other.
Chronic kidney disease (CKD) patients often are advised to restrict their dietary intake of some nutrients to slow disease progression and prevent complications. Sodium intake should be limited to 2-2.4 g/day to reduce fluid accumulation and assist in blood pressure control, while phosphorus should be restricted to 800-1,000 mg/day to reduce renal osteodystrophy and calcification.
Jonathan B. Jaffery, MD, and Virginal L. Hood, MBBS, MPH, of the University of Wisconsin Medical School in Madison, surveyed the sodium and phosphorus content of foods listed on the National Kidney Foundation Web site for patients (J Ren Nutr. 2006;16:332-336). These lists give lower-sodium and lower-phosphorus alternatives for foods with high contents of these nutrients. The sodium and phosphorus content of these foods was quantified using the U.S. Department of Agriculture Nutrient Database.
The authors found that a few lower-sodium food alternatives were a little higher in phosphorus content than their regular-salt food. Three of the 47 foods had a phosphorus content higher by more than 10 mg per serving; these were chicken noodle soup (12 mg), canned snap peas (13 mg), and canned green peas (57 mg) respectively.
A similar comparison of sodium content of nine pairs of higher- and lower-phosphorus alternative foods found that in four food pairs, sodium was higher in the lower-phosphorus choice by 11, 22, 49, and 410 mg per serving. These pairs were cola versus ginger ale (11 mg), lima beans versus mixed vegetables (22 mg), unsalted peanuts versus light salt microwave popcorn (49 mg), and grits versus oatmeal (410 mg).
“The renal diet,” Drs. Jaffery and Hood wrote, “is extraordinarily complex and is prescribed to a population that almost universally experiences multiple medical problems. Identifying foods that fulfill all the desired criteria remains a huge challenge, and following a low-phosphorus diet may be particularly challenging, given that phosphorus content is not typically readily available on nutrition labeling.”
However, overall there were relatively few low-sodium foods that contained higher phosphorus levels and vice versa. Only seven food alternatives of the 56 pairs studied posed this problem. Within the low-sodium choices, the differences in phosphorus were 12, 13, and 57 mg, only a small part of the 800-1,000 mg daily goal. Within the lower-phosphorus choice pairs, sodium was greater in only four pairs at 11, 22, 49, and 410 mg per serving; the first three also were only a small part of a 2,000 mg per day goal. Only the 410 mg difference between grits and oatmeal was highly significant at 40%-50% of recommended intake.
A dietary sodium guideline of 2,000-2,400 mg per day is likely to be only one third to one half
of patients’ free-choice intakes. Fortunately, the nutrient label requires listing the sodium content per serving and the percent of recommended daily intake. This way, people can compare labels of several brands to determine the lower-sodium alternative. Unprocessed foods should be emphasized because they contain little sodium. Phosphorus intake, however, is not listed on the label, so additional information must be used to make dietary choices that are lower in this nutrient.
The foods naturally highest in phosphorus are the high-protein foods such as meat, dairy products, and legumes. The phosphorus in meat and dairy food is predominantly in the form of phosphate, and up to 70% may be absorbed by the body. Beef and chicken contain about 8 mg phosphorus per gram of protein, whereas low-fat milk has 28 mg phosphorus per gram of protein. Balancing intake of high-quality protein while adhering to phosphorus limitations is difficult. About 10% of the phosphorus in legumes is in the form of phytate and only about 1-2% of dietary phytate is absorbed by human beings.
Processed foods often contain phosphate additives, which must be listed on the label in the ingredient list. In particular, meat and cheese, which are already high in phosphorus, may be processed with additional phosphate salts. Although the phosphorus amounts are not listed, patients can quickly learn to pick between similar foods with no phosphate additives. Patients will find that not just colas contain phosphates, but other soft drinks as well, and this can be ascertained from the label.
In summary, patients should first look for lower-sodium foods and those without phosphorus additives, but must also be cautious about amounts of meat and dairy because of their relatively high phosphorus content.