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The dietary sodium prescription has been influenced by many factors in the past 50 years, but it continues to depend on the presence or absence of edema and hypertension and the kidney's ability to conserve and excrete sodium. In the 1950s and 1960s, medications to control hypertension were limited, and 250- to 1000-mg sodium diets were used to control severe edema and elevated blood pressure (28). Distilled water was specified in these diets, as many sources of well water and municipal water contained excessive amounts of sodium. Vegetables like celery, beets and carrots were limited because of their natural sodium content. Aggressive diuretic therapy and frequent episodes of vomiting and diarrhea because of uremia caused the amount of dietary sodium prescribed to be adjusted frequently. Because newer antihypertensive agents have become available and biomedical technology has improved dialysis techniques, dietary sodium prescriptions have increased to 2000-3000 mg/day. In addition, the changing American food supply has affected dietary sodium intake significantly; this will be discussed later.

As the level of protein in the renal diet became more liberalized, potassium control also became an issue for patients receiving maintenance hemodialysis. When routine dialysis increased to two to three times per week, patients began to feel better and their appetite increased. A dietary potassium intake of 2-3 g/day was the maximum amount recommended in all meal plans for patients receiving maintenance hemodialysis (29). However, dietitians realized that many patients who were receiving maintenance peritoneal dialysis did not require a potassium restriction and intake could be liberalized based on the patient's serum potassium levels.

Food lists with low and medium potassium levels were developed using standard reference materials. High potassium food lists were also developed and patients were advised not to consume these foods. Tsaltas reported that leaching or dialyzing vegetables reduced the potassium and sodium content. Patients were taught to peel, slice, and soak vegetables in water and then boil in large amounts of water to reduce potassium by approximately 50% (30). Since this initial work, Burrowes and Ramer (31) found that the most effective method for removing potassium from tuberous root vegetables was the double cook method. This method involves placing the vegetable in a 2:1 water to vegetable ratio, bring the water to a boil, drain the water, replace with fresh water, and then bring to a boil again until the vegetable is cooked.

Fluid and mineral recommendations were reviewed in the early literature. In 1927, Norman and coworkers reviewed 165 cases in which a diet restricted in salt and water proved effective for cases of obstinate edema due to nephritis (32). Yet in another article, the same authors provide contradictory statements. They recommended a 40-g protein diet for nephritis if there was retention of protein derivatives such as urea, and large amounts of fluid to wash out the poisonous substances (33). A few years later in 1931, Lashmet and Enke discussed the Neutral Diet in the treatment of nephritic edema. They theorized that edema was not due to the failure of the kidneys to excrete water or chloride, but due to the alkaline ash content of the diet. It was felt that an acid ash diet would decrease edema. Lashmet and Enke recommended 45-50g of protein, 2000 calories, a slight excess of acid ash, 10-15 g of ammonium chloride in 0.5 capsules with meals and 5000 mL of fluid daily (34,35).

The machines used initially for hemodialysis did not have volumetric controls, and fluid removal was not consistent. Fluid limits were very strict in the early days of dialysis, especially for patients who received dialysis infrequently or who had limited residual kidney function. Patients were instructed often to drink only what was necessary to take their medications and/or to satisfy their thirst with the fluid or moisture in food. These early diets specified fluid limits of 500-800 mL/day plus urine output (28). This amount was increased in 1998 to 1000 mL/day plus output (36).

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