Low protein diets are commonly prescribed for patients with idiopathic calcium nephrolithiasis,
who account for > 80% of new diagnoses of kidney stones. This dietary advice is supported
by metabolic studies and epidemiologic observational studies but has not been evaluated
in a controlled trial. Using 1983-1985 data from three Northern California Kaiser
Permanente Medical Centers, the authors randomly assigned 99 persons who had calcium
oxalate stones for the first time to a low animal protein, high fiber diet that contained
approximately 56-64 g daily of protein, 75 mg daily of purine (primarily from animal
protein and legumes), one-fourth cup of wheat bran supplement, and fruits and vegetables.
Intervention subjects were also instructed to drink six to eight glasses of liquid
daily and to maintain adequate calcium intake from dairy products or calcium supplements.
Control subjects were instructed only on fluid intake and adequate calcium intake.
Both groups were followed regularly for up to 4.5 years with food frequency questionnaires,
serum and urine chemistry analysis, and abdominal radiography; and they were urged
to comply with dietary instructions. In the intervention group of 50 subjects, stones
recurred in 12 (7.1 per 100 person-years) compared with two (1.2 per 100 person-years)
in the control group; both groups received a mean of 3.4 person-years of follow-up
(p = 0.006). After adjustment for possible confounding effects of age, sex, education,
and baseline protein and fluid intake, the relative risk of a recurrent stone in the
intervention group was 5.6 (95% confidence interval 1.2-26.1) compared with the control
group. The authors conclude that advice to follow a low animal protein, high fiber,
high fluid diet has no advantage over advice to increase fluid intake alone.