The production of p-cresol sulfate and indoxyl sulfate in vegetarians versus omnivores

Abstract

Background and objectives: The uremic solutes p-cresol sulfate (PCS) and indoxyl sulfate (IS) are generated by colon bacteria acting on food components that escape absorption in the small bowel. The production of these potentially toxic compounds may thus be influenced by diet. This study examined whether production of PCS and IS is different in vegetarians and omnivores.

Design, setting, participants, & measurements: The production of PCS and IS was assessed by measuring their urinary excretion rates in participants with normal kidney function. Studies were carried out in 15 vegetarians and 11 individuals consuming an unrestricted diet. Participants recorded food intake over 4 days and collected urine over the final 2 days of each of two study periods, which were 1 month apart.

Results: Average PCS excretion was 62% lower (95% confidence interval [95% CI], 15-83) and average IS excretion was 58% lower (95% CI, 39-71) in vegetarians than in participants consuming an unrestricted diet. Food records revealed that lower excretion of PCS and IS in vegetarians was associated with a 69% higher (95% CI, 20-139) fiber intake and a 25% lower (95% CI, 3-42) protein intake. PCS and IS excretion rates varied widely among individual participants and were not closely correlated with each other but tended to remain stable in individual participants over 1 month.

Conclusions: PCS and IS production rates are markedly lower in vegetarians than in individuals consuming an unrestricted diet.

Figure 1.

Urinary PCS and IS excretion. Average daily excretion rates for PCS (left panel) and IS (right panel). Values obtained for individual vegetarian participants in period 1 and period 2 are represented by triangles connected by broken lines. Values obtained for unrestricted participants are represented by circles connected by solid lines. PCS, p-cresol sulfate; IS, indoxyl sulfate.

Figure 2.

The relation of PCS and IS excretion in individual participants. PCS and IS excretion rates were not closely correlated. Values are for period 1 in vegetarian (triangles) and unrestricted (circles) participants (r²=0.06; P=0.21). Results for period 2 were similar (not shown). PCS, p-cresol sulfate; IS, indoxyl sulfate.

Figure 3.

The relation of PCS and IS excretion to urea nitrogen excretion and to fiber intake. PCS and IS excretion rates were only weakly correlated with protein intake as reflected by urine urea nitrogen excretion (upper panels) and with fiber intake as obtained by dietary history (lower panels). Values are for period 1 for vegetarian (triangles) and unrestricted (circles) participants with the following correlations: PCS versus urine urea nitrogen (r²=0.14; P=0.06); IS versus urine urea nitrogen (r²=0.16; P=0.04); PCS versus dietary fiber, (r²=0.10; P=0.12); and IS versus dietary fiber (r²=0.11; P=0.10). PCS, p-cresol sulfate; IS, indoxyl sulfate; UreaN, urea nitrogen.