The Production of p-Cresol Sulfate and Indoxyl Sulfate in Vegetarians Versus Omnivores

Discussion

Treatment of ESRD is now focused on removing uremic solutes by dialysis. In theory, treatments that reduce solute production could also reduce solute levels and ameliorate uremic illness. One barrier to the development of such treatments has been limited knowledge of the factors that influence solute production.

This study focused on the production of two uremic solutes, PCS and IS, which have recently been the subject of intense study (4,5). There is substantial, albeit inconclusive evidence, that both PCS and IS contribute to illness in dialysis patients (4,6–10). Their dialytic clearance is greatly restricted by protein binding, which could make maneuvers that lower their production particularly valuable. PCS and IS are also the best known members of the class of uremic solutes derived from the action of colonic microbes (2,3,14). Because such solutes are made in an isolated compartment by nonmammalian metabolism, their production could prove simpler to suppress than the production of other uremic solutes.

One obvious means to influence the production of colon-derived solutes is manipulation of the diet. This study showed that average excretion of PCS and IS was approximately 60% lower in vegetarians than in individuals eating an unrestricted diet. Urinary excretion is considered to reflect solute production in normal participants, there being no other known route of removal from the plasma except dialysis (15,16).

Previous studies, however, identified mechanisms that could account for reduced production of these solutes in vegetarians (2). One potential mechanism by which a vegetarian diet could reduce PCS and IS production is by increasing the dietary intake of fiber. Reported average fiber intake in our study was approximately 70% higher in vegetarians than in participants consuming an unrestricted diet. This difference is similar to that observed in a large study of US vegetarians (17). The term fiber includes various forms of carbohydrate that escape digestion in the small intestine. Fermentation of some of this carbohydrate by colon microbes provides energy to the host in the form of short-chain fatty acids. Colon microbes are also supplied with amino acids in the form of incompletely digested proteins, sloughed intestinal cells, and secretions (18,19). PCS is the sulfate conjugate of p-cresol, which is formed by microbes from phenylalanine and tyrosine; IS is the sulfate conjugate of indoxyl, which is formed by microbes from tryptophan. The portion of these amino acids converted to PCS and IS may be reduced by increasing fiber intake (2). Increasing fiber intake provides increased substrate for microbial fermentation so that amino acids are consumed in microbial growth rather than broken down to waste solutes. In addition, a reduced colon transit time may limit conversion of amino acids to waste solutes when fiber intake is high (20).

A second mechanism by which a vegetarian diet could reduce PCS and IS production is by reducing protein intake. In this study, average reported protein intake was approximately 25% lower in vegetarians than in participants consuming an unrestricted diet. This difference, which was confirmed by measurement of urine urea nitrogen excretion, is again similar to that observed in a large study of US vegetarians (17). Lowering dietary protein can reduce delivery of amino acids to the colon and thereby limit their availability for conversion to PCS and IS (18).

The extent to which a higher fiber intake and a lower protein intake accounted for lower PCS and IS production in vegetarians cannot be determined from our results. The differences we observed, however, are larger than might be expected based on short-term studies in which fiber and protein intake have been manipulated. Ingestion of 20 g/d of the indigestible carbohydrate oligo-fructose inulin for 4 weeks was found to reduce PCS production by 20%–30% in normal volunteers (21). The same treatment was subsequently shown to decrease plasma PCS levels by about 20% without altering IS levels in hemodialysis patients (7). Available studies of the effect of protein intake are of shorter duration and have documented changes in PCS production, assessed as urinary p-cresol excretion, only with a reduction of protein intake to 20 g/d or an increase of protein intake to approximately 125 g/d (22,23).

Several factors may have contributed to our vegetarian participants having lower PCS and IS production rates than would be expected based on available studies in which fiber and protein intake were manipulated. Higher fiber and lower protein intakes were combined in vegetarian participants. This could cause larger changes in microbial metabolism than would be achieved by separate manipulation of either fiber or protein alone. The types of fiber consumed by the vegetarian participants may have been particularly effective in promoting microbial growth and PCS and IS production may also have been influenced by other differences in vegetarian and unrestricted diets that we did not explore, including differences in the type of protein ingested (17,24). Finally, our vegetarian participants had adhered to meat- and fish-free diets for many years, whereas experimental studies of dietary manipulation have necessarily been of limited duration.

Over the long term, vegetarian and unrestricted diets may have influenced PCS and IS production by altering the colon microbial flora, or microbiome, as well as by providing different nutrients to colon microbes on a day-to-day basis. DNA sequencing technology has greatly increased our knowledge of the microbiome, which dwarfs its human host in cell number and even more in gene number (25,26). Recent studies have documented major differences in the composition of the colon microbiome in individuals consuming different diets (27). Other studies have shown that there is enormous dissimilarity in the microbiome between individuals but that the composition of the microbiome in an individual tends to remain stable over long periods. These findings, coupled with knowledge that the enzymes that produce p-cresol and indole are irregularly present among the myriad types of colon microbes, could account for many features of PCS and IS production (28,29). As observed in prior studies, we found that PCS and IS production rates were poorly correlated in individual participants, which could be explained by differences in the microbial flora of different individuals (30). In addition, the variation in PCS and IS production among individuals was much larger than the variation in urea production, and the correlations between PCS and IS production and protein and fiber intake were weak. Although there were wide individual variations in PCS and IS production among individual participants, the rates of PCS and IS production tended to remain stable in each individual.

Some other differences were noted in the two groups. Vegetarian participants had lower urinary sulfate excretion, which can be accounted for by lesser intake of sulfur-containing amino acids associated with lesser protein intake. Vegetarian participants also had a lower urinary creatinine excretion, which can be accounted for by lesser intake of creatinine in the form of cooked meat and also by a slightly higher percentage of women among the vegetarian participants (31). A particularly interesting finding was that average phosphate excretion was lower in vegetarian participants. Moe et al. (32) recently found that plasma phosphate was lower and urinary phosphate tended to be lower in patients with chronic renal insufficiency on a vegetarian diet than on a meat diet with the same phosphorous content. This may reflect lesser absorption of phosphate from plant foods, in which a large portion of phosphate is in the form of phytate, than from meat to which phosphate salts are now often added as preservatives (33). Dietary recommendations for patients with renal insufficiency may require modification in light of these findings (32,34).

Our study has notable limitations. PCS and IS are only two of what is likely a very large number of uremic solutes made by colon microbes, and we do not know the extent to which these solutes are toxic (14). Our study was performed in participants with normal renal function, and we cannot be sure that a vegetarian diet has a similar effect on PCS and IS production in patients with renal failure. We did not assess fecal solute excretion, and thus could have failed to detect differences in colonic absorption of unconjugated PCS or IS precursors or intestinal secretion of the conjugates. Most importantly, our study does not reveal what feature(s) of the vegetarian participants' diets contributed most to their lower levels of PCS and IS production.

In conclusion, we found that in individuals with normal renal function production of the uremic solutes PCS and IS averaged approximately 60% lower in long-time vegetarians than in participants eating an unrestricted diet. This finding supports hope that production of these solutes could be reduced in dialysis patients. Dialysis patients are already subject to burdensome dietary restrictions, and few of them would likely chose to become vegetarians. Further study could, however, allow the effects of the vegetarian diet to be replicated by selective prebiotic, probiotic, or pharmacologic therapies.