The effect of saccharin on the microbial metabolism of tryptophan in man

Assessing the in vivo data on low/no-calorie sweeteners and the gut microbiota

Low/no-calorie sweeteners (LNCS) are continually under the spotlight in terms of their safety and benefits; in 2014 a study was published linking LNCS to an enhanced risk of glucose intolerance through modulation of the gut microbiota. In response, an in-depth review of the literature was undertaken to evaluate the major contributors to potential changes in the gut microbiota and their corresponding sequelae, and to determine if consuming LNCS (e.g., acesulfame K, aspartame, cyclamate, neotame, saccharin, sucralose, steviol glycosides) contributes to changes in the microbiome based on the data reported in human and animal studies. A few rodent studies with saccharin have reported changes in the gut microbiome, but primarily at high doses that bear no relevance to human consumption. This and other studies suggesting an effect of LNCS on the gut microbiota were found to show no evidence of an actual adverse effect on human health. The sum of the data provides clear evidence that changes in the diet unrelated to LNCS consumption are likely the major determinants of change in gut microbiota numbers and phyla, confirming the viewpoint supported by all the major international food safety and health regulatory authorities that LNCS are safe at currently approved levels.

Saccharin mechanistic data and risk assessment: urine composition, enhanced cell proliferation, and tumor promotion

Sodium saccharin (NaSac) produces bladder tumors consistently in male rats only after lifetime exposure that begins at birth. NaSac is not metabolized and is negative in most genotoxicity tests. NaSac-induced cell damage and proliferation have been proposed as important factors in tumor promotion, and dose-response information demonstrating a threshold for these parameters is available. One theory proposes that high levels of NaSac, combined with protein in a high Na+, high pH environment found only in the male rat, form toxic microscopic crystals; therefore, NaSac-induced tumors may not be relevant to human carcinogenesis.

A data-derived safety (uncertainty) factor for the intense sweetener, saccharin

An increased incidence of bladder cancer is found when male rats are fed high dietary concentrations of sodium saccharin (3% or more) from birth. This toxicity has been used as the basis for the development of a data-derived safety factor. Such an effect would attract an extra factor (10-fold) for nature of toxicity and in the absence of other data would result in a high overall safety factor. However the extensive mechanistic database on sodium saccharin allows an assessment of the potential relevance of the effect for humans. In addition the effect is only seen under specific conditions in rats, i.e. largely with the sodium salt and with a commercial rat diet. The effect is not related to the concentration of saccharin in the rat urine or bladder so that toxicokinetic considerations are simplified. The extensive animal database allows the determination of data-derived factors for inter-species differences in both toxicokinetics and toxicodynamics. Based on this analysis an overall safety factor of 50 (which includes the factor of 10 for severity of effect) would appear appropriate at the present time. This factor, and the ADI which would result from its application, are consistent with the absence of an association between the consumption of artificial sweeteners and bladder cancer in humans.

A review and biological risk assessment of sodium saccharin

Dietary sodium saccharin is associated with bladder tumors when fed at high levels to the male rat. Under these conditions urinary pH, sodium concentration, and volume are elevated and proliferative changes are present in the urothelium. Extensive epidemiological studies have shown that saccharin does not increase the risk of bladder cancer in humans and laboratory investigations have shown that sodium saccharin is not mutagenic and does not bind to DNA. Recent research indicates that the urothelium in male rats is damaged under conditions of high urinary pH and sodium levels by a mechanism that involves alpha 2u-globulin and possibly silicate crystalluria. These studies and their implications for human health risk are reviewed.

Metabolism of drugs and other xenobiotics in the gut lumen and wall

Metabolism in the gut lumen and wall can decrease the bioavailability and the pharmacological effects of a wide variety of drugs. Bacterial flora in the gut, the environmental pH and oxidative or conjugative enzymes present in the intestinal epithelial cells can all contribute to the process. Bacterial biotransformation is greatest in the colon, while gut wall metabolism is generally highest in the jejunum and decreases distally. Gut wall metabolism may be induced or inhibited by dietary or environmental xenobiotics or by co-administered drugs. Recent evidence suggests that some drugs, food-derived mutagens and other xenobiotics can be metabolized by gut flora and/or gut wall enzymes to reactive species which may cause tumors.

The health risks of saccharin revisited

Almost from its discovery in 1879, the use of saccharin as an artificial, non-nutritive sweetener has been the center of several controversies regarding potential toxic effects, most recently focusing on the urinary bladder carcinogenicity of sodium saccharin in rats when fed at high doses in two-generation studies. No carcinogenic effect has been observed in mice, hamsters, or monkeys, and numerous epidemiological studies provide no clear or consistent evidence to support the assertion that sodium saccharin increases the risk of bladder cancer in the human population. Mechanism of action studies in the one susceptible species, the rat, continue to provide information useful in assessing potential risk to the human from saccharin consumption. Unlike typical carcinogens which interact with DNA, sodium saccharin is not genotoxic, but leads to an increase in cell proliferation of the urothelium, the only target tissue. It also appears that the effect of saccharin is modified by the salt form in which it is administered, despite equivalent concentrations of saccharin in the urine. The chemical form of saccharin in the urine is unaffected, and there is no evidence for a specific cell receptor for the saccharin molecule. Changes in several urinary parameters, such as pH, sodium, protein, silicates, volume, and others, appear to influence the reaction of the urothelium to sodium saccharin administration. Silicon-containing precipitate and/or crystals appear to be generated in the urine under specific circumstances, acting as microabrasive, cytotoxic material. Using a mathematical model of carcinogenesis, which encompasses the temporal dynamics and complexity of the process at a cellular level, including spontaneous genetic transitions, it has been shown that the effects of sodium saccharin can be explained entirely in terms of its non-genotoxic influence on cell proliferation. In interpreting these analytical studies in the human context, particularly as they pertain to the urinary milieu which appears to be pivotal in the effect of sodium saccharin, we are led to the conclusion that there is a threshold effect in male rats and that an effect on the human urothelium is unlikely at even the highest levels of human consumption.

Microbial amino acid metabolites and bladder cancer: no evidence of promoting activity in man

1. Indole, p-cresol and phenol are microbial amino acid metabolites which show co-carcinogenic or promoting activity in animal studies. Their involvement in the development of human bladder cancer has been determined by measuring the urinary excretion of indican (indoxyl sulphate) and conjugated phenols. 2. Thirty-two patients (22 males, 10 females) with histologically confirmed carcinoma of the urinary bladder and a similar number of age and sex matched controls took part in the study. The excretion of indican, p-cresol and phenol showed wide interindividual variability, but did not differ significantly between the two groups. 3. The findings indicate that these endogenous metabolites do not contribute significantly to the development of human bladder cancer.

The effect of saccharin ingestion on the excretion of microbial amino acid metabolites in rat and man

Low dietary levels of sodium saccharin (0-2%) fed to male rats for 6 weeks produced a dose-related increase in the urinary excretion of p-cresol, a major microbial metabolite of tyrosine. Some animals fed higher levels of saccharin (5-7.5%) for 6 weeks excreted increased amounts of p-cresol, but many excreted negligible amounts so that the overall dose-response relationship was bell shaped. After 20 weeks of exposure, all rats in the higher dose groups showed increased p-cresol excretion and by 26 weeks the 7.5% saccharin group showed a 36-fold increase over animals fed the 0% saccharin diet. The urinary excretion of phenol, another microbial amino acid metabolite, was constant in animals fed dietary levels of saccharin below 2% for 6 weeks, but was virtually abolished at higher levels. The excretion of indican (formed from indole, a microbial metabolite of tryptophan) was increased by saccharin in a dose-related fashion at all time points, but showed only a 3-fold increase at 7.5% compared with the 0% group. p-Cresol may therefore prove more sensitive than indican as an indicator of altered microbial metabolism due to saccharin. In a separate study the effect of 7.5% saccharin on p-cresol and indican excretion was shown to be largely reversible and the excretion of phenol increased rapidly when saccharin was withdrawn from the diet. Chronic saccharin administration to man at high doses (1 g/day for 4 weeks) had no perceptible effect on the excretion of these three metabolites.