Childhood cancer in relation to cured meat intake: review of the epidemiological evidence

Over the past two decades a series of epidemiological studies have examined the relationship between consumption of cured meats during pregnancy and the subsequent risk of brain tumors, as well as other cancers, in the offspring. The research was prompted in large part by experimental investigations showing that transplacental exposure to certain N-nitroso compounds, i.e., nitrosoureas, could produce brain tumors in laboratory animals. Fourteen such epidemiological studies, 13 of which used the case-control approach, are reviewed here. Most of the studies showed no significant association between total cured meat intake and childhood cancer risk but more found positive than negative relationships. Furthermore, several studies reported significant positive associations for maternal and sometimes childhood or paternal consumption of one or more cured meats, with odds ratios of twofold or greater reported among the highest consumers. On the other hand, a correlation analysis found no positive concordance between temporal trends from the 1970s to 1990s in childhood brain cancer rates and cured meat consumption, inasmuch as cancer rates rose over time while residual nitrite levels in cured meats fell sharply. Because of the potential for bias, especially recall bias, and/or confounding, the relatively weak magnitude of the associations reported, and the inconsistency between study findings, at this time it cannot be concluded that eating cured meat has increased the risk of childhood brain cancer or any other cancers. Moreover, although N-nitroso compounds are sometimes found in cured meats or may be formed endogenously, there is no empirical evidence that eating cured meats results in human neural nitrosourea exposure. Nevertheless, the hypothesis that eating nitrite-cured meats may influence childhood and perhaps adult brain cancer cannot be dismissed. Unbiased evaluation of the hypothesis may derive from the conduct of cohort studies, where the interview-derived information on cured meat intake precedes, or is not otherwise associated with, the diagnosis of cancer.

Synergistic interaction between Helicobacter pylori gastritis and diet in gastric cancer

Infection with Helicobacter pylori increases the risk of gastric cancer. One possible mechanism is the higher likelihood of malignant transformation due to inflammatory responses in the epithelium. An alternative explanation is that these inflammatory responses induce chronic gastritis associated with decreased acidity in the stomach, which in turn increases the endogenous formation of carcinogenic N-nitroso compounds. Inflammatory responses seem to trigger two different causal pathways: one for the diffuse type of gastric cancer and the other for the intestinal type. The striking geographic variability in intestinal gastric cancer can be explained by the synergistic interaction between H. pylori infection and dietary factors, such as intake of salt and ascorbic acid. Screening and eradication of this organism, together with appropriate dietary modifications, offer promise in countries with a high prevalence of H. pylori infection and high risk of gastric cancer, but the safety of such interventions needs to be ensured.

Dose-dependent effect of dietary meat on endogenous colonic N-nitrosation

Human male volunteers were studied in a metabolic facility whilst they were fed randomized controlled diets. In eight volunteers there was a significant increase in faecal apparent total N:-nitroso compounds (ATNC) and nitrite excretion (P < 0.0001 and P = 0.046, respectively) when randomized doses of meat were increased from 0 to 60, 240 and 420 g/day over 10 day periods. Mean (+/- SE) faecal ATNC levels were 54 +/- 7 microg/day when the diets contained no meat, 52 +/- 11 microg/day when the diets contained 60 g meat/day, 159 +/- 33 microg/day with 240 g meat and 199 +/- 36 microg/day with 420 g meat. Higher concentrations of NOC were associated with longer times of transit in the gut (r = 0.55, P = 0.001) and low faecal weight (r = -0.51, P = 0.004). There was no significant decline in levels in individuals fed 420 g meat for 40 days. The exposures found on the higher meat diets were comparable with other sources of N:-nitroso compounds (NOC), such as tobacco smoke. Many NOC are known large bowel initiators and promotors in colon cancer, inducing G-->A transitions in codons 12 and 13 of K-ras. Endogenous NOC formation, combined with prolonged transit times in the gut, may explain the epidemiological associations between high meat/low fibre diets and colorectal cancer risk.

Evaluation of fecal mutagenicity and colorectal cancer risk

Colorectal cancer is one of the most common internal malignancies in Western society. The cause of this disease appears to be multifactorial and involves genetic as well as environmental aspects. The human colon is continuously exposed to a complex mixture of compounds, which is either of direct dietary origin or the result of digestive, microbial and excretory processes. In order to establish the mutagenic burden of the colorectal mucosa, analysis of specific compounds in feces is usually preferred. Alternatively, the mutagenic potency of fecal extracts has been determined, but the interpretation of these more integrative measurements is hampered by methodological shortcomings. In this review, we focus on exposure of the large bowel to five different classes of fecal mutagens that have previously been related to colorectal cancer risk. These include heterocyclic aromatic amines (HCA) and polycyclic aromatic hydrocarbons (PAH), two exogenous factors that are predominantly ingested as pyrolysis products present in food and (partially) excreted in the feces. Additionally, we discuss N-nitroso-compounds, fecapentaenes and bile acids, all fecal constituents (mainly) of endogenous origin. The mutagenic and carcinogenic potency of the above mentioned compounds as well as their presence in feces, proposed mode of action and potential role in the initiation and promotion of human colorectal cancer are discussed. The combined results from in vitro and in vivo research unequivocally demonstrate that these classes of compounds comprise potent mutagens that induce many different forms of genetic damage and that particularly bile acids and fecapentaenes may also affect the carcinogenic process by epigenetic mechanisms. Large inter-individual differences in levels of exposures have been reported, including those in a range where considerable genetic damage can be expected based on evidence from animal studies. Particularly, however, exposure profiles of PAH and N-nitroso compounds (NOC) have to be more accurately established to come to a risk evaluation. Moreover, lack of human studies and inconsistency between epidemiological data make it impossible to describe colorectal cancer risk as a result of specific exposures in quantitative terms, or even to indicate the relative importance of the mutagens discussed. Particularly, the polymorphisms of genes involved in the metabolism of heterocyclic amines are important determinants of carcinogenic risk. However, the present knowledge of gene-environment interactions with regard to colorectal cancer risk is rather limited. We expect that the introduction of DNA chip technology in colorectal cancer epidemiology will offer new opportunities to identify combinations of exposures and genetic polymorphisms that relate to increased cancer risk. This knowledge will enable us to improve epidemiological study design and statistical power in future research.

Safety of proton-pump inhibitors: the acid test

It is well established that atrophic gastritis, pernicious anaemia and partial gastrectomy are associated with an increased risk of gastric carcinoma. These conditions all result in a low acid output state. Although the mechanism of gastric carcinogenesis is not known, the model proposed by Correa has provided the most attractive explanation to date. The suggestion is that prolonged hypochlorhydria predisposes to gastric carcinoma by an increase in the production of carcinogenic N-nitroso compounds. This hypothesis has led to concerns about the safety of long-term acid suppression in the management of a variety of acid-related gastrointestinal conditions.

Endogenous N-nitroso compounds, and their precursors, present in bacon, do not initiate or promote aberrant crypt foci in the colon of rats

Processed meat intake is associated with increased risk of colorectal cancer. This association may be explained by the endogenous formation of N-nitroso compounds (NOC). The hypothesis that meat intake can increase fecal NOC levels and colon carcinogenesis was tested in 175 Fischer 344 rats. Initiation was assessed by the number of aberrant crypt foci (ACF) in the colon of rats 45 days after the start of a high-fat bacon-based diet. Promotion was assessed by the multiplicity of ACF (crypts per ACF) in rats given experimental diets for 100 days starting 7 days after an azoxymethane injection. Three promotion studies were done, each in 5 groups of 10 rats, whose diets contained 7%, 14%, or 28% fat. Tested meats were bacon, pork, chicken, and beef. Fecal and dietary NOC were assayed by thermal energy analysis. Results show that feces from rats fed bacon-based diets contained 10-20 times more NOC than feces from control rats fed a casein-based diet (all p < 0.0001 in 4 studies). In bacon-fed rats, the amount of NOC input (diet) and output (feces) was similar. Rats fed a diet based on beef, pork, or chicken meat had less fecal NOC than controls (most p < 0.01). No ACF were detected in the colon of bacon-fed uninitiated rats. After azoxymethane injection, unprocessed but cooked meat-based diets did not change the number of ACF or the ACF multiplicity compared with control rats. In contrast, the bacon-based diet consistently reduced the number of large ACF per rat and the ACF multiplicity in the three promotion studies by 12%, 17%, and 20% (all p < 0.01). Results suggest that NOC from dietary bacon would not enhance colon carcinogenesis in rats.

Sex differences in lung CYP1A1 expression and DNA adduct levels among lung cancer patients

Several epidemiological studies have indicated that female tobacco smokers may be at higher risk of lung cancer than males. In a study of lung cancer cases, we have found that female smokers had a significantly higher level of aromatic/hydrophobic DNA adducts in their nontumor lung tissue (15.39+/-9.47 adducts/10(8) nucleotides, n = 29) than male smokers (12.08+/-8.14, a = 93; P = 0.047). Females had significantly higher levels of adducts/pack-year (females 0.95+/-0.82 adducts/pack-year and males 0.46+/-0.46; P = 0.0004) and adducts/cigaret/day (females 1.48+/-1.29 and males 0.89+/-0.74, P = 0.015). By quantitative reverse transcription-PCR, it was found that female smokers exhibited a significantly higher expression level of lung CYP1A1 (494+/-334 CYP1A1 mRNA/10(6) glyceraldehyde-3-phophate dehydrogenase mRNA, n = 15) compared with males (210+/-208, n = 12; P = 0.016). Furthermore, for both sexes combined a significant correlation between CYP1A1 expression and DNA adduct level was found (r = 0.50, P = 0.009). In conclusion, the observed sex difference in aromatic/hydrophobic DNA adduct levels may at least in part be explained by different levels of CYP1A1 expression.

Natural and man-made mutagens and carcinogens in the human diet

Around 40% of human cancers may relate to dietary factors, including both exogenous and endogenous mutagens. Of exogenous factors, alcohol, certain metals and certain pesticides (both naturally produced or manufactured by the chemical industry), N-nitroso compounds, heterocyclic amines and polycyclic aromatic hydrocarbons are all probable human carcinogens. Despite current negative publicity, genetic engineering appears to be a more precise process and no more likely to lead to cancer risks than conventional breeding processes. Many traditional assessments of cancer hazard from endogenous or exogenous chemicals ignore the presence of modifying factors in the human diet. For example, dietary fat and dietary fibre probably either enhance or protect against cancer, depending upon the exact amounts and chemical nature of the fat fibre. Considerable numbers of other types of antimutagen/anticarcinogens have been identified, with varying modes of action. Additionally, there is an interaction of dietary factors with genetics. Epidemiology will always be important in assessing relative risks, but it is essential to continue developing more sensitive biomonitoring methodologies. It would be desirable to compare precise measures of individual exposure to dietary carcinogens with levels of oxidative damage and evidence of genotoxic effects in a given tissue. Such experimental approaches might be expected to lead a better understanding of the interplay between different dietary factors and also between diet, hereditary and the environment.

N-Nitroso compounds in the diet

N-Nitroso compounds were known almost 40 years ago to be present in food treated with sodium nitrite, which made fish meal hepatotoxic to animals through formation of nitrosodimethylamine (NDMA). Since that time, N-nitroso compounds have been shown in animal experiments to be the most broadly acting and the most potent group of carcinogens. The key role of nitrite and nitrogen oxides in forming N-nitroso compounds by interaction with secondary and tertiary amino compounds has led to the examination worldwide of foods for the presence of N-nitroso compounds, which have been found almost exclusively in those foods containing nitrite or which have become exposed to nitrogen oxides. Among these are cured meats, especially bacon-and especially when cooked; concentrations of 100 micrograms kg(-1) have been found or, more usually, near 10 micrograms kg(-1). This would correspond to consumption of 1 microgram of NDMA in a 100-g portion. Much higher concentrations of NDMA (but lower ones of other nitrosamines) have been found in Japanese smoked and cured fish (more than 100 micrograms kg(-1)). Beer is one source of NDMA, in which as much as 70 micrograms l(-1) has been reported in some types of German beer, although usual levels are much lower (10 or 5 micrograms l(-1)); this could mean a considerable intake for a heavy beer drinker of several liters per day. Levels of nitrosamines have been declining during the past three decades, concurrent with a lowering of the nitrite used in food and greater control of exposure of malt to nitrogen oxides in beer making. There have been declines of N-nitroso compound concentrations in many foods during the past two decades. The small amounts of nitrosamines in food are nonetheless significant because of the possibility-even likelihood-that humans are more sensitive to these carcinogens than are laboratory rodents. Although it is probable that alkylnitrosamides (which induce brain tumors in rodents) are present in cured meats and other potentially nitrosated products in spite of much searching, there has been only limited indirect evidence of their presence.

Risk of colorectal and other gastro-intestinal cancers after exposure to nitrate, nitrite and N-nitroso compounds: a follow-up study

N-nitroso compounds are potent carcinogens detected in foodstuffs. The importance of dietary nitrosamines in relation to human cancer development is, however, uncertain. We studied the relationship between intake of nitrates, nitrites and N-nitrosodimethylamine (NDMA) and risk of cancers of the gastro-intestinal tract in a cohort of 9,985 adult Finnish men and women. During a follow-up period of up to 24 years, 189 gastro-intestinal cancer cases were diagnosed in the cohort, initially free from cancer. Intake of nitrate, nitrite and NDMA were estimated, based on food-consumption data from a 1-year dietary history interview covering the total diet of the participants. A significant positive association was observed between intake of NDMA and subsequent occurrence of colorectal cancer with a relative risk (RR) between the highest and lowest quartiles of intake of 2.12 [95% confidence interval (CI) 1.04-4.33]. Of various sources of N-nitroso compounds, intake of smoked and salted fish was significantly (RR = 2.58, 95% CI 1.21-5.51) and intake of cured meat was non-significantly (RR = 1.84, 95% CI 0.98-3.47) associated with risk of colorectal cancer. No similar association was observed for intake of other fish or other meat. No significant associations were observed between NDMA intake and cancers of the head and neck combined or of the stomach or between nitrate or nitrite intake and risk of cancers of the gastro-intestinal tract. Our results are in line with the idea that N-nitroso compounds can induce colorectal cancer in humans.

N-nitroso compounds and Helicobacter pylori in the gastric remnant

AIMS AND BACKGROUND

The aims of this study were to investigate the role of N-nitroso compounds (NOC) and Helicobacter pylori (H. pylori) in gastric stump carcinogenesis.

METHODS AND STUDY DESIGN

Analyses of biochemical parameters such as pH and NOC concentration were carried out on 65 fasting gastric juice samples obtained at endoscopy from 45 patients previously submitted to partial gastrectomy for benign peptic ulcer disease (23 Billroth I, 22 Billroth II/Reichel-Polya) and 20 normal controls. Biopsy specimens were taken to determine histology and H. pylori status.

RESULTS

Significantly higher mean pH values and NOC concentrations were found in partial gastrectomies compared to normal controls. In relation to surgical methods, higher mean pH values and NOC concentrations were observed in the gastric juice of patients with Billroth II compared to Billroth I gastrectomies. Independently of the type of surgical reconstruction, higher mean NOC levels were recorded in patients with more severe histological changes and H. pylori infection.

CONCLUSIONS

All these data suggest that high levels of NOC in gastric juice and H. pylori infection could be cofactors in gastric stump carcinogenesis.

Maternal diet during pregnancy and risk of brain tumors in children

Nine studies of childhood brain tumors and maternal diet during pregnancy have focused on foods related to the N-nitroso-compound(NOC) hypothesis. An association between frequent consumption of cured meat by pregnant women and increased risk is a consistent finding in most of the studies. The data on fruit and vegetable consumption are less consistent, but suggest decreased risk. Studies that assess all aspects of maternal diet during pregnancy are needed to determine whether the observed associations remain after adjustment for other aspects of diet. Such comprehensive studies also may elucidate other dietary factors that affect the risk of brain tumors in children.

A methodological note on the selection of friends as controls

A number of issues inherent in the selection of friends as controls in case-control studies are illustrated in the light of a recent dietary study, based on hospital and friend controls. Preselection of certain characteristics of the controls by the interviewee is almost unavoidable. The choice of controls to be used in a case-control study must reflect the nature and type of hypothesis which is being tested. Advantages and drawbacks of potential control groups must be weighted against each other.

Dietary nitrosamines, heterocyclic amines, and risk of gastric cancer: a case-control study in Uruguay

To study the effects of dietary chemicals like nitrosodimethylamine and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine, resulting from the cooking method of red meat, on gastric carcinogenesis, a case-control study was conducted in Uruguay, a country with areas of high rates of gastric cancer. The study involved 340 cases and 698 controls, who were interviewed between January 1993 and December 1996. Dietary nitrosodimethylamine was associated with an increased risk of gastric cancer [odds ratio (OR) = 3.6, 95% confidence interval (CI) = 2.4-5.5], whereas dietary 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (a potent mutagen derived from the frying and broiling of red meat) showed an OR of 3.9 (95% CI = 2.3-6.4). Both chemicals displayed independent effects, and its interaction followed a multiplicative model with an elevated OR of 12.7 (95% CI = 7.7-21.2). These results suggest that salted and barbecued meat, frequent items in the Uruguayan diet, and the resulting chemicals from the cooking methods of both types of meat are significantly associated with a high risk of stomach cancer.

Dietary nitrates, nitrites, and N-nitroso compounds and cancer risk: a review of the epidemiologic evidence

Experimental animal studies have shown N-nitroso compounds (NOC) to be potent carcinogens. Epidemiologic evidence of the carcinogenic potential of dietary NOC and precursor nitrates and nitrites in humans remains inconclusive with regard to the risk of stomach, brain, esophageal, and nasopharyngeal cancers. Inadequate available data could obscure a small to moderate effect of NOC.

Maternal exposure to prescription and non-prescription pharmaceuticals or drugs of abuse and risk of craniosynostosis

BACKGROUND

The pre- and peri-natal drug exposures reported by women participating in a case-control study of children in Colorado were examined for association with infant craniosynostosis.

METHODS

Mothers of case and control children underwent a standardized telephone interview and obstetric and newborn medical record review. The interview included questions on the use of prescription and non-prescription drugs, nutritional supplements, recreational and drugs of abuse. The mother's obstetric record was abstracted for information about pharmacologic agents taken before and during the antepartum period.

RESULTS

There were no statistically significant risk ratios associating craniosynostosis with prenatal exposure to hydantoin, valproic acid, or cocaine. Mothers of four case children and one control child reported specific exposure to retinoic acid. There were no statistically significant increases in the odds ratio (OR) for any suture type among children exposed to hypoxigenic agents, sympathomimetic or parasympatholytic agents, or metal-containing agents. The OR was 1.87 (lower bound of the two-sided 95% test-based confidence interval (CI): 1.08) among children with sagittal/lambdoid suture synostosis who were exposed to nitrosatable drugs chlorpheniramine, chlordiazepoxide, and nitrofurantoin compared to controls.

CONCLUSIONS

Certain nitrosatable drugs may be associated with increased risk of infant sagittal/lambdoid craniosynostosis. A possible mechanism related to ischaemia/reperfusion injury is suggested.

[Colorectal cancer: controversial role of meat consumption]

Diet is supposed to influence the colorectal cancer etiology, but the precise causative factors are yet unknown. International ecological studies show a strong correlation between meat consumption and the colorectal cancer incidence. Most case-control studies (22 of 29) show an increased risk to develop a colorectal cancer for those eating higher amounts of meat. In contrast, only 2 out of the 5 best prospective cohort studies have shown this positive association for red meat. Two studies out of 4 show an association with processed meat. Consumption of white meat or of fish is not associated with a high risk, and might even reduce the occurrence of colorectal cancer. Several plausible hypotheses concerning the link between meat and colon carcinogenesis have been suggested. They involve saturated fat, protein, iron, heterocyclic amines produced by cooking, and N-nitroso compounds. High fat diets may promote cancer because they have a high caloric content, or because they lead to increased levels of bile acids in the colonic lumen. Six experimental studies are published on the effect of meat, or meat fractions, on the colon tumor incidence in rodents initiated with chemical carcinogens. Data from these studies do not support the belief that red meat (beef) has a specific effect on intestinal carcinogenesis. Instead, diets containing beef meat (cooked or raw) decrease carcinogenesis when compared to control diets containing similar amounts of fat and protein of vegetal origin. However, high fat or high protein diets often increase carcinogenesis when compared to low fat or low protein diets. Thus, one cannot state, nor exclude, that meat promotes colorectal cancer.

N-nitroso compounds and man: sources of exposure, endogenous formation and occurrence in body fluids

Based on recent analytical data, total human exogenous exposure to N-nitrosamines is estimated to be 1.10 mumol/day; the major exposure sources are the diet (0.79 mumol/may, 80-120 micrograms/day; 72%), occupational exposure (0.15-0.30 mumol/day; 25%), cigarette smoking (0.02 mumol/day, 3.4 micrograms/day; 2%), and miscellaneous minor sources, including pharmaceutical products, cosmetics, indoor and outdoor air (0.001 mumol/day, 0.1 micrograms/day; 1%). Excretion of apparent total N-nitroso compounds (ATNC) in healthy adults is estimated to be 1.30 +/- 1.05 mumol/day in urine and between 1.56 +/- 1.56 and 3.17 +/- 2.58 mumol/day in faeces. The excretion of volatile N-nitrosamines (N-nitrosodimethylamine), and N-nitrosamine acids and their derivatives (N-nitrososarcosine, N-nitrosoproline, N-nitrosothiazolidine-4-carboxylic acid and N-nitroso-2-methylthiazoline-4-carboxylic acid) accounts for approximately 0.03% and 16.0% of urinary ATNC, respectively. 4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanol and its O-glucuronide conjugate, two metabolites of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone present in urine of smokers, account for 0.2% of the urinary ATNC response; < 1.5 of the excretion of currently identified N-nitroso compounds in urine. The remaining N-nitroso compounds excreted in urine and those present in faeces are still unidentified. A crude mass balance between exogenous exposure and excretion in urine and faeces indicates that 45-75% of the total human exposure to N-nitroso compounds results through in vivo formation.

Immunotoxic effects of smokeless tobacco on the accessory cell function of rat oral epithelium

Smokeless tobacco (ST) is known to adversely effect the oral mucosa, but knowledge about the influence on immune defence is limited. Few studies have investigated the effect of ST on the local immune response. In the present study, we have assessed the effect of a crude Swedish moist snuff (SS) extract, alkaloids, and nitrosamines on T-cell mitogenic response to Con A using epithelial cells, including Langerhans cells, of the rat oral mucosa as accessory cells. SS extract at a concentration of 4% reduced the T-cell proliferation by 50% (IC50 = 4%). Pretreatment of either oral epithelial cells or T-cells with SS extract also gave a significant inhibition of T-cell proliferation. This effect was not obtained following preincubation with SS components as alkaloids and different tobacco-specific nitrosamines (TSNA). None of the tested compounds were found to possess any mitogenic properties. This in vitro study showed that SS extract can evoke an immunosuppressive effect on mitogen-driven T-cell proliferation using cells from oral epithelium as accessory cells. This effect was more pronounced when SS extract was employed compared to when the single SS components were used alone.