Heme-induced biomarkers associated with red meat promotion of colon cancer are not modulated by the intake of nitrite

Dietary inclusion of nitrite-containing frankfurter exacerbates colorectal cancer pathology and alters metabolism in APCmin mice

Colorectal cancer (CRC) is the second most prevelant malignancy in Europe and diet is an important modifiable risk factor. Processed meat consumption, including meats with preservative salts such as sodium nitrite, have been implicated in CRC pathogenesis. This study investigated how the CRC pathology and metabolic status of adenomatous polyposis coli (APC) multiple intestinal neoplasia (min) mice was perturbed following 8 weeks of pork meat consumption. Dietary inclusions (15%) of either nitrite-free pork, nitrite-free sausage, or nitrite-containing sausage (frankfurter) were compared against a parallel control group (100% chow). Comprehensive studies investigated: gastrointestinal tract histology (tumours), aberrant crypt foci (ACF), mucin deplin foci (MDF), lipid peroxidation (urine and serum), faecal microbiota, and serum metabolomics (599 metabolites). After 8 weeks mice consuming the frankfurter diet had 53% more (P = 0.014) gastrointestinal tumours than control, although ACF and MDF did not differ. Urine and serum lipid peroxidation markers were 59% (P = 0.001) and 108% (P = 0.001) higher, respectively in the frankfurter group. Gut dysbiosis was evident in these mice with comparably fewer Bacteriodes and more Firmicutes. Fasting serum levels of trimethylamine N-oxide (TMAO) and numerous triglycerides were elevated. Various serum phosphotidylcholine species were decreased. These results demonstrate that nitrite-containing sausages may exaccerbate the development of CRC pathology in APC^Min mice to a greater extent than nitrite-free sausages, and this is associated with greater lipid peroxidation, wide-ranging metabolic alternation and gut dysbiosis.

Meat and colorectal cancer in Middle Eastern and North African countries: update of literature review

Background

This review discusses the findings from epidemiological studies that have examined the possible role of meat and colorectal cancer (CRC) risk in Middle Eastern and North African (MENA) countries.

Methods

We conducted a literature search in the PubMed, Clinical Trials, Google Scholar, Science Direct, and Cochrane databases for observational studies that investigated the association between meat and CRC risk in adults from the MENA region.

Results

Eleven studies were included in this review. For red meat overall, significant associations were found. Regarding beef meat intake, the study included found controversial results with OR = 0.18 (95% CI 0.03–0.09). A positive association was observed between chicken and CRC risk, at OR = 2.52 (95% CI 1.33–4.77) to OR = 4.00 (95% CI 1.53–10.41) to OR = 15.32 (95% CI 3.28–71.45). A significant association was observed between processed meat intake and CRC risk, OR = 9.08 (95% CI 1.02–80.58).

Conclusion

This is the first literature review which illustrated the association between meat consumption and CRC risk in MENA region. We concluded that these studies included in this review have been controversial and not sufficient to establish a clear relationship between CRC and meat consumption in the MENA region. Further studies are necessary to be carried out in this region, with a larger sample size and submit to rigorous criteria. This review will help researchers to improve the quality of future studies about the association between CRC and nutritional diet in general and meat in particular.

A Review of the In Vivo Evidence Investigating the Role of Nitrite Exposure from Processed Meat Consumption in the Development of Colorectal Cancer

The World Cancer Research Fund (WCRF) 2007 stated that the consumption of processed meat is a convincing cause of colorectal cancer (CRC), and therefore, the public should avoid it entirely. Sodium nitrite has emerged as a putative candidate responsible for the CRC-inducing effects of processed meats. Sodium nitrite is purported to prevent the growth of Clostridium botulinum and other food-spoiling bacteria, but recent, contradictory peer-reviewed evidence has emerged, leading to media reports questioning the necessity of nitrite addition. To date, eleven preclinical studies have investigated the effect of consuming nitrite/nitrite-containing meat on the development of CRC, but the results do not provide an overall consensus. A sizable number of human clinical studies have investigated the relationship between processed meat consumption and CRC risk with widely varying results. The unique approach of the present literature review was to include analysis that limited the human studies to those involving only nitrite-containing meat. The majority of these studies reported that nitrite-containing processed meat was associated with increased CRC risk. Nitrite consumption can lead to the formation of N-nitroso compounds (NOC), some of which are carcinogenic. Therefore, this focused perspective based on the current body of evidence links the consumption of meat containing nitrites and CRC risk.

Red meat and colon cancer: A review of mechanistic evidence for heme in the context of risk assessment methodology

On October 26, 2015, IARC published a summary of their findings regarding the association of cancer with consumption of red meat or processed meat (IARC 2015; The Lancet Oncology 2015). The Working Group concluded that there is limited evidence in human beings for carcinogenicity from the consumption of red meat and inadequate evidence in experimental animals for the carcinogenicity of consumption of red meat. Nevertheless, the working group concluded that there is strong mechanistic evidence by which ingestion of red meat can be linked to human colorectal cancer and assigned red meat to Group 2A "probably carcinogenic to humans". The Working Group cited supporting mechanistic evidence for multiple meat components, including those formed from meat processing, such as N-nitroso compounds (NOC) and heterocyclic aromatic amines, and the endogenous compound, heme iron. The mechanism of action for each of these components is different and so it is critical to evaluate the evidence for each component separately. Consequently, this review critically examined studies that investigated mechanistic evidence associated with heme iron to assess the weight of the evidence associating exposure to red meat with colorectal cancer. The evidence from in vitro studies utilized conditions that are not necessarily relevant for a normal dietary intake and thus do not provide sufficient evidence that heme exposure from typical red meat consumption would increase the risk of colon cancer. Animal studies utilized models that tested promotion of preneoplastic conditions utilizing diets low in calcium, high in fat combined with exaggerations of heme exposure that in many instances represented intakes that were orders of magnitude above normal dietary consumption of red meat. Finally, clinical evidence suggests that the type of NOC found after ingestion of red meat in humans consists mainly of nitrosyl iron and nitrosothiols, products that have profoundly different chemistries from certain N-nitroso species which have been shown to be tumorigenic through the formation of DNA adducts. In conclusion, the methodologies employed in current studies of heme have not provided sufficient documentation that the mechanisms studied would contribute to an increased risk of promotion of preneoplasia or colon cancer at usual dietary intakes of red meat in the context of a normal diet.

Association between red meat consumption and colon cancer: A systematic review of experimental results

A role for red and processed meat in the development of colorectal cancer has been proposed based largely on evidence from observational studies in humans, especially in those populations consuming a westernized diet. Determination of causation specifically by red or processed meat is contingent upon identification of plausible mechanisms that lead to colorectal cancer. We conducted a systematic review of the available evidence to determine the availability of plausible mechanistic data linking red and processed meat consumption to colorectal cancer risk. Forty studies using animal models or cell cultures met specified inclusion criteria, most of which were designed to examine the role of heme iron or heterocyclic amines in relation to colon carcinogenesis. Most studies used levels of meat or meat components well in excess of those found in human diets. Although many of the experiments used semi-purified diets designed to mimic the nutrient loads in current westernized diets, most did not include potential biologically active protective compounds present in whole foods. Because of these limitations in the existing literature, there is currently insufficient evidence to confirm a mechanistic link between the intake of red meat as part of a healthy dietary pattern and colorectal cancer risk. Impact statement Current recommendations to reduce colon cancer include the reduction or elimination of red or processed meats. These recommendations are based on data from epidemiological studies conducted among cultures where meat consumption is elevated and consumption of fruits, vegetables, and whole grains are reduced. This review evaluated experimental data exploring the putative mechanisms whereby red or processed meats may contribute to colon cancer. Most studies used levels of meat or meat-derived compounds that were in excess of those in human diets, even in cultures where meat intake is elevated. Experiments where protective dietary compounds were used to mitigate the extreme levels of meat and meat-derived compounds showed protection against colon cancer, with some essentially negating the impact of meat in the diet. It is essential that better-designed studies be conducted that use relevant concentrations of meat or meat-derived compounds in complex diets representative of the foods consumed by humans.

Mechanisms Linking Colorectal Cancer to the Consumption of (Processed) Red Meat: A Review

Colorectal cancer (CRC) is the third most commonly diagnosed cancer in the world. The vast majority of CRC cases have been linked to environmental causes rather than to heritable genetic changes. Over the last decades, epidemiological evidence linking the consumption of red and, more convincingly, of processed red meat to CRC has accumulated. In parallel, hypotheses on carcinogenic mechanisms underlying an association between CRC and the intake of red and processed red meat have been proposed and investigated in biological studies. The hypotheses that have received most attention until now include (1) the presence of polycyclic aromatic hydrocarbons and heterocyclic aromatic amines, two groups of compounds recognized as carcinogenic, (2) the enhancing effect of (nitrosyl)heme on the formation of carcinogenic N-nitroso compounds and lipid peroxidation. However, none of these hypotheses completely explains the link between red and processed red meat intake and the CRC risk. Consequently, scientists have proposed additional mechanisms or refined their hypotheses. This review first briefly summarizes the development of CRC followed by an in-depth overview and critical discussion of the different potential carcinogenic mechanisms underlying the increased CRC risk associated with the consumption of red and processed red meat.

DNA adductomics to study the genotoxic effects of red meat consumption with and without added animal fat in rats

Digestion of red and processed meat has been linked to the formation of genotoxic N-nitroso compounds (NOCs) and lipid peroxidation products (LPOs) in the gut. In this study, rats were fed a meat based diet to compare the possible genotoxic effects of red vs. white meat, and the interfering role of dietary fat. To this purpose, liver, duodenum and colon DNA adductomes were analyzed with UHPLC-HRMS. The results demonstrate that the consumed meat type alters the DNA adductome; the levels of 22 different DNA adduct types significantly increased upon the consumption of beef (compared to chicken) and/or lard supplemented beef or chicken. Furthermore, the chemical constitution of the retrieved DNA adducts hint at a direct link with an increase in NOCs and LPOs upon red (and processed) meat digestion, supporting the current hypotheses on the causal link between red and processed meat consumption and the development of colorectal cancer.

Consumption of Red/Processed Meat and Colorectal Carcinoma: Possible Mechanisms Underlying the Significant Association

Epidemiology and experimental studies provide an overwhelming support of the notion that diets high in red or processed meat accompany an elevated risk of developing pre-neoplastic colorectal adenoma and frank colorectal carcinoma (CRC). The underlying mechanisms are disputed; thus several hypotheses have been proposed. A large body of reports converges, however, on haem and nitrosyl haem as major contributors to the CRC development, presumably acting through various mechanisms. Apart from a potentially higher intestinal mutagenic load among consumers on a diet rich in red/processed meat, other mechanisms involving subtle interference with colorectal stem/progenitor cell survival or maturation are likewise at play. From an overarching perspective, suggested candidate mechanisms for red/processed meat-induced CRC appear as three partly overlapping tenets: (i) increased N-nitrosation/oxidative load leading to DNA adducts and lipid peroxidation in the intestinal epithelium, (ii) proliferative stimulation of the epithelium through haem or food-derived metabolites that either act directly or subsequent to conversion, and (iii) higher inflammatory response, which may trigger a wide cascade of pro-malignant processes. In this review, we summarize and discuss major findings of the area in the context of potentially pertinent mechanisms underlying the above-mentioned association between consumption of red/processed meat and increased risk of developing CRC.

Dietary nitrate improves vascular function in patients with hypercholesterolemia: a randomized, double-blind, placebo-controlled study

BACKGROUND

The beneficial cardiovascular effects of vegetables may be underpinned by their high inorganic nitrate content.

OBJECTIVE

We sought to examine the effects of a 6-wk once-daily intake of dietary nitrate (nitrate-rich beetroot juice) compared with placebo intake (nitrate-depleted beetroot juice) on vascular and platelet function in untreated hypercholesterolemics.

DESIGN

A total of 69 subjects were recruited in this randomized, double-blind, placebo-controlled parallel study. The primary endpoint was the change in vascular function determined with the use of ultrasound flow-mediated dilatation (FMD).

RESULTS

Baseline characteristics were similar between the groups, with primary outcome data available for 67 patients. Dietary nitrate resulted in an absolute increase in the FMD response of 1.1% (an ∼24% improvement from baseline) with a worsening of 0.3% in the placebo group (P < 0.001). A small improvement in the aortic pulse wave velocity (i.e., a decrease of 0.22 m/s; 95% CI: -0.4, -0.3 m/s) was evident in the nitrate group, showing a trend (P = 0.06) to improvement in comparison with the placebo group. Dietary nitrate also caused a small but significant reduction (7.6%) in platelet-monocyte aggregates compared with an increase of 10.1% in the placebo group (P = 0.004), with statistically significant reductions in stimulated (ex vivo) P-selectin expression compared with the placebo group (P < 0.05) but no significant changes in unstimulated expression. No adverse effects of dietary nitrate were detected. The composition of the salivary microbiome was altered after the nitrate treatment but not after the placebo treatment (P < 0.01). The proportions of 78 bacterial taxa were different after the nitrate treatment; of those taxa present, 2 taxa were responsible for >1% of this change, with the proportions of Rothia mucilaginosa trending to increase and Neisseria flavescens (P < 0.01) increased after nitrate treatment relative to after placebo treatment.

CONCLUSIONS

Sustained dietary nitrate ingestion improves vascular function in hypercholesterolemic patients. These changes are associated with alterations in the oral microbiome and, in particular, nitrate-reducing genera. Our findings provide additional support for the assessment of the potential of dietary nitrate as a preventative strategy against atherogenesis in larger cohorts. This trial was registered at clinicaltrials.gov as NCT01493752.

Effects of hemin and nitrite on intestinal tumorigenesis in the A/J Min/+ mouse model

Red and processed meats are considered risk factors for colorectal cancer (CRC); however, the underlying mechanisms are still unclear. One cause for the potential link between CRC and meat is the heme iron in red meat. Two pathways by which heme and CRC promotion may be linked have been suggested: fat peroxidation and N-nitrosation. In the present work we have used the novel A/J Min/+ mouse model to test the effects of dietary hemin (a model of red meat), and hemin in combination with nitrite (a model of processed meat) on intestinal tumorigenesis. Mice were fed a low Ca2+ and vitamin D semi-synthetic diet with added hemin and/or nitrite for 8 weeks post weaning, before termination followed by excision and examination of the intestinal tract. Our results indicate that dietary hemin decreased the number of colonic lesions in the A/J Min/+ mouse. However, our results also showed that the opposite occurred in the small intestine, where dietary hemin appeared to stimulate tumor growth. Furthermore, we find that nitrite, which did not have an effect in the colon, appeared to have a suppressive effect on tumor growth in the small intestine.

Fat content and nitrite-curing influence the formation of oxidation products and NOC-specific DNA adducts during in vitro digestion of meat

The effects of fat content and nitrite-curing of pork were investigated on the formation of cytotoxic and genotoxic lipid oxidation products (malondialdehyde, 4-hydroxy-2-nonenal, volatile simple aldehydes), protein oxidation products (protein carbonyl compounds) and NOC-specific DNA adducts (O⁶-carboxy-methylguanine) during invitro digestion. The formation of these products during digestion is suggested to be responsible for the association between red meat and processed meat consumption and colorectal cancer risk. Digestion of uncured pork to which fat was added (total fat content 5 or 20%), resulted in significantly higher lipid and protein oxidation in the mimicked duodenal and colonic fluids, compared to digestion of pork without added fat (1% fat). A higher fat content also significantly favored the formation of O⁶-carboxy-methylguanine in the colon. Nitrite-curing of meat resulted in significantly lower lipid and protein oxidation before and after digestion, while an inconsistent effect on the formation of O⁶-carboxy-methylguanine was observed. The presented results demonstrate that haem-Fe is not solely responsible for oxidation and nitrosation reactions throughout an invitro digestion approach but its effect is promoted by a higher fat content in meat.

Nitrite curing of chicken, pork, and beef inhibits oxidation but does not affect N-nitroso compound (NOC)-specific DNA adduct formation during in vitro digestion

Uncured and nitrite-cured chicken, pork, and beef were used as low, medium, and high sources of heme-Fe, respectively, and exposed to an in vitro digestion model simulating the mouth, stomach, duodenum, and colon. With increasing content of iron compounds, up to 25-fold higher concentrations of the toxic lipid oxidation products malondialdehyde, 4-hydroxy-2-nonenal, and other volatile aldehydes were formed during digestion, together with increased protein carbonyl compounds as measurement of protein oxidation. Nitrite curing of all meats lowered lipid and protein oxidation to the level of oxidation in uncured chicken. Strongly depending on the individual fecal inoculum, colonic digestion of beef resulted in significantly higher concentrations of the NOC-specific DNA adduct O(6)-carboxymethyl-guanine compared to chicken and pork, whereas nitrite curing had no significant effect. This study confirms previously reported evidence that heme-Fe is involved in the epidemiological association between red meat consumption and colorectal cancer, but questions the role of nitrite curing in this association.

Accumulation of promutagenic DNA adducts in the mouse distal colon after consumption of heme does not induce colonic neoplasms in the western diet model of spontaneous colorectal cancer

SCOPE

Red meat is considered a risk factor for colorectal cancer (CRC). Heme is considered to promote colonic hyperproliferation and cell damage. Resistant starch (RS) is a food that ferments in the colon with studies demonstrating protective effects against CRC. By utilizing the western diet model of spontaneous CRC, we determined if feeding heme (as hemin chloride) equivalent to a high red meat diet would increase colonic DNA adducts and CRC and whether RS could abrogate such effects.

METHODS AND RESULTS

Four groups of mice: control, heme, RS and heme + RS were fed diets for 1 or 18 months. Colons were analyzed for apoptosis, proliferation, DNA adducts "8-hydroxy-2-deoxyguanosine" and "O(6) -methyl-2-deoxyguanosine" (O(6) MeG), and neoplasms. In the short term, heme increased cell proliferation (p < 0.05). Changes from 1 to 18 months showed increased cell proliferation (p < 0.01) and 8-hydroxy-2-deoxyguanosine adducts (p < 0.05) in all groups, but only heme-fed mice showed reduced apoptosis (p < 0.01) and increased O(6) MeG adducts (p < 0.01). The incidence of colon neoplasms was not different between any interventions.

CONCLUSION

We identified heme to increase proliferation in the short term, inhibit apoptosis over the long term, and increase O(6) MeG adducts in the colon over time although these changes did not affect colonic neoplasms within this mouse model.