A large number of studies of differing designs have investigated the relationship between human processed meat consumption and CRC.
Table 2;
Table 3, respectively, describe all prospective studies and case-control studies conducted thus far.
Supplementary Tables S1 and S2 include more information about the studies presented in
Table 2 and
Table 3. Of the 49 human studies identified, 23 found that processed meat consumption was linked with CRC [
32,
33,
34,
35,
36,
37,
38,
39,
40,
41,
42,
43,
44,
45,
46,
47,
48,
49,
50,
51,
52,
53,
54], 25 found no link [
55,
56,
57,
58,
59,
60,
61,
62,
63,
64,
65,
66,
67,
68,
69,
70,
71,
72,
73,
74,
75,
76,
77,
78,
79], and one study found processed meat to be protective for CRC [
80]. It is, therefore, difficult to draw any definitive conclusions from these studies.
Although an equal number of studies prove/disprove the link between processed meat and CRC, those studies which found a relationship generally involved more participants; this explains why numerous meta-analyses have concluded that processed meat is a risk factor for CRC [
5,
6,
7]. One study reported no association between processed meat consumption and CRC in men (
n = 241), but reported a beneficial association between processed meat and CRC in women (
n = 197) [
69]. Two further prospective studies found that processed meat consumption was associated with CRC in men but not in women [
37,
59]. These findings were supported by three further studies (
n = 119,260) indicating no effect of processed meat in female cases and controls [
32,
55,
79]. A study of almost 48,000 American men reported that the association between processed meat consumption and CRC did not reach significance (
p = 0.06) [
73].
Studies Focusing on Nitrite-Containing Meat and CRC
Of the human studies discussed above, 17 studies investigated nitrite-containing processed meats (
Table 4). Five studies found nitrite-containing processed meats to have no effect on CRC [
43,
56,
57,
59,
62] and one study found nitrite-containing processed meat was protective [
80]. In contrast, a total of 11 studies found that nitrite-containing processed meat increases the risk of CRC [
34,
35,
36,
37,
38,
39,
42,
45,
48,
49,
53]. This indicates that the proposed causal relationship between processed meat is potentially skewed by the intake of nitrite-containing processed meats.
Of the prospective studies discussed (
n = 5), three used the International Classification of Diseases criteria to confirm that the participant met the conditions of a positive diagnosis of CRC [
49,
57,
59]; two studies did not specify which criteria they used. Wu et al. [
48] consulted the patients’ medical records. Takachi et al. [
56] conducted a linkage study, where the researchers had access to a local cancer registry. Of the case-control studies (
n = 12), ten studies recruited participants that had histologically confirmed adenocarcinoma; only one stated that the International Classification of Diseases criteria was applied to their study population [
80]. Dales et al. [
62] recruited hospitalized CRC patients, whilst Young and Wolf [
37] recruited participants from the Wisconsin Cancer Reporting System.
All studies used a food frequency questionnaire (FFQ) to capture processed meat consumption in their respective cohorts; only two of the studies set out with the intention of investigating the role of nitrite in CRC development [
59,
62] and both of these found no relationship. No study considered the concomitant consumption of haem proteins, and only one study considered the effect of nitrosamine exposure [
59] levels; however, no study directly measured that. The FFQs varied in design and detail. Three studies recorded information on portion size [
43,
45,
57], and the remainder did not. Researchers invited participants to describe their consumption habits in a number of ways, Dales et al. [
62] included eight different frequencies, ranging from “never” to “at least once a day.” Four studies provided their participants with six frequencies to choose from [
34,
37,
48,
80]. Two studies used five categories of frequency [
42,
57], Lohsoonthorn et al. [
53] used four categories; the remainder of the studies did not specify how many categories were available (
n = 10). Only one study described their FFQ as validated [
39]. It has been shown that illness substantially effects dietary intake, and it is, therefore, crucial that all studies investigating the causative effect of habitual dietary pattern, consider the period prior to illness. Of the studies that recruited participants with existing CRC (
n = 12), seven studies stated that participants were instructed to record dietary information on periods prior to illness. These periods ranged from weekly to over the course of the participant’s lifetime.
The above analysis of prior studies clearly indicates that there is a need for methodical studies which specifically investigate nitrite exposure, and which control for confounding factors, such as haem, and saturated fat intake. As a pre-requisite, a well-designed, validated FFQ focusing on processed meat must include portion size. An early report concluded that although processed meat consumption was linked to CRC development [
36], nitrite was not responsible. This conclusion was based on the finding that meat with no nitrite had a higher relative risk than meat with nitrite [
36]. The authors postulated that meat consumption was acting as a surrogate measure of saturated fat consumption, and that in fact, saturated fat intake could be responsible for the positive association [
36]. Further investigation found that the frequency of consumption of nitrite-containing foods and high fat containing foods were not different between cases or controls [
62]. This hypothesis is disputed by a study that found controlling for meat intake substantially decreased the association between fat intake and CRC development, suggesting that meat intake was responsible for the relationship [
81].
Positive relationships between CRC and the consumption of salami [
38], sausages [
39], ham [
34], and bacon [
45] have been reported, although Sato et al. [
57] found no relationship between CRC and sausage or ham intake. The content of a sausage differs greatly depending on the location it is being manufactured in. Sausages made in continental Europe tend to contain sodium nitrite, whilst British/Irish sausages do not. As the aforementioned studies that measured sausage intake were conducted in Argentina and Japan, respectively, it is difficult to determine the proportion of nitrite-containing sausages that the populations actually consumed.
In a prospective study of 9985 participants, it was reported that
N-nitrosodimethylamine consumption was positively related to CRC development; when locating the origin of the
N-nitrosodimethylamine, there was a strong significant association between intake of smoked and salted fish and risk of CRC; however, the association between intake of cured meat and sausages with CRC was not significant [
59]. A case-control study found that CRC patients were more likely to consume processed lunchmeat at various stages of their life compared with controls, and furthermore, it was speculated that additives such as nitrites may be responsible for this association [
37]. Pierre et al. (2013) [
22] conducted the only human intervention study in this area. Seventeen males were asked to abstain from meat and antioxidants for a 7-day control period, following which they consumed 180 g of ham per day for 4 days; after a washout period, the same participants consumed 180 g of tocopherol enriched ham a day for 4 days, and finally, 180 g of ham and 500 mg of calcium per day. During the ham-only period, participants had significantly higher faecal levels of apparent total N-nitroso compounds (ATNC) and thiobarbituric acid reactive substances (TBARS) than during the control periods. The addition of calcium to the diet mitigated this rise. There was no change in the faecal water cytotoxicity or 1,4-Dihydroxynonane mercapturic acid (DHN-MA) following any treatment. None of the markers measured in that study are validated markers of colorectal cancer; however, they are the best available without imaging or visualising the colon. The existing human evidence indicates that nitrites, through their exogenous and endogenous conversion to NOCs, are an important contributing factor in the proposed causal link between processed meat consumption and CRC.