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Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. References

Background

: The published epidemiological studies of chronic Helicobacter pylori infection and gastric cancer yield conflicting results, so there is uncertainty as to whether any material association exists and, if so, how strong it is.

Aim

: To review these studies quantitatively.

Methods

: A systematic review of sero-epidemiological studies published before 1998 of H. pylori and gastric cancer, as identified by computer-assisted literature searches of relevant journals, reference lists and discussions with authors. All relevant studies identified were included, subdivided by study design. The following was abstracted from published reports: adjusted odds ratio (or, in prospective studies, the risk ratio) and confidence interval, study design, type of controls, mean age, mean duration of follow-up, assay methods, location of study, and degree of adjustment for confounders.

Results

: The 34 retrospective studies included in total 3300 gastric cancers, but their controls were of uncertain validity. The 10 ‘nested’ case–control comparisons in prospective studies included in total only 800 gastric cancers, and combined analysis of them yielded a risk ratio of 2.5 (95% CI: 1.9–3.4; 2P < 0.00001) for gastric cancer in people seropositive for H. pylori antibodies.

Conclusions

: The prospective studies suggest that gastric cancer is 2 or 3 times as common in those chronically infected by H. pylori, but to help investigate causality, further observational studies are still needed, as are large-scale randomized trials of whether antibacterial regimens reduce the eventual incidence of gastric cancer.


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. References

When Helicobacter pylori was first identified, those that discovered it suggested that it might cause gastric cancer,1 which kills about a million people a year worldwide.2H. pylori is found in the stomachs of about one-third of the adults in developed countries (where it has been decreasing in prevalence) and in about two-thirds of the adults in developing countries.3 It is strongly correlated with lower socioeconomic status, usually acquired in early life, probably spread from person to person, and generally persists until old age unless eradicated with specific antibiotic treatment.4 A proportion of infected people develop peptic ulceration.5 There is, however, a great deal of uncertainty as to whether the infection can also predispose to gastric cancer. In 1994 the International Agency on Research in Cancer declared H. pylori a ‘carcinogen’,6 but a consensus panel of the National Institutes of Health disagreed.5 A number of additional studies have been reported recently, but expert opinions continue to differ, with claims ranging from a nine-fold association7 to no important association at all.8 To help clarify the actual evidence, this article provides a systematic review (meta-analysis) of the epidemiological studies.

METHODS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. References

Epidemiological and clinical studies published between 1983 and 1998 that reported on H. pylori status in cases with gastric cancer and in controls without the disease were sought by Medline searches, scanning of reference lists, hand-searching of gastroenterology, epidemiology and other relevant journals, and discussions with authors of relevant reports. Combinations of key words were used in the searches (e.g. Helicobacter pylori, Campylobacter pylori, gastric adenocarcinoma, gastric cancer, stomach cancer).9[10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52]–53 (One study of 40 cases was found to be ineligible as it selected only controls who were known to be infected with H. pylori.54) Non-English articles were translated and abstracted according to a fixed protocol. The following information was obtained for each study (from published reports or from the investigators): adjusted odds ratio (or, in prospective studies, the risk ratio) and confidence interval; number of cases and controls; study design (‘nested’ case–control study or retrospective study); type of controls (prospective studies with ‘internal’ controls; healthy volunteers; hospital in-patients; patients with gastric diseases); mean age of cases (or, for prospective studies, mean age at entry and mean follow-up duration); assay methods; geographical location of study; and degree of adjustment for confounders denoted by: + age and sex only; ++ these plus smoking; +++ these plus socioeconomic status; ++++ these plus information on dietary habits. For prospective studies, combination of results involved inverse-variance-weighted log risk ratios. Heterogeneity was assessed with standard χ2-tests.

RESULTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. References

Thirty-four retrospective epidemiological studies, involving in total about 3300 cases, have reported on the possible association between H. pylori and gastric cancer. Of these, the nine (including 571 cases) in which gastric biopsy was used to assess H. pylori status are not reviewed because the detection method is insensitive.34[35][36][37][38][39][40][41]–42 This leaves 25 retrospective case-control studies of H. pylori seropositivity (with 2700 cases). The results of each are given in Figure 1, but in general the controls (particularly those who had gastric disease) were of uncertain validity, and no formal combination of the published findings is appropriate. In addition, there were 10 ‘nested’ case–control comparisons within prospective studies (with about 800 cases, Figure 2) of H. pylori seropositivity and gastric cancer. These prospective studies avoid many of the problems of control selection, but it is still difficult to adjust properly for potential confounding factors. All made adjustments for age and sex, and several also attempted adjustment for smoking, social class or diet. The weighted mean age at baseline of the participants in the 10 studies was 56 years, with a weighted mean follow-up of 8 years (range 2–14 years). There was no evidence of significant heterogeneity among the prospective studies (χ29 = 14.5, > 0.05), and a combined analysis was done that yielded a risk ratio of 2.5 (95% CI: 1.9–3.4; 2P < 0.00001). Subdivision of the published prospective evidence by possibly relevant characteristics (e.g. study location, anatomical site of gastric cancer, duration of follow-up) is not possible because the data are too sparse, or reported in insufficient detail, or both.

image

Figure 1. . Odds ratios in retrospective sero-epidemiological studies of H. pylori and gastric cancer. Areas of black squares are proportional to numbers of cases. Degree of adjustment: + age and sex only; ++ these plus smoking; +++ these plus socioeconomic status; ++++ these plus information on dietary habits.

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image

Figure 2. . Risk ratios in prospective sero-epidemiological studies of H. pylori and gastric cancer.

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DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. References

The present meta-analysis differs from a recent review of H. pylori and gastric cancer55 in two main ways. First, whereas the present review involves a total of 44 different studies including more than 4000 cases, the previous report identified only 19 studies with 2500 cases, leaving it prone to substantial ‘publication bias’.56 Second, the previous review statistically combined the results of all 19 identified studies regardless of the study design. This is inappropriate because retrospective studies of H. pylori and gastric cancer suffer from important methodological limitations due to the use of inappropriate controls and limited adjustment for possible confounding factors. Figure 1 illustrates just such distortions: the use of healthy volunteers can over-represent controls from higher social classes, thereby tending to overestimate the strength of the association of H. pylori with gastric cancer, whereas the use of controls with conditions that are themselves associated with H. pylori infection (e.g. peptic ulceration or chronic gastritis) can lead to underestimation. Moreover, studies that failed to adjust appropriately for age, sex, cigarette smoking and indicators of socioeconomic status, either because such characteristics were not recorded or because they were not measured accurately, were prone to inflated estimates.

The prospective studies, by contrast, use internal controls ‘nested’ within cohorts from which blood samples were taken some years before the onset of clinical disease ( Figure 2). These studies should be more informative than retrospective reports because they avoid selection bias, assess infection before the onset of clinical disease, and assess H. pylori status using serum IgG antibodies (raised concentrations of which are usually reliable indicators of chronic gastric infection57 and, in the absence of specific treatment, generally persist without much fluctuation58). Figure 2 suggests a less important role for H. pylori than claimed in some previous commentaries.6 Moreover, there is still the possibility of some ‘publication bias’56 (whereby some of the studies with less extreme associations may not yet have been published), and of some residual confounding. Conversely, the infection’s role may have been underestimated by studies from developing countries that tested for H. pylori antigens from Western populations (because the prevalence of antigens can vary substantially between regions), or by studies that failed to exclude cancers reported in the first few years of follow-up (because disease itself may render some cases sero-negative).59

If even a two-fold relative risk could be confirmed (or, if stronger associations could be reliably demonstrated with specific H. pylori subtypes, such as cytotoxin-positive strains60), the effect would be sufficient to be of some practical relevance, for H. pylori would then be responsible for about one-quarter of the one million gastric cancer deaths each year. Large randomized trials with prolonged follow-up could ascertain whether some of these deaths might be avoidable by antibiotic treatment in middle age, but, even collectively, the presently recruiting trials of H. pylori eradication are too small (and, at present, too brief) to detect even a 25% reduction in the incidence of gastric cancer.61 In the UK, for example, a trial of H. pylori eradication at age 60 might have to randomize about 100 000 individuals (four times as many as are now being randomized in total worldwide) with at least one decade of follow-up to achieve statistically reliable results.62 Even larger numbers might be needed if there is a delay of some years before benefits emerge, if antibiotic treatments become more widely used for ‘non-ulcer dyspepsia’ or if gastric cancer rates fall more steeply than anticipated.

Whether or not the relatively weak association described in Figure 2 reflects causality, H. pylori cannot plausibly account for the sharp variations in gastric cancer mortality between different populations (e.g. 20-fold higher in certain parts of China than in the USA63) or between past and present (e.g. the five-fold decrease in Scotland between 1950 and 199064). Also, gastric cancer is twice as common in men as in women and may occur less frequently in people with duodenal ulceration, whereas H. pylori occurs roughly equally in both sexes and is strongly associated with peptic ulceration.65 These observations imply that important cause(s) of gastric cancer still await discovery.

In conclusion, published prospective sero-epidemiological studies indicate that gastric cancer is 2 or 3 times as common in those chronically infected by H. pylori, but it is not known how well publication bias and confounding have been controlled.

ACKNOWLEDGEMENTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. References

Rory Collins, Richard Doll, Hellen Gelband, Richard Peto and Martin Vessey commented helpfully; Helgi Sigvaldason provided additional details from a study published as an abstract; Paul Appleby plotted the figures; Young-Ha Go and Francesca Signorelli translated articles. The study was supported by a Frohlich award.

References

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS
  5. RESULTS
  6. DISCUSSION
  7. ACKNOWLEDGEMENTS
  8. References
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