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1 School of Population Health, University of Western Australia, Perth Australia; 2 CancerCare Manitoba; Manitoba Department of Health, Winnipeg, Manitoba, Canada; and 3 Surveillance and Risk Assessment Division, Centre for Chronic Disease Prevention and Control, Health Canada, Ottawa, Canada
Requests for reprints: Lin Fritschi, University of Western Australia, School of Population Health M435, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. Phone: 61-8-9380-1308; Fax: 011-61-8-9380-8145. E-mail: lin.fritschi{at}uwa.edu
| Abstract |
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| Introduction |
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We recently completed an analysis of Canadian National Enhanced Cancer Surveillance System (NECSS) data which examined the risk of leukemia, multiple myeloma, and non-Hodgkin lymphoma (NHL) in animal-related occupations (7). An unexpected finding of that analysis was a substantial reduction in the risk for leukemia and NHL among those in fishing occupations [leukemia odds ratio (OR) = 0.4; NHL OR = 0.6]. The effect seemed stronger among fishers rather than fish processors and increased with time exposed. We hypothesized that this association may have been due to increased dietary fish intake in people who work in fish-related occupations.
There are no current effective preventive messages for leukemia and NHL and more evidence is needed on the existence and extent of any protective effect of fish intake.
Therefore, this study was undertaken to determine whether fish intake was protective against leukemia, lymphoma, and myeloma and whether the finding of a protective effect of fish-related occupations on the risk of these diseases was due to dietary intake of fish.
| Materials and Methods |
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A self-completed 60-item food frequency questionnaire (FFQ) was included in the NECSS questionnaire. The FFQ was modeled on two extensively validated FFQs (8, 9) from the United States and was altered very slightly to reflect Canadian national food disappearance patterns. A Canadian nutrient database was used to calculate nutrient values (10). To minimize the impact of seasonality, cases were collected over a continuous 2- to 4-year period: for NHL, three randomly selected cases out of every five cases over 2 years; for leukemia, every eligible case over 2 years; and for multiple myeloma, every case over 4 years. Controls were collected evenly over a 1-year period in the mid-year of the study.
The information collected on dietary fish intake related to diet 2 years (and 20 years) before ascertainment (diagnosis date for cases and date of contact for controls). It included: number of servings per month of fresh, frozen or canned fish and smoked, salted or dried fish; total energy (kilojoules) intake; and total fat intake. Using total energy intake and total fat intake estimated by the FFQ, we calculated the proportion of total fat (% fat) and proportion of total energy (% energy) obtained from fresh fish, smoked fish, and all fish.
Subjects were also asked how their current diet compared to their intake 20 years ago. The relevant question for this study was: "Compared to 2 years ago, 20 years ago I used to consume X chicken or fish" where X = much less, somewhat less, about the same, somewhat more, much more. Because chicken and fish were combined in this question, we could not use it directly, but repeated the analyses above using only those subjects who stated that they ate about the same chicken and fish now that they did 20 years ago.
Subjects also completed an occupational history comprising all jobs held for 12 months or more (job title, industry, major tasks performed, and start and end date). Jobs were coded manually according to whether exposure to fish was likely, whether fish exposure was as packaging or fishing, and the duration of fish exposure (7). The coder was blind to case or control status. Fish exposure was aggregated for each subject by combining multiple jobs with fish exposure into total duration exposed.
The three case groups and the controls were compared using means and cross tabulations with regard to: intake of fresh and smoked fish, energy, and fat; age; sex; body mass index (BMI); smoking status (current/ex/never); and whether the interview was conducted with the subject or a proxy.
Multiple logistic regression was used to compare controls with cases of all three cancers combined (referred to as all LH cancers) and then each cancer separately according to quartiles of servings of fresh fish and smoked fish eaten per week, percentage of total fat intake from fresh and smoked fish, and percentage of total energy intake from fresh and smoked fish. Models were adjusted for age, sex, smoking, BMI, and proxy status. Trend tests were used in conjunction with the categorical analysis to examine dose-response relationships. Because of the small numbers of subjects who ate smoked fish, cancer risk was also examined according to whether subjects ever ate smoked fish. All analyses were repeated stratifying for sex.
To determine whether the protective effect of a fish-related occupation was due to diet, multiple logistic regressions were completed for fish work alone, for fish work adjusting for the above co-variates, and for fish work and dietary fish intake adjusting for co-variates.
| Results |
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Compared to the subjects with cancer, controls were more likely to be a current smoker, had a lower BMI, and were less likely to have had proxy respondents answering the questionnaire (Table 1). Leukaemia cases were more likely than the other groups to be male and have a proxy respondent. Myeloma cases were older and less likely to have ever smoked. There was no association between level of education and cancer status. Controls were more likely to have been in a fish-related occupation.
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The three eastern provinces (Nova Scotia, Prince Edward Island and Newfoundland; n = 1044) had slightly different sampling schemes to the rest of the country. Nova Scotia collected additional controls to complement an extended lung cancer data collection. Newfoundland recruited many of their cases while they attended the cancer clinic, and Prince Edward Island took all cases for all three cancers over 3 years because it has a population of only 100,000. Restricting the analysis to subjects from these provinces resulted in similar results for fresh fish and all fish, although none were statistically significant (data not shown). Results for smoked fish were close to unity and were not statistically significant.
To determine if the protective effect of working with fish was due to the fish workers eating more fish in their diet, the regression analyses were repeated including a variable for whether a subject had ever worked with fish. These analyses showed that the effect of working with fish was independent of the effect of fish intake (Table 5).
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| Discussion |
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0.5 may be important in reducing cancer risk; however, Western diets tend to result in a much lower ratio (11). Estimated energy intakes were low in this study (8405 kj/day for controls) which is probably due to the FFQ design. Assuming underestimation of energy intake was consistent for all respondents, this does not present a problem for our analyses, as to overcome differences in total food intake we examined fish intake relative to total energy intake and relative to total fat intake. One shortcoming of our dietary assessment is that we were not able to separate the consumption of fatty fish, which are richer in EPA and DHA, from lean fish types. However, much of the fish consumed in our Canadian study population is likely to be fatty fish, as these are fished from cold waters.
We found the protective effect of fish to be much stronger in males than females for all three cancers. Case-control studies in Italy reported the risks for myeloma and NHL separately by sex (1). For myeloma, there was a protective effect for both sexes (OR = 0.8 for males and 0.7 for females) although it was not statistically significant. For NHL, there was a protective effect for males (OR = 0.8) but not for females (OR = 1.0) as in our study. However, a case-control study in Japan (12) found a protective effect of eating fish dishes more than three times a week on NHL to be stronger in females.
Leukemia and fish intake was examined in a cohort study in the United States which found no association between fish and seafood intake and later development of all leukemias combined (6). However, when acute myeloid leukemia (AML, n = 48) and chronic lymphocytic leukemia (CLL, n = 58) were examined separately, there was a nonsignificant decrease of AML risk with increasing fish consumption, and a nonsignificant increase of CLL risk. A small case-control study of leukemia in those under 25 years old near a nuclear reprocessing plant found a raised risk with consumption of local fish and seafood but this result was not adjusted for any potential confounders (5).
There have been three case-control studies of myeloma which have reported ORs for fish consumption (1, 2, 4). These have all shown a statistically significant protective effect of fish consumption with the OR for the highest tertile or quartile similar to our finding of about 0.60.7. However, a report of a cluster of seven myelomas in a small Japanese village found the only common factor to be that five of the seven cases had been fishermen for long periods (13). The authors suggest that contaminated seafood may have been the causal factor.
Two case-control studies (1, 2) and one cohort study (3) have found nonsignificant decreases in the risk of NHL with the highest group of fish consumption at a similar level to ours. Another case-control study (14) reported that fish consumption was not associated with NHL risk, but did not show the effect estimates. All these studies were much smaller than ours, ranging from 104 to 429 cases. No relation was found between NHL and consumption of fish or of fish n-3 fatty acids in the Nurses' Health Study (15).
Animal studies have found that consumption of fish oils increases survival of animals with cancer (16).
None of the indicators of fish intake examined in this study could explain our finding of a protective effect of working in a fish-related occupation on risk of leukemia, multiple myeloma, and NHL. We had previously hypothesized (7) that the effect we found was due to diet, as it is known that fishers tend to eat more fish than the general population. Those in fish-related occupations in our study did eat more fish than the rest of the subjects, but when we adjusted for the amount of fish in their diet, the effect of occupation did not disappear. There have been four previous cohort studies of fishers in Scandinavian countries (1720). Generally, the SMRs and SIRs reported in these studies have been between 1.0 and 2.0 for leukemia, around 1.0 for NHL, and between 1.0 and 4.0 for myeloma. All reported results have wide CIs because they are based on small numbers of cases or deaths. The authors of one of the studies have hypothesized that the increase in risk was due to increased intake of persistent organochlorine compounds found in the Baltic Sea (18). A study of commercial fishermen in Atlantic Canada found an increased risk of lymphoma and leukemia when compared to the general population, after adjusting the SMRs so that the all cause SMR was equal to 1 (21). When they examined SMRs by age group, fishers under the age of 45 had a higher risk than older fishers.
One possible explanation of this finding is that people in fish-related occupations may be more likely to eat lobster, crab, mussels, or other shellfish that might provide some protective effect. We were unable to evaluate this component of diet because our FFQ did not capture seafood intake. No previous publications have specifically addressed this issue. Another possible explanation is the discordance in time between the fish-related work and the diet. The fish-related occupation may have been held at any time during the subject's life, while the diet information referred to a time 2 years before the questionnaire was completed. It may well be that the diet questionnaire did not therefore measure the true level of fish intake at the time of the fish-related job.
Apart from these two explanations, it is not clear what other factors may be responsible for our finding among Canadians in fish-related occupations. Fishing is a difficult job which involves heavy physical stress, long working hours including shift work, exposure to noise, extreme weather, vibration, heat, and cold (22). Their working environment has been described as "A cramped mini-factory which pitches and rolls" (22). Similarly, fish processing involves hard, repetitive boring tasks. Few studies have been done evaluating whether high levels of physical activity protect against LH cancers, and the ones which have been published have found no association between exercise and NHL (23, 24). It may be that despite the large sample size and the robustness of the finding within our data, that it is a chance finding which is not repeatable within other data sets.
Advantages of this study include the large numbers of cases and controls, the nationwide population-based sample frame, the case-by-case assessment of occupational fish exposure, and the ability to adjust for a range of potential confounding factors. Selection bias was probably minimal because the response rate for cases and controls was respectable, and proxy respondents allowed us to include data from deceased subjects, at least in the largest province. Although about 10% of subjects were missing FFQ data, inclusion was not associated with case status or with fish occupation. Some of the analyses, especially of multiple myeloma, were based on smaller numbers of exposed cases, and should be interpreted with caution.
In summary, in a very large case-control study, we have found a strong protective effect of fresh fish intake for leukemia, myeloma, and NHL which is consistent with previous literature.
| Footnotes |
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Note: The Canadian Cancer Registries Epidemiology Research Group comprises a Principal Investigator from each of the Provincial Cancer Registries involved in the National Enhanced Cancer Surveillance System: Dagny Dryer, MD, Prince Edward Island Cancer Registry; Bertha Paulse, MSc, BN, Newfoundland Cancer Foundation; Ron Dewar, MSc, Nova Scotia Cancer Registry; Nancy Kreiger, PhD, Cancer Care Ontario; Erich Kliewer, PhD, CancerCare Manitoba; Diane Robson, BA, Saskatchewan Cancer Foundation; Shirley Fincham, PhD, Division of Epidemiology, Prevention and Screening, Alberta Cancer Board; and Nhu Le, PhD, British Columbia Cancer Agency.
Received 8/28/03; revised 11/ 4/03; accepted 11/19/03.
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