Dietary factors associated with a low risk of colon cancer in coloured west coast fishermen
Schloss, I.; Kidd, M.S.; Tichelaar, H.Y.; Young, G.O.; O'Keefe, S.J.
South African Medical Journal 87(2): 152-158
1997
ISSN/ISBN: 0256-9574 PMID: 9107220 Document Number: 240
Objective: To examine the relative roles of dietary fibre, anti-oxidant vitamins and fish oils in the relatively low incidence of colon cancer in colored West Coast fishermen. Design: Dietary intake survey, based on food frequency questionnaire and household surveys. Blood sampling for vitamin and fatty acid concentrations. Setting: Isolated West Coast fishing villages (sample population) and urban Cape Town inhabitants (controls). Participants: 101 male and female West Coast fishermen over the age of 40 years, and 99 age- and sex-matched urban whites. Outcome measures: Dietary analysis by 'Foodfinder' technique, and nutrient blood levels. Statistical analysis using SASR program version 6. Results: An analysis of the number of recorded cases of colorectal cancer over a period of 5 years leading up to the study confirmed a lower rate of colorectal cancer in the West Coast fishermen than in the Cape Town population: fishermen 6 cases/120,000, urban whites 677/2 million. A significantly higher proportion of fishermen were smokers, had hypertension and gave a history of previous tuberculosis infections. Vitamin supplementation was noted in one-third of whites but in no fishermen. The results of the dietary analysis (mean (SD)) demonstrated that fishermen consumed less fibre (9.9 (4.7) g/d v. 17.4 (7.2) g/d; P lt 0.01) and less fruit and vegetables (190 g/d v. 365 g/d; P lt 0.001), with a lower intake of vitamin C, thiamin, riboflavin, vitamin B-6 and folate. Sodium intake was higher in fishermen, whereas intakes of potassium, magnesium, calcium, iron, zinc and copper were lower. In addition, the daily intake of omega-3 fatty acids (fish oils), docosahexaenoic acid (C22:6) and eicosapentaenoic acid (C20:5) in the West Coast population was considerably higher (0.71 (0.51) g/d v. 0.14 (0.09) g/d; P lt 0.0001 and 0.37 (0.3) g/d v. 0.05 (0.05) g/d; P lt 0.0001, respectively) than in the urban population owing to their higher intake of fish (110 g v. 30 g/d; P lt 0.001). Plasma fatty acids correlated positively with these dietary intake figures, showing fishermen to have higher levels of circulating omega-3 fatty acids, C20:5 (3.9 (1.8)% v. 0.66 (0.29)%; P lt 0.001) and C22:6 (5.6 (1.8) v. 2.9 (0.08)%; P lt 0.001) and lower levels of omega-6 fatty acids, C18:2 and C20:3 (29.7 (4.7)% v. 33.5 (5.6)%; P lt 0.05 and 0.85 (0.48)% v. 1.5 (0.47%; P lt 0.001, respectively) compared with the urban group. Conclusion: The low incidence of colorectal carcinoma in West Coast fishermen might be explained by the protective effects of fish (omega-3) oils, but not by other 'protective' dietary items such as fibre, anti-oxidant vitamins or calcium. The high incidence of hypertension may be related to a higher sodium intake, accompanying high consumption of salted fish, or to the higher prevalence of tobacco smoking.
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