Dietary factors associated with a low risk of colon cancer in coloured west coast fishermen.

OBJECTIVE

To examine the relative roles of dietary fibre, anti-oxidant vitamins and fish oils in the relatively low incidence of colon cancer in coloured 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/120000, 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 < 0.01) and less fruit and vegetables (190 g/d v. 365 g/d; P < 0.001), with a lower intake of vitamin C, thiamin, riboflavin, vitamin B6 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 < 0.0001 and 0.37 (0.3) g/d v. 0.05 (0.05) g/d; P < 0.0001, respectively) than in the urban population owing to their higher intake of fish (110 g v. 30 g/d; P < 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 < 0.001) and C22:6 (5.6 (1.8) v. 2.9 (0.08)%; P < 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 < 0.05 and 0.85 (0.48)% v. 1.5 (0.47)%; P < 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.