Dietary fiber showed no preventive effect against colon and rectal cancers in Japanese with low fat intake: an analysis from the results of nutrition surveys from 23 Japanese prefectures

It is currently believed that dietary fiber and dietary fat are the most significantly associated dietary factors in colorectal carcinogenesis [1‐3]. Current epidemiological evidence would suggest that dietary fat is a more significant risk factor for colorectal cancer than the evidence indicating dietary fiber to be a preventative factor [1‐4]. Furthermore, since Fuchs' report [5] in 1999, the reported protective effect of dietary fiber on colorectal carcinogenesis has led many researchers to question its real benefit. However, care should be taken in ascertaining the effect of dietary factors in carcinogenesis, including colorectal carcinogenesis, as the protective effects of dietary factors on carcinogenesis may not only be based upon the balance of the overall diet, but also on the interaction of specific dietary components, such as dietary fiber, nutrients and other foods. Furthermore, Fuchs' results generated many points of controversy against the study, such as the inaccuracy of the dietary survey (using a food-frequency questionnaire); the basic values for fiber may not merit such analysis (the data on fiber are drawn from a 1976 article by Southgate et al., which gives fiber values for a limited number of foods in the United Kingdom); and the possible effect of fiber consumed early in life on the risk of colorectal cancer cannot be ruled out; no assessment of colonic function was taken into consideratioon (so the Fuchs' article leaves unchallenged the evidence that bulkier stools and faster transit through the gut reduce the risk of colorectal cancer) [6] and so on. It is thus logical to attempt to extend Fuchs' cohort study to include Japanese subjects, whose intake of fat is lower than the US subjects, and to see if the study arrives at the same conclusions. Furthermore, care should be taken in estimating the result of intervention studies [7, 8], and further studies are thus required in other national groups.

As a nation, Japan is unique in that accurate regional dietary data are available, because many prefectures have periodically performed dietary surveys using similar methods. Japan is therefore an appropriate model to analyze geographically the relationship between dietary habit and colorectal cancer.

To evaluate the association between diet and colorectal cancer, we examined the correlation between dietary intake around 1990 and the standardized mortality ratios (SMRs) of colon and rectal cancer between 1995 and 1997 in 23 Japanese prefectures.

Assuming that diet is a factor involved in the development of colon and rectal cancers, any dietary-related effect would be expected to become apparent after a certain latent period, the duration of which is however unknown. Furthermore, the period between the development of colon and rectal cancers and death due to these disease is also unknown. For this study, the authors essentially defined the period in which dietary intake is able to have an influence on death from colon cancer as approximately 10 years. This is a limitation of this study, because it is clear that 10 years is too short. Furthermore, there were differences in the survey year among the 23 prefectures examined up to a maximum of 4 years. This is also a potential limitation.

From the previous case-control and cohort studies in Japan concerning fiber source foods and their preventive effect on colon and rectal cancer, three reports showed significant preventive effects in vegetables and fruits from a total of nine reports, and two out of five reports on rice showed a preventive effect. As for beans or seaweeds, one and two reports from respective totals of five and three showed significant preventive effects [13]. Only two reports from Ohta and colleagues examined the relationship of pure dietary fiber on colorectal cancer or colon polyps. The authors reported that dietary fiber intakes in patients with colorectal cancer and colon polyp were higher than those in controls [10, 14]. On the other hand, one report from a total of five on dietary fat showed a promoting effect. As mentioned above, there are not many studies which show significant relationships between the effects of either dietary fiber or fat on colorectal cancer in Japan.

In this study, it was shown that there were significant relationships between fat intake and colon and rectal cancers, but no relationship between dietary fiber intake and colon or rectal cancers, thus demonstrating that the promoting effect of dietary fat was greater than the protective effect of dietary fiber. Furthermore, seaweeds and beans also demonstrated a carcinogenic promoting relationship with colon cancer even though these are well known as a source of dietary fiber. Hence, our data failed to show any protective effect of dietary fiber on colorectal carcinogenesis in subjects in the low fat intake group (Japanese; 25% in 1977–1978 and 29% in 1994–1995 in men and women, respectively) [15] in this analysis, while Fuchs showed the same result in subjects in the high fat intake group (US subjects; 42% and 40% in 1977–1978 and 34% and 32% in 1994–1995 in men and women, respectively) [16]. However, we could not divide the total fat intake into animal fat and plant fat, so that it is unclear which type of fat has more influence on these cancers.

On the other hand, vitamin A (including carotenes) [17, 18] and vitamin C [19, 20] are believed to offer beneficial dietary factors against colorectal cancer through their anti-oxidative action, though the preventive effects of these dietary factors have not been reported in Japan. In this study, we also obtained similar results, even though the correlation coefficients were lower than those of fat.

The current results revealed that the number of nutrients and food groups that were significantly related with colon cancer was greater than those related to rectal cancer. This would suggest that colon cancer is affected by environmental factors, including the dietary factor, more than rectal cancer.