Discussion
DIETARY FACTORS
Ecological studies consistently find that relatively
high levels of risk of CRC are associated
with high intakes of meat and fat and low
intakes of fibre.6 17 18 COMA reported that the
balance of evidence is that fish consumption is
not associated with CRC.1 However, there
seems to be an additive eVect of meat and fish
(table 2), which may reflect the overall intake of
animal protein and fat.
Vegetarians are generally at lower risk of
CRC than meat eaters.19 The apparently
protective eVect of a vegetarian diet, and
specifically of fibre, might be attributable to
plant components of diet, perhaps antioxidants,
or it might result from lack of exposure
to meat and fat. The Working Group on Diet
and Cancer of the Committee on Medical
Aspects of Food and Nutrition Policy
(COMA) reported that the summarised evidence
on antioxidants is inconclusive.1 In our
study, simple correlation showed an association
between high incidence rates of CRC
and low levels of cereal and vegetable
consumption. However, multivariate modelling
showed that high levels of meat consumption
rather than low levels of cereal and
vegetables were the important factors. Vegetables
played a part in our model when olive oil
was omitted, but lost significance when it was
included.
The association between CRC and fruit in
our study, after adjusting for meat, fish and
olive oil, was unexpected. This association may
be confounded with some aspect of aZuence.
The COMA report concluded that there is
limited and inconsistent evidence about fruit
consumption and CRC.1
Olive oil was found to be negatively associated
with CRC after allowing for the other
dietary factors. The moderating influence of
the olive oil eVect may be best understood by a
consideration of how olive oil and bile salts
might interact together.