H Gilbert Welch and his colleagues describe wonderfully the problems of assessing the effects of dietary fat in The Washington Post
"The best data to date suggesting the potential for diet -- or any lifestyle alterations -- to affect cancer risk is limited. The single notable exception is smoking: There is no doubt that not smoking dramatically lowers cancer risk.
The effect of diet on cancer is likely to be small for most people because diet is so heterogeneous and the effect of any given food may depend on its interaction with other foods. And the smaller the effect, the harder it is to demonstrate statistically. So it is really not surprising that results of research about diet and cancer flip-flop. Low-fat diets probably do lower the risk of breast cancer -- but the effect on risk is small -- particularly for women with no prior history of the disease. Changing diet to reduce breast cancer -- or any other cancer -- is a personal decision, not an imperative."
Its very counter-intuitive. All the five-a-day campaigning is partly premised on the idea that better diet will reduce cancer risk. Yet the evidence about dietary fat is not quite there.
The trouble is that there is a gulf between what we know for certain and what we might presume. That said, few doctors would advise their patients anything other than switching to healthier diets for the prevention of a range of other diseases as well as cancer.