One that prevents hyperglycemia.

Since cancer cells can't typically burn fat, they depend on glucose for their energy supply, the Warburg Effect ( http://en.wikipedia.org/wiki/Warburg_effect ). If you have high blood glucose levels then you are potentially feeding the cancer cells.

Jaminet has this to say: "Higher blood glucose levels provide more fuel to tumour cells, stimulate their proliferation and hasten the progression of the cancer. If high blood glucose induces cells to start burning glucose, it may actually cause cancer by switching them from normal metabolism to tumour cell metabolism."

Some of this is obviously speculation, but it seems like a sensible explanation given that cancer is a "disease of civilisation", which is especially prevalant in our "55% energy from carb" society.

So cutting carbs (and preventing hyperglycemia) might be beneficial. Cutting them further and inducing ketosis might have an even stronger preventative influence on cancer.

Jaminet again: "In contrast, ketogenic diets which keep blood glucose levels low and provide ketone bodies to mitochondria have shown remarkable effectiveness in slowing cancer progression. In vitro, ketone bodies have been shown sometimes to restore mitochondria to health, triggering cancer cell death. Ketogenic diets have been shon in clinical trials to slow the progress of brain cancer, and clinical trials of other cancers are in progress."

Jaminet suggests a low-carb, low-protein, ketogenic day once a week as a hedge answer cancer.

A diet high in plant flavonoids, low in sugar, low in PUFAs, with good O3 and CLA input and hormone-free meats seems to be what is recommended to me as best practices. I just had a breast cancer tumor removed a few weeks ago. While I run into a few patients and books that push a vegetarian diet, even some of those concede that meat is not as big of a problem as the hormones and lack of CLA in most available meats.

A few answers have said that avoidance of PUFAs is important. I don't advocate sucking down fish oil, but I think avoidance of linoleic acid is the important part, not avoidance of all PUFA's. I think linoleic acid is a strong promoter of cancer.

Korion cited this study involving rats red different oils and given the carcinogen dimethylbenz(a)anthracene. The authors stated "Mammary tumorigenesis was very sensitive to linoleate intake".

In this study, rats were fed corn oil, coconut oil, and a fish oil and exposed to a carcinogen that induces mammary tumors. The fish oil and coconut oil resulted in the same amount of mammary tumors, while the corn oil produced significantly more tumors. The authors stated "The protective effect of menhaden oil and coconut oil may be due, at least in part, to the decreased linoleic acid content of these diets relative to the corn oil diet".

In a study in which rats injected with the carcinogen azoxymethane, rats fed corn oil and safflower oil (high in linoleic acid) had increased numbers of colon tumors, while rats fed coconut oil, olive oil, and MCT oil (low in linoleic acid) did not.

In another study in which rats were fed sunflower, rapeseed, olive, and coconut oil, the authors wrote "Our results indicate that a high linoleic acid diet may contribute to oxidative stress in the liver of female rats leading to a marginal increase in oxidative DNA-damage".

In perhaps my favorite of these studies, rats were fed corn, olive, coconut, and safflower and cancer was initiated using N-nitrosomethylurea. There was a direct correlation between the linoleic acid content of the diet and tumor incidence. The study's authors suggest linoleic acid increases tumor incidence via its role as a prostaglandin precursor.

Of course, these studies were all performed in rats given a carcinogen. The practical application of these studies is thus a bit limited, but this study is an interesting one. In a controlled clinical trial involving 846 men in which vegetables oils were substituted for saturated fats in the diet, cancer increased notably in the vegetable oil group.

There's a lot to diet and cancer (and is what got me into studying nutrition), but I think one important component that deserves attention is avoidance of linoleic acid.

Avoidance of PUFA is likely the most important thing of all, see this study. Things like cascara, aspirin, ... can help.