Kristi Funk is a breast cancer surgeon, famous (I assume) for treating Angelina Jolie and Sheryl Crow (I assume these are famous people). She has a new book to promote and has been doing publicity for it. So there are plenty of interviews with her where she give advice about how to look after your breasts.
One of the pieces of advice is that diet is a major cause of breast cancer. In this interview in The Times (paywalled if you've exceeded your weekly quota) she argues for a vegan diet:
Pretty much overnight she changed her family's diet to avoid dairy and meat.
She describes meats as carcinogens:
...processed meats such as sausages and, ham and bacon, which are carcinogens...
She thinks that:
it is crystal clear that the body's cellular response to animal protein and fat is nothing but dangerous.
Her interviewer recognises that this is not mainstream opinion:
The Cancer Research UK website advises women that research on the link between diet and cancer is "inconclusive and inconsistent".
But the interviewer still quotes some specific claims about diet that have what look like very significant numbers in them:
Those eating a teaspoon a day of ground flaxseed for a year showed ... reduction in biomarkers for cancer of 80 percent.
One study showed that those adding [olive oil] to their diet are 68% less likely to get breast cancer.
Funk appears to be making strong claims (but this is just an interview and doesn't come with references so they are hard to validate). The specific, if scattershot, references to the relationship between diet and cancer risk seem like big claims (a 68% or 80% reduction is a very significant reduction).
But mainstream medicine (like Cancer Research UK) don't seem to agree and have not concluded that the links with diet are either large or significant. Moreover, nutritional epidemiology is a notoriously unreliable field (Ionnadis described it as "at the bottom of the septic tank of science") where almost any result is possible and almost every result is wrong. And Funk has, according to this blogger, referred patients to acupuncturists and Ayurvedic specialists (neither field being well known for their use of high quality evidence.)
It is possible she is cherry-picking radical results that don't represent the collective knowledge we have about the relationship between diet and cancer. She claims that animal products are basically toxic. She is clearly making a strong claim that vegan diets lead to a large reduction in breast cancer. Are her claims credible?
To keep the question simple: is there a strong and consistent relationship between switching to a vegan diet and a large reduction in breast cancer incidence?