You need to worry about cancer because you have a roughly four in 10 chance of coming down with invasive cancer. (Skin cancers like squamous cell and basal cell are quite common, but rarely invasive.)
Dr. David Gorski is a breast cancer surgeon. He’s looked at the scientific literature on the linkage between diet and exercises, and the risk of developing cancer.
Here’s his conclusion from his review at Science-Based Medicine:
“You can reduce your risk of cancer by staying active and exercising, eating a healthy diet with a lot of plant-based foods and minimizing intake of processed meats, limiting alcohol consumption (although I think the WCRF/AICR guidelines go a bit too far in saying that you shouldn’t drink at all if possible), and maintaining a healthy weight. (Of course, if you stay active and eat a healthy diet, maintaining a healthy weight will probably not be a problem.) Conceptually, it’s easy to do. In practice, as I’m discovering, it’s anything but easy.”
Source: Diet and exercise versus cancer: A science-based view « Science-Based Medicine
The Mediterranean diet seems to protect against cancer.
Steve Parker, M.D.
PS: One of the reasons I write diet books is that I want to keep you from getting cancer.
Paleo Diabetic 
Gorski wrote in the above blog:
“Such were my thoughts over the weekend as I got into a Twitter exchange with an advocate of integrative medicine who was touting the benefits of diet as a cancer preventative and how a course in nutrition “opened her eyes.” That in and of itself wasn’t particularly annoying, although I strongly suspect that the nutrition course she took was not given by actual registered dietitians or other experts in science-based nutrition (she wouldn’t say when questioned)”
Here we go again, the same old mantra “science based evidence”, “registered blah blah”, “experts” and “science based nutrition”. By intoning these mantras the author hopes that we’ll instantly accept what they say as the truth and nothing but the truth.
Unfortunately experience often proves the opposite. When I was first dx’d with T2 the “expert” and “registered” dietician said I could eat wholemeal toast for breakfast without any adverse results.
Except the opposite was the case and after several readings of >10 (180) the penny finally dropped.
As a diabetic you soon begin to realise that the “experts” are often anything but expert in their chosen field.
That’s not to say I’m anti allopathic med. I take my Gliclazide every day but I also take some supplements such as Vitamin B12 Complex that I feel are beneficial.
As regards diet and exercise well talk about the blindingly obvious as regards health benefits although saying that D & E can “reduce your risk of cancer” is so vague as to be almost meaningless.
This is where the old thorny causation vs correlation problem kicks in. Too many variables and not enough data quoted to make any meaningful judgements. For instances the type of person that religiously exercises and watches what they eat every day may very well be a “healthy personality” type anyway who would not get cancer anyway. Whereas the person that is a slob in many ways may have a personality that encourages cancer and other diseases.
Now unlike Gorski I’m not putting this forward as “evidence” – just saying that we need to keep it open as to what causes cancer cos in reality very little is known as to what causes cancer and thus not much can be said about prevention.
Thanks for contribuiting, Sunshine.