Even after over 40 years of research, there is no definitive answer on what causes cancer. Sure, we know that direct exposure to toxic chemicals can cause cancer, and many people believe that the pesticides and preservatives in our food supply are culprits.
This could be, but cancer has been around a very long time. The first writings about cancer came from the Ancient Greeks. The Egyptians also experienced cancer. Presumably, there weren't any toxic chemicals being added to the food supply or lurking in the workplaces in those ancient time periods. So what caused cancer back then?
Again, there's no definitive answer, but there are few facts which make interesting "food for thought" on the subject of what causes cancer:
Dr. Tom Cowan has written an interesting article on a possible link. In 1842, a French physician name Stanislas Tanchou presented a paper titled "Memoir on the Frequency of Cancer" to the Paris medical society. He claimed that he could predict the exact cancer rates in every major European city over the next 50 years based on the extent of "civilization" of the city. Tanchou showed that the rates of cancer were higher in cities and lower in rural populations. He predicted that as civilization increased, the rates of cancer would increase. One of the factors involved in industrialization and the infiltration of civilization is a higher consumption of processed food products containing sugar and grain.
Dr. Tanchou predicted that cancer would never be found in hunter-gatherer societies. Hunter gatherer tribes known around the world then were being studied, and various researchers reported remarkably that no cases of cancer were found. It was only after these populations adopted a Western grain/sugar based diet (the diet of civilization) that cancer appeared.
Cancer rates are very rare and even non-existent in primitive cultures who consume a hunter-gatherer diet (meaning mostly animal foods, some plant foods, but no grain).
Here's an excellent interview with author Gary Taubes in which he discusses the fact that native peoples eating traditional diets don't get cancer:
People with celiac disease, a specific autoimmune reaction to wheat gluten, are known to have hugely increased rates of many types of cancer. Intestinal cancers are found up to 100 times as frequently in celiacs as in people without celiac disease. They also experience 12 times the risk of esophageal cancer, and breast cancer is twice as common in celiacs.
In 1924, the German Nobel laureate Dr. Otto Warburg first published his papers on a phenomenon he saw in fast-growing tumors. He found that cancer cells appeared to fuel themselves exclusively through glycolysis. Glycolysis is the metabolic process of breaking sugar down for fuel purposes. Cancer cells thrive on sugar and it's the only fuel they can use to stay alive. Warburg believed that this difference in the cellular fuel process of a cancer cell was the key to understanding what causes cancer.
In contrast, healthy cells can use either glucose or ketone bodies from fatty acids (from ketosis) to make energy and thrive.
Low levels of vitamin D in the bloodstream are associated with higher risks for many types of cancer. There's been a lot of research done on this topic lately, and the strongest associations between low vitamin D levels and cancer were found with colon and breast cancers. However, there are also associations with low vitamin D and non-Hodgkin lymphoma, ovarian cancer and kidney cancer. In America, the USDA food pyramid counsels people to avoid the foods that are rich in vitamin D, including natural fats like butter and egg yolks. Fatty fish is another source and fears about mercury contamination have decreased consumption of these foods as well. In addition, Americans are counseled to avoid sun exposure. All of these factors point toward an epidemic of vitamin D deficiencies in the American population.
Vegetable oils such as corn oil increase the growth rates of tumors. Vegetable oils are polyunsaturated fats (PUFAs for short) and high in Omega 6 fatty acids. PUFAs are highly volatile, and easily oxidized by light and heat factors. When a fat oxidizes (the scientist's word for going rancid) in the body, it causes a release of free radicals which inflame the cellular structures involved. This "lipid peroxidation" process is at the root of the inflammation involved in many disease processes. In contrast, saturated fats such as butter, coconut oil and lard don't oxidize easily, and cause less inflammation and free radical formation in the body.
Chronically high levels of blood sugar and circulating insulin are linked to higher rates of cancer. Switching to a ketogenic diet is an effective cancer treatment because it reduces both blood sugar and insulin levels.
Other Factors to Consider
What about all the money spent on cancer research? Billions of dollars have been spent on the question of what causes cancer. And sadly, the results of all that spending are pretty dismal.
Cancer drugs are more damaging to many people than the original cancer, and none of these drugs offer a cure or even better survival rates, 50 years into the cancer fight. At least one researcher has written that people diagnosed with cancer actually live longer if they avoid the mainstream cancer treatments.
Cancer treatments using the Ketogenic Diet are much more promising. Basically, the idea is to starve the cancer of the sugar it needs to live. The ketogenic does that pretty effectively and without damaging the other healthy cells. There are quite a few studies being conducted testing this theory.
When you look at the facts above, it seems pretty clear that a high carbohydrate, grain and sugar based diet has at least a plausible connection to what causes cancer.
It's just this basic. People who eat a diet free of grain and sugar rarely get cancer. Humans who eat a grain and sugar laden diet develop cancer at higher rates.
This would explain why the Greeks and Egyptians had cancer even though they didn't have the processed foods, environmental toxins and chemical preservatives we have today.
Bread dipped in olive oil made up a large part of each meal for the ancient Greek peoples. The Ancient Egyptians' diet consisted mostly of bread and beer, both made from grain.
Resources for Further Reading
Here's a video from Craig Thompson, MD, the current president of the Memorial Sloan-Kettering Cancer Center, on his staff's current thinking about what causes cancer. The whole video is interesting, but at about the 17:40 minute mark, the rubber hits the road. He talks about how cancer cells mutate to absorb more glucose than normal, and how the overconsumption of carbohydrates dramatically increases your cancer risk. This parallels Otto Warburg's "cancer cells need sugar" hypothesis.
Here's another paper which discusses the "Warburg Effect" and cancer as a metabolic disease.
This paper discusses "How a Low Carbohydrate, High Protein Diet Slows Tumor Growth and Prevents Cancer Initiation."
Dr. Dominic D'Agostino, an Assistant Professor and cancer researcher at the Hyperbaric Biomedical Research Laboratory at the University of South Florida has a KetoNutrition blog on which he writes about ketogenic diets for cancer treatment.