What causes cancer? In 2015, researchers updated the landmark 1981 study from the U.S. Congress Office of Technology Assessment noting that the original estimates had “generally [hold] true for 35 years.” At 35% of the attributable risk, tobacco was the single largest contributor to cancer. But very close behind, was our diet, which researchers estimated contributed between 30–60% of the risk. Generally accepted as true, the far more contentious question is “What part of the diet contributed to the risk of cancer?”
The legendary Irish surgeon Denis Burkitt noted in 1973 that many diseases characteristic of modern Western civilization were noticeably absent where he worked in rural Africa. Cancer, specifically colorectal cancer was one of these diseases. Burkitt hypothesized that diet was the main differentiating factor, and specifically, the fiber. The traditional African diet contained a lot of fiber — a lot. This bulked up the stool, leading to frequent and large-volume bowel movements. Big lush piles of poo. Not the measly, rabbit pellet stools of the European ex-pats. Perhaps the regular movement of the stool cleared the intestinal system, preventing decaying and putrefying of foods inside the colon, which might be carcinogenic. The high stool volume meant frequent “cleansing” bowel movements. Eating more fiber was enthusiastically championed as an easy way to improve health and reduce cancer. From the 1970s to the early 2000s, people heartily ate more ‘roughage’ in the hopes of preventing cancer. But those efforts were largely futile.
By the mid 1990s, studies established that eating more dietary fiber played little or no role in reducing the risk of colon cancer. The Nurse’s Health Study, with over 88,000 women over 16 years of follow up, found that those women eating the most fiber had essentially risk of cancer as those eating the least. Other studies were equally discouraging. The Toronto Polyp Prevention Group, the Australian Polyp Prevention Project, and a randomized controlled trial published in year 2000 in the New England Journal of Medicine all confirmed that a high fiber diet did not reduce cancer.
The next suspect was dietary fat, particularly saturated fats. There was no real reason to suspect that dietary fat should cause cancer. After all, humans had been eating fats, including saturated fats such as animal fats (e.g., meat, dairy) and plant fats (e.g., coconut oil, olive oil) for millennia.
Nobody had any idea how dietary fat caused cancer. Anecdotally, there were few observations suggesting that people who ate a lot of fat got a lot of cancer. The Inuit ate plenty of animal fats from whale and seal and South Pacific Islanders ate lots of coconuts, full of coconut oil, a highly saturated fat. Both populations enjoyed low cancer rates despite eating plenty of fat for centuries, and the vegetarians of India, eating very low-fat diets of mostly grain weren’t protected from cancer. But it didn’t really matter, because from the 1970s to the 2000s, the entire scientific world had hopped on the fat-is-bad bandwagon, so they logically concluded that it probably caused cancer. Who needs proof if you have dogma? Blame-dietary-fat-for- everything-bad was the name of the game. So, play on!
The Women’s Health Initiative randomized women to a low fat diet and measured their risk of breast and colon cancer. Unfortunately, in both cases, a low fat diet failed to prevent or reduce these cancers. What was next?
The next thought was that perhaps cancer was due to some vitamin deficiency, just as scurvy was a disease of too little vitamin C. First up was vitamin A and its precursor beta carotene. In a 1996 controlled trial, participants were randomly given supplements to see if this would prevent lung cancer. Unfortunately, the results were not good. The group that took vitamin A supplements didn’t get less cancer, they got more cancer.
Next up was vitamin B. Several studies were done, and when the results were analyzed, once again the results were not good. Those taking more vitamin B didn’t get less cancer, they got more cancer.
The same disappointing results were seen with vitamin C, with no evidence of decreased cancer. However, at least here, there was no indication of increased cancer. A metanalysis of all the randomized studies done using vitamin C supplements showed no overall benefit.
Most recently, there was high hopes that vitamin D supplements would decrease the risk of cancer. Alas, recent studies also show no reduction in cancer rates with the sunshine vitamin.
Next up? The potent antioxidant vitamin E, which had also failed in preventing heart disease. Sadly, Vitamin E supplements did not reduce cancer risk.
Diet and Cancer
So, after years of intensive and expensive medical research, we were left with some wholly unsatisfying conclusions.
1. Diet plays a large role in cancer
2. Lack of fiber does not cause cancer.
3. Excess dietary fat does not cause cancer
4. Vitamin deficiency does not cause cancer.
While vague, these important conclusions have literally cost decades of research time and millions of dollars. But one hugely important question remains unanswered here. If all these dietary factors did not cause cancer, what did? The answer would not become apparent until the mid-2000s.
Obesity and Cancer
The Cancer Prevention Study II, a large prospective cohort study, began in 1982. This massive scientific undertaking required 77,000 volunteers simply to enroll all the participants, which numbered over 1 million. The participants (average age: fifty-seven) were healthy and free of any detectable cancer at the beginning of the study. Every two years, they were tracked to see who had died and why. In 2003, the data reached a then-novel and stomach-churning conclusion: obesity, already a well-known risk factor for diabetes, heart disease, and stroke, also significantly raised the risk of cancer.
Not all cancers are related to obesity. Lung cancer, for example is largely caused by tobacco smoke, and whether you are overweight or not makes little difference to the risk of lung cancer. However, a number of other cancers were strongly influenced by obesity, including the very common breast and colorectal cancers.
Currently the World Health Organization lists at least 13 different types of cancer as obesity related, with several more that are suggestive. The recognition that obesity plays a large role in cancer causation is at once both good and bad news.
The bad news is that obesity epidemic that is currently rampaging throughout the world has shown little sign that it is abating. The good news is that if we can provide reasonable strategies to attain a normal weight, we can reduce the risk of these cancers that are causing so much suffering. Intermittent fasting is one such ancient strategy that may have found its time. Read more in the new book, The Cancer Code.