Before we wade too deeply into what the research suggests, note that there are no large, well-controlled diet studies.
The challenge with diet studies in cancer, in particular, is that they rely mainly on observational studies, which have many weaknesses. Among them is recall bias, or the ability of subjects to remember what they did.
Durability is also a problem; the studies are not long enough, especially with cancer, which may take decades to develop. Patient adherence and confounding factors, like family history, are other challenges, as are study designs and end points of the studies. Let’s look at the evidence.
Dr. Dean Ornish, a professor of medicine at UC San Francisco Medical School, has done several well-designed pilot studies with prostate cancer.
His research focuses on how lifestyle affects genes. In one of the studies, results of comprehensive lifestyle modifications, including diet, showed a significant increase in telomere length over a five-year period.
Telomeres are found on the end of our chromosomes; they help prevent the cell from aging, becoming unstable and dying. Shorter telomeres may have an association with diseases, such as cancer.
Interestingly, the better patients adhered to the lifestyle modifications, the more telomere growth they experienced. However, in the control group, telomeres decreased in size over time.
In an earlier study with 30 patients, there were over 500 changes in gene expression in the treatment group. Of these, 453 genes were down-regulated, or turned off, and 48 genes were up-regulated, or turned on over just a three-month period with lifestyle modifications.
In both studies, the patients had prostate cancer that was deemed at low risk of progressing to advanced or malignant prostate cancer. These patients had refused immediate conventional medical therapy.
Both studies involved intensive lifestyle modifications that included a low-fat, plant-based, vegetable-rich diet.
The EPIC trial
Considered the largest of the nutrition studies is the European Prospective Investigation into Cancer and Nutrition (EPIC). Results showed that fruit may have a statistically significant impact on lung cancer, with a 40 percent decrease in the risk of developing lung cancer in those that were in the highest quintile of fruit consumption, compared to those in the lowest quintile.
However, oddly, vegetables did not show an impact. The results were most pronounced in the northern European region.
Ironically, it seems that some other studies, mostly smaller studies, show potentially beneficial effects from fruits and vegetables. This may be because it is very difficult to run an intensive, well-controlled, large study.
A meta-analysis involving a group of 24 case-control studies and 11 observational studies looked at the effects of cruciferous vegetables, also sometimes referred to as dark-green, leafy vegetables. It showed a significant reduction in colorectal cancer.
In another study, cruciferous vegetables were shown to significantly decrease the risk of developing multiple cancers, including esophageal, oral cavity/pharynx, breast, kidney and colorectal. There was also a trend that did not reach statistical significance for preventing endometrial, prostate, liver, ovarian and pancreatic cancers.
The most interesting part is that the comparison was modest, contrasting consumption of at least one cruciferous vegetable a week with none or less than one a month.
In conclusion, it would appear that the data are mixed in terms of the effectiveness of fruits and vegetables in preventing cancer or its progression. The large studies have flaws, and pilot studies require larger studies to validate them.
However, imperfect as they are, there are results that indicate that diet modification may have an impact on preventing cancer or its progression.