In the past several decades, there has been considerable interest in lycopene-rich foods, particularly tomatoes and tomato products, in lowering a man’s risk of getting prostate cancer. In the previous AICR report, the strength of evidence for a benefit was viewed as “probable” for lycopene-rich foods, but in the latest round, the recommendation was lowered to “limited, no conclusion.”
To understand this change, it is important to examine the nature of the evidence used to reach the new conclusion. Most of the evidence is based on studies that record what men are eating, or measure blood lycopene levels, and then follow the men for any diagnosis of prostate cancer.
Then dietary or blood factors are linked to risk of cancer diagnosis. Statistical methods are used to account for other factors. Because these studies are examining associations, which may not necessarily be causal, other considerations such as biologic plausibility are taken into account in formulating the conclusions.
A big difference between the earlier studies and more recent studies is that the former were conducted in the era prior to widespread PSA screening, while the latter studies were conducted primarily in populations where PSA screening was highly prevalent. The high prevalence of PSA screening has two important influences on the study of prostate cancer:
- PSA screening leads to the detection of a wider spectrum of cancers, many of which are indolent and would never had come to clinical light without the PSA test.
- The cancers are diagnosed typically earlier in their natural history, at a time before they have revealed any indication of aggressive behavior, such as metastasis.
These two factors are important because they have made it more difficult to uncover risk factors for the important subset of prostate cancers that would progress. We may be mixing cancers that are not influenced by diet and those that are.
In the earlier studies prior to PSA screening, which were more promising for a benefit of lycopene rich foods, the cancers detected were much more aggressive when diagnosed. Some of these complexities introduced by PSA screening can potentially be taken into account in study design and analyses, but frequently the required information on PSA screening history, particular type of prostate cancer, treatments, and follow-up to metastasis is not available.
It is important to note that the report did not conclude that tomato products are not beneficial, but rather, that the data are too mixed at this time to reach a conclusive decision. In fact, some data are very promising.
Tomato products or lycopene have strong anti-cancer effects in a number of animal models of prostate cancer. A recent human study showed that high intake of dietary lycopene, largely from tomatoes, was strongly correlated with less blood vessel formation in prostate cancers. New blood vessel formation is critical to feed prostate cancers, and in fact, in this study, cancers with more new blood vessel formation were much more likely to progress to metastasis than those with few new vessels.
We are finding that just pooling all prostate cancers as one entity is an inadequate way to study it. Newer studies are now considering specific factors in the prostate cancer, such has specific types of genetic damage or blood formation. I expect that firmer conclusions on dietary factors and prostate cancer can be reached over the next several years.
Edward Giovannucci, MD, ScD, is Professor in the Departments of Nutrition and Epidemiology at the Harvard School of Public Health, and an Associate Professor of Medicine at the Harvard Medical School/Brigham and Women’s Hospital. His research focuses on how nutritional, environmental and lifestyle factors relate to various malignancies, especially those of the prostate and colorectum. He was the Panel lead for the Continuous Update Report on Diet, Nutrition, Physical Activity and Prostate Cancer.