A Conversation with Dr. Walter Willett about Diet and Cancer
Dr. Walter Willett, chair of the department of nutrition at the Harvard School of Public Health, recently delivered a lecture titled "Diet & Cancer: The Fourth Paradigm" on the NIH campus in Bethesda, MD. An archived videocast of the lecture, sponsored by NCI's Cancer Prevention Fellowship Program, is available online.
How have views on diet and cancer evolved over the last 40 years?
When I started in this area in the 1960s, the thinking revolved around carcinogens in food. These were chemicals produced by high temperatures, such as with barbecuing, that had been shown to cause DNA mutations in animal models and test systems. In fact, this topic has not been totally resolved, but if carcinogens in food were a major problem for humans, we probably would have seen more evidence than we have. This was the first paradigm.
What were the second and third paradigms?
The second paradigm was the idea that fat in the diet is a major cause of cancer. There was never any strong evidence for this idea, but it was repeated so often that it became dogma in the 1980s and 1990s. For conditions such as heart disease and diabetes, the type of fat in the diet is quite important. But the hypothesis that the percentage of calories from fat in the diet is an important determinant of cancer risk, at least during midlife and later, is not supported by the data.
The third paradigm was that fruits and vegetables dramatically reduce risks of cancer. But, as the prospective data came in, the results just did not support this idea either. That's not to say there's no benefit from fruits and vegetables, but [the benefit is] probably very small and limited to certain foods and certain cancers. Overall, we just don't see a relationship.
That brings us to the fourth paradigm.
The fourth paradigm is that a major cause of cancer is excessive adiposity [obesity]. This paradigm, also referred to as positive energy balance, is here to stay, because the evidence is overwhelming from all types of studies. These findings have coalesced from research over the last 10 to 15 years, but the evidence to support this idea actually goes back to animal studies in the 1930s. In a sense, it was right there in front of us all the time.
Can you put the role of obesity and cancer risk in context?
On a population level, the number of cases of cancer attributable to people being overweight and obese is about equal to the number attributable to current smoking. This is in part because smoking is going down and obesity is going up; in terms of importance within a population, they are in the same ballpark. However, on an individual basis, the cancer risk due to smoking remains substantially higher than that due to obesity.
Are you looking for clues to obesity and cancer in younger people?
Yes, this is one of the new frontiers in cancer research. Until now, we've been looking at a pretty narrow period of life—essentially around the time people are getting cancer. But we have lots of epidemiologic hints that many factors operate earlier in life and maybe even across generations.
What are you learning?
For the first time, we are getting a pretty good look at the diets of adolescents and cancer incidence. In the Nurses' Health Study 3, we have retrospectively collected details about high school diets from participants, who were 25 to 42 years old at the time of enrollment. They weren't so far beyond their high school diets, so we have some data that is recalled pretty well.
In addition, we collected data from their mothers about their mothers' experiences with the pregnancy resulting in our participant and the participant's infant feeding patterns and diet and activity before age 5. We're just starting to get follow-up from that information. This approach can piece together the lifespan, which I think will be necessary for a thorough look at diet and cancer.
If money weren't a factor, what kind of study would you launch?
The most critical missing elements in the research right now are the dimensions of time—the time when we start the studies, and the length of follow up. The ideal study would start during pregnancy (perhaps even before) and would collect data about maternal diets and then continue to collect data and follow participants—in other words, a birth cohort.
Are these studies under way?
There have been some attempts in this country to develop a birth cohort that was big enough to look at cancer, but the studies sank under the weight of huge budgets. On the other hand, there are more than 300,000 participants in birth cohorts in Scandinavian countries. The next generations of scientists will be the ones to analyze the results in terms of cancer. I've been involved in this work and am hoping to see some of the results myself.