Many studies have tried to determine whether eating specific foods increases or decreases a person's risk of getting cancer. Unfortunately, different studies have conflicting results, so it is hard to know what effect foods or dietary supplements have on cancer risk. A common problem is that when studies find that people who eat more of a certain food seem to have lower rates of a certain cancer, it can be difficult to tell whether those people also were different in terms of other risk factors (such as where they live, how much they smoke and drink, and so forth). Often, when doctors do a controlled trial (see What Participants Need to Know About Clinical Trials) and randomly give some people a seemingly helpful food or supplement, the studies do not show a beneficial effect. Some foods and supplements have been studied more than others, and many studies are ongoing.
Although the effects of specific foods and supplements on the risk of getting cancer are unclear, there is good evidence that obesity increases the risk of a number of cancers.