Observational cohort studies show an association of smoking with prostate cancer incidence and mortality. Ill-defined exposure categories in many cohort studies suggest that pooled data underestimate risk.

In vivo and in vitro studies suggest that dairy products, calcium, and dietary vitamin D inhibits the development of colorectal cancer (CRC). A meta-analysis was performed to evaluate this relationship in observational studies. Data from 60 epidemiological studies enrolling 26,335 CRC cases were pooled using a general variance-based meta-analytic method. Summary relative risk (RR) estimates and 95% confidence intervals (CIs) were calculated for the highest vs. the lowest intake categories. Sensitivity analyses tested the robustness of these summary effect measures and the statistical heterogeneity. The summary RR for high milk and dairy product intake, respectively, on colon cancer risk was 0.78 (95% CI = 0.67-0.92) and 0.84 (95% CI = 0.75-0.95). Milk intake was unrelated to rectal cancer risk. High calcium intake had a greater protective effect against tumors of the distal colon and rectal cancer vs. proximal colon. The risk reduction associated with calcium was similar for dietary and supplemental sources. Vitamin D was associated with a nonsignificant 6% reduction in CRC risk. Higher consumption of milk/dairy products reduces the risk of colon cancer, and high calcium intake reduces the risk of CRC. Low vitamin D intake in the study populations may limit the ability to detect a protective effect if one exists.

During the last decade, approaches to evidence-based medicine, with its heavy reliance on the randomized clinical trial (RCT), have been adapted to nutrition science and policy. However, there are distinct differences between the evidence that can be obtained for the testing of drugs using RCTs and those needed for the development of nutrient requirements or dietary guidelines. Although RCTs present one approach toward understanding the efficacy of nutrient interventions, the innate complexities of nutrient actions and interactions cannot always be adequately addressed through any single research design. Because of the limitations inherent in RCTs, particularly of nutrients, it is suggested that nutrient policy decisions will have to be made using the totality of the available evidence. This may mean action at a level of certainty that is different from what would be needed in the evaluation of drug efficacy. Similarly, it is judged that the level of confidence needed in defining nutrient requirements or dietary recommendations to prevent disease can be different from that needed to make recommendations to treat disease. In brief, advancing evidence-based nutrition will depend upon research approaches that include RCTs but go beyond them. Also necessary to this advance is the assessing, in future human studies, of covariates such as biomarkers of exposure and response, and the archiving of samples for future evaluation by emerging technologies.