A wide range of experimental approaches has been taken to test the effects of vegetables and fruit on risk factors for disease in humans (Table 2 ). Design methodologies used include multiple-arm trials, randomized crossover studies, and more compromised designs such as nonrandomized crossovers and pre- and posttreatment analyses. Duration of treatment varies from a single dose to years, depending on the mechanism of interest; however, study periods are a typically 2 wk to 1 mo in length and long-term effects are not well studied. Stringency of dietary control ranges from addition of supplements to a habitual diet to provision of all food for the duration of a treatment. Studies of this type, especially ones providing a controlled diet, are time and labor intensive. They can require a significant commitment by study participants. As a result, sample sizes are often small and may lack the statistical power to detect an effect. Despite the challenges, human experimental dietary studies must be rigorously designed and executed to provide meaningful data and a useful link between population- and laboratory-based studies.