VITamins And Lifestyle cohort study: study design and characteristics of supplement users.

Vitamin and mineral supplements are among the most commonly used drugs in the United States, despite limited evidence on their benefits or risks. This paper describes the design, implementation, and participant characteristics of the VITamins And Lifestyle (VITAL) Study, a cohort study of the associations of supplement use with cancer risk. A total of 77,738 men and women in western Washington State, aged 50-76 years, entered the study in 2000-2002 by completing a detailed questionnaire on supplement use, diet, and other cancer risk factors, and 70% provided DNA through self-collected buccal cell specimens. Supplement users were targeted in recruitment: 66% used multivitamins, 46% used individual vitamin C, 47% used individual vitamin E, and 46% used calcium, typically for 5-8 of the past 10 years. Analyses to identify confounding factors, the main study limitation, showed that regular nonsteroidal anti-inflammatory drug use, intake of fruits and vegetables, and recreational physical activity were strongly associated with supplement use (p < 0.001). The authors describe a follow-up system in which cancers, deaths, and changes of residence are tracked efficiently, primarily through linkage to public databases. These methods may be useful to other researchers implementing a large cohort study or designing a passive follow-up system.