A comparison of prospective and retrospective assessments of diet in a study of colorectal cancer

Dietary factors are widely studied as risk factors for colorectal cancer, w ith much information from case-control studies. We evaluated the validity o f dietary data from a retrospective case-control study of diet and colorect al cancer As part of the alpha-Tocopherol, beta-Carotene Cancer Prevention Study. diet was assessed at baseline and after diagnosis for colorectal can cer cases and at baseline and regularly during the trial for a random contr ol group. The dietary assessment referred to the previous 12 months (in cas es before diagnosis). In the two dietary assessments, the cases reported a greater increase in consumption of fruits and dairy products and a decrease in consumption of potatoes. Accordingly, relative risks for colorectal can cer by baseline dietary data differed markedly from odds ratios from case-c ontrol data; e.g., relative risk for a 652-mg increase in calcium intake wa s 0.79 (95% confidence interval = 0.48-1.30) in case-cohort analysis vs. an odds ratio of 1.57 (95% confidence interval = 1.06-2.33) for case-control analysis. The most likely explanation is the influence of current diet on r ecall of prediagnosis diet and effects of occult cancer on diet in the year before cancer diagnosis, which have implications fbr interpretation of cas e-control studies in evaluating associations between diet and colorectal ca ncer.