- Calcium supplements to the western-style diet may reduce the risk for colorectal neoplasia. Using rectal epithelial proliferation (REP) measurements as a biomarker of response to intervention, the authors evaluated the effects of 1-year calcium supplementation in adenoma patients and its possible interactions with the patients' dietary and lifestyle habits. Consenting adenoma patients, without a family history of colorectal neoplasia, were randomly selected to receive 3.75 g calcium carbonate (1.5 g Ca2+) daily or to receive no treatment. All had their long-term dietary and lifestyle habits assessed and their REP labeling index (LI) evaluated before and at end of follow-up. The change in LI was compared between groups, and statistical associations were examined between mean nutrient consumption and treatment effect and between lifestyle and treatment effect. Fifty-two adenoma patients (33 treated and 19 untreated) completed intervention and follow-up. There were no significant differences between study groups in age, weight, cigarette smoking, or medication use. The LI decreased in 58% of calcium-intervened patients and in only 26% of nonintervened patients (P = 0.04); the mean LI x 100 (+/- standard deviation) of the former fell from 5.04 +/- 1.93 to 4.54 +/- 1.58, and rose from 4.32 +/- 1.58 to 4.93 +/- 1.58 in the latter (P = 0.04). A lower fat, a higher carbohydrate, fiber, or fluid intake each interacted with the calcium supplementation to decrease the LI (P = 0.02, 0.001, 0.02, and 0.08, respectively). Long-term calcium supplements significantly suppressed REP in adenoma patients, and long-term dietary habits contributed to this effect. Patient diet should be assessed when researchers use REP as a biomarker in calcium chemoprevention studies. Study results indicated that relevant dietary counseling may be useful in addition to calcium supplements in persons at increased risk for colorectal neoplasia.