Abstract Little is known about the effect of preventive diets on colonic nutrient concentrations. This study randomized 120 persons at increased risk of colon cancer to a Mediterranean versus a Healthy Eating diet for six months. The former targeted increases in whole grains, fruits, vegetables, monounsaturated, and n3 fats. The Healthy Eating diet was based on Healthy People 2010 recommendations. At baseline, dietary fat and carotenoid intakes were poorly associated (Spearman ρ < 0.4) with serum and colon concentrations. Strong associations were observed between serum and colon measurements of β-cryptoxanthin (ρ = 0.58; P < 0.001), α-carotene (ρ = 0.48; P < 0.001), and β-carotene (ρ = 0.45; P < 0.001). After six months, the Healthy Eating intervention increased serum lutein, β-, and α-carotene significantly (P < 0.05). In the Mediterranean arm, the significant increases were in serum lutein, β-cryptoxanthin, β-carotene, monounsaturated, and n3 fats. A significant group-by-time interaction (P = 0.03) was obtained for monounsaturated fats. Colonic increases in carotenoids and n3 fats were significant only in Healthy Eating arm, whereas the group-by-time interaction was significant for β-carotene (P = 0.02) and α-carotene (P = 0.03). Changes in colon concentrations were not significantly associated with reported dietary changes. Changes in colon and serum concentrations were strongly associated for β-cryptoxanthin (ρ = 0.56; P < 0.001) and α-carotene (ρ = 0.40; P < 0.001). The associations between colonic and serum concentrations suggest the potential use of using serum concentration as a target in dietary interventions aimed at reducing colon cancer risk. Cancer Prev Res; 6(6); 558–65. ©2013 AACR.