Calorie Intake during Mammary Development Influences Cancer Risk: Lasting Inhibition of C3H/HeOu Mammary Tumorigenesis by Peripubertal Calorie Restriction¹

To test for a relationship between peripubertal calorie intake, mammary development, and tumorigenesis, weanling C3H/HeOu mice were separated into 3 groups: fed diet either ad libitum (AL) and designated group AL (n = 60); fed a similar, calorie-restricted (CR) diet only during mammary development when 4–12 weeks old and then subsequently fed ad libitum when ≥13 weeks old (group CR_4–12, n = 24); or continuously calorie restricted (group CR, n = 60). Eight weeks of peripubertal calorie restriction provided CR_4–12 mice with lasting protection from mammary tumorigenesis (P = 0.004) and lowered cumulative tumor incidence by 33% compared to AL mice. Sustained calorie restriction of group CR mice further reduced mammary tumor incidence compared to both AL (P = 0.000001) and CR_4–12 mice (P = 0.009). Calorie intake significantly influenced mammary development and cellular proliferation. Compared to the mammary development of AL mice, calorie restriction reduced the diameter of ductal end buds (189 µm compared to 146 µm; P < 0.01), lowered the end bud [³H]thymidine labeling index from ≥20 to ≤13% (P < 0.001), delayed end bud migration and mammary glandular growth (P < 0.01), reduced alveolar budding (P < 0.001), reduced the proportion of alveoli containing at least one [³H]thymidine labeled cell from ≥50 to ≤22% (P < 0.001), and lowered the alveolar [³H[thymidine labeling index of labeled alveoli from ≥14 to ≤7% (P < 0.001). These findings link peripubertal calorie intake, mammary development, and carcinogenic risk, and show that the abrogation of mammary tumorigenesis by calorie restriction is partially attributable to influences on mammary development.