Influence of dietary fat, caloric restriction, and voluntary exercise on N-nitrosomethylurea-induced mammary tumorigenesis in rats.

The effect of dietary fat, energy restriction, and exercise on N-nitrosomethylurea (NMU:CAS:684-93-5)-induced mammary tumorigenesis in female F344 rats was investigated. Rats were fed the NIH-07 diet until N-nitrosomethylurea administration on Day 50 of age, when they were transferred to six treatment groups. Three sedentary groups were fed either high-fat (20%, w/w), medium-fat (10%), or low-fat (5%) diets ad libitum (HFAL, MFAL, LFAL, respectively); two sedentary groups were fed high fat and medium fat diets restricted to 75% of the food consumed by their ad libitum counterparts (HFR, MFR), and one group was fed a HFAL diet but allowed free access to an activity wheel (HFEX). Tumor yields among the three ad libitum sedentary groups were significantly greater in the HFAL and MFAL groups when compared to the LFAL group. Dietary restriction reduced tumor yields by more than 90% of ad libitum controls regardless of fat intake. Voluntary exercise reduced tumor yields and delayed time of tumor appearance in HFEX animals to levels similar to those found in LFAL animals. Animals with voluntary access to exercise wheels averaged between 1.03 and 2.85 miles/day, consumed more food (+18%), and exhibited greater weight gain (+13%) than their sedentary counterparts. Restricted animals exhibited significantly decreased body weight gains (-15%) compared to their ad libitum counterparts, but no differences in weight gains were detected among the HFAL, MFAL, and LFAL groups, despite widely varying amounts of fat intake. Body composition studies indicated that body fat content was not influenced by the quantity of fat consumed in the diet, but was significantly reduced by caloric restriction (-20 to 26%) and exercise (-20%). While the precise mechanisms underlying the tumor-promoting effects of HFAL diets and the antipromoting effects of energy restriction and exercise remain to be elucidated, available evidence suggests that these effects are not due to alterations in energy homeostasis per se, but may instead be exerted indirectly, and perhaps independently via endocrine, paracrine, or neurohormonal mechanisms.