Effect of type and amount of dietary fat on the enhancement of rat mammary tumorigenesis by exercise.

The effect(s) of treadmill exercise and type and amount of dietary fat on the process of mammary tumorigenesis was investigated. Female Sprague-Dawley rats were fed a purified 5% fat diet (AIN-76A) from 21 to 64 days of age. At 50 days of age each rat was intubated p.o. with 5 mg 7,12-dimethylbenz(a)anthracene (DMBA). Fourteen days after DMBA, the rats were randomized into one of three diet groups: 5% fat as corn oil, 24.6% fat as corn oil, or 24.6% fat as a mixture of palm (21.8%) and corn oil (2.8%). The combination of palm and corn oil provided the same amount of linoleic acid per g as the 5% corn oil diet. Half the animals receiving each diet were exercised on a treadmill at a speed of 20 m/min, 1 degree incline, 15 min/day, 5 days/week, and were designated as the moderate intensity treadmill exercise group (MITE). The remaining animals were exercised at a speed of 2 m/min, 1 degree incline, 15 min/day, 5 days/week, and were designated as the low intensity treadmill exercise group (LITE). The experiment was terminated 154 days after DMBA was administered. The median tumor-free time was significantly shortened in MITE rats receiving the 24.6% fat, corn oil-formulated diet in comparison to LITE rats receiving the same diet (43 day vs. 62 day, P = 0.028). Similarly, tumor appearance was more rapid in MITE rats consuming the low fat corn oil diet in comparison to the low fat diet-fed LITE group (57 day vs. 67 day, P = 0.046). Exercise exerted no effect on the rate of tumor appearance in rats that received the 24.6% palm and corn oil mixture, (58 day, MITE, vs. 62 day, LITE, P = 0.502). Mean body weight gains were similar among groups, although MITE rats consistently weighed more than LITE rats consuming the same diet. Gross carcass composition was unaffected by either the level of exercise or the amount of dietary fat consumed. The data indicate that moderate intensity treadmill exercise for a short duration, that is without effect on carcass fat content, can stimulate mammary tumorigenesis in rats fed low or high fat diets. This effect can be influenced by the type of dietary fat ingested.