Exercising for food: bringing the laboratory closer to nature.

Traditionally, exercise physiology experiments have borne little resemblance to how animals express physical activity in the wild. In this experiment, 15 adult male rats were divided into three equal-sized groups: exercise contingent (CON), non-exercise contingent (NON) and sedentary (SED). The CON group was placed in a cage with a running wheel, where the acquisition of food was contingent upon the distance required to run. Every 3 days the distance required to run to maintain food intake at free feeding levels was increased by 90% in comparison to the previous 3 days. The NON group was housed identically to the CON group, but food acquisition was not dependent upon running in the wheel. Finally, the SED group was kept in small cages with no opportunity to perform exercise. A two-way ANOVA with repeated measures was used to determine significant differences in responses between the experimental phases and treatment groups, and ANCOVA was used to analyse growth and tissue mass variables with body length and body mass used separately as covariates. A post hoc Tukey's test was used to indicate significant differences. A Pearson's correlation was used to test the relationship between the distance travelled by the animal and the distance/food ratio. The level of significance was set at P<0.05 for all tests. The CON group showed the hypothesized correlation between distance required to run to obtain food and the mean distance travelled (P<0.001), during 45 days in the contingency phase. This group showed a decrease in body mass, rather than an increase as shown by NON and SED groups. The CON group had a significantly lower body temperature (P<0.05) and adiposity (P<0.05) when compared with the other two groups for the same body size. The present experimental model based on animals choosing the characteristics of their physical exercise to acquire food (i.e. distance travelled, speed and duration) clearly induced physiological effects (body characteristics and internal temperature), which are useful for investigating relevant topics in exercise physiology such as the link between exercise, food and body mass.