Effect of an Isocaloric High Fat Diet on Initiation of Puberty in Osborne-Mendel Rats1

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Abstract
The relationships between age, body weight, naso-anal length, food intake, caloric intake and the Quetelet index of obesity were studied for 2 groups (12/group) of female Osborne-Mendel (O-M) weanling rats fed high fat (20% corn oil) and low fat (5% corn oil) diets until confirmation of first estrus by vaginal examination. The diets were isocaloric; corn oil was substituted for corn starch in the high fat diet. The rats fed high fat diets (HFD) had vaginal opening (VO) and first estrus (FE) significantly earlier (P<0.001) than did rats fed low fat diets (LFD). Sixty-six percent of the HFD rats experienced VO and FE on the same day, compared with 27% of the LFD rats. Caloric intake/100 g BW was approximately the same for both groups, increasing until 25 days of age and then decreasing to the onset of puberty. When the animals were grouped by day prior to the event of VO, the mean values for the index of obesity displayed a linear increase for the 2 groups (HFD: r = 0.97; LFD: r = 0.98). Three days before VO a significant difference was observed in the index of obesity (LFD, 47.7 ± 1.4 SEM; HFD, 46.4 ± 0.7; P<0.05). The linear equations for both groups intersected at VO, suggesting that HFD and LFD rats had a remarkably similar index of obesity at time of puberty. These findings suggest that a metabolic signal, related to body fatness, may be involved with the onset of puberty.