The linear regression of body weight and age in intact, ovariecotomised, and estrogen treated rats: some applications and implications
Clark, R.G.; Tarttelin, M.F.
Growth 42(2): 113-127
1978
ISSN/ISBN: 0017-4793 PMID: 680577 Document Number: 134309
Many variables, including rat body weight (BWt), are sequentially measured in individuals yet the information obtained is often poorly utilized. Many advantages result if a linear relationship is established, on the original or transformed scales, between the variable and time. For individual rats log BWt, which is normally distributed, is linearly related to the reciprocal of age. Slopes (rates of BWt gain) and constants (ultimate BWt) then serve as data. Linearity extended from 4 wk onwards in ovariectomized (OvX) and intact rats, implying that puberty does not affect BWt and that BWt increases before puberty following pre-pubertal OvX; the ovary inhibiting growth before puberty. For individual rats weaning BWt was correlated with rate of BWt gain but not with ultimate BWt, indicating post-weaning compensatory growth which continues to maturity. Pre-treatment and post-treatment measurements were made on individuals, allowing within-animal comparisons and regression. The BWt response to OvX or estrogen treatment depended on the pre-treatment rate of BWt gain and ultimate BWt, implying that the BWt response entails a form of compensatory growth. OvX at 3 ages (day 2, week 4 and week 7) produced different rates of BWt gain but similar ultimate BWt. These results support the hypothesis that ultimate BWt is predetermined, but can be modified by treatment.