Instraspecies Relationship between Fasting Heat Production and Body Weight: A Reevaluation of W.75

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Abstract
The use of W.75 as an estimator of fasting heat production (HP) for male and female rats, chickens, rabbits, dogs, humans, cattle and for ewes was studied using data obtained from the literature. Regression coefficients for models which related HP to body weight (W) were calculated from data for each population of animals. Highly significant (P<.001) equations were obtained for animals in each species-sex subclass, thus nullifying the common assumption that the narrow W ranges within species prevent the determination of meaningful relationships between HP and weight. Data from various sources within species-sex subclasses did not differ with respect to b1 for the model: log HP = b0 + b1 (log W). However, source of data did affect b0 for dogs, ewes, women and steers. Weight exponents (b1 values) were significantly different for the sexes within the chicken, dog, and human species. Tests for equality of weight exponents showed that exponents derived for species-sex-source, sex-source within species, and species-source within sex subclasses were not equal (P<.001). Thus, a universal equation which relates HP to W should not be declared the best fitting equation for species-sex subclasses. It is concluded that W.75, which was developed from data of species varying widely in W, ignores effects other than weight which are peculiar to specific populations. Therefore many researchers have inappropriately adjusted biological data for variation in weight by dividing response criteria by W.75. A more appropriate adjustment would be to include Wb or log W as a covariable in the statistical model. Copyright © 1976. American Society of Animal Science . Copyright 1976 by American Society of Animal Science