Tissue-Specific Mass Changes During Fasting: The Protein Turnover Hypothesis

Abstract

Phenotypic flexibility in organ size is common in animals during fasting and is especially remarkable in birds during their migrations. This phenotypic flexibility is often explained in relation to changes in functional demands in response to environmental change. A new hypothesis suggests that the rate of tissue-specific degradation during fasting is related to the tissue-specific rate of protein turnover. This hypothesis predicts allometric scaling of tissue-specific protein turnover rate across species and so provides estimates of the rate of tissue degradation for a specific tissue in relation to other tissues within a given animal. We used carbon isotopic incorporation rate as a proxy for protein turnover and then predicted that organ size changes during fasting in migratory birds would occur in the following rank order from most to least reduced: small intestine, liver, kidney, gizzard, heart, flight, and leg muscles. Furthermore, the hypothesis that protein turnover determines the degree of tissue-specific mass loss during fasting predicts that rate of change is independent of activity levels of tissues. In the following contribution we discuss the proposed hypothesis in the context of other competing hypotheses and review its support given the existing data on tissue turnover and mass change.