Carbon isotopes in exhaled breath track metabolic substrates in brown bears (Ursus arctos)

Abstract

Dietary carbon is oxidized and exhaled as CO2, thus 13Cbreath values can provide information on diet and substrate use for energy. However, physiological phenomena such as fat deposition and fasting can alter values of 13Cbreath such that interpretation of source contributions may be unclear. Consequently, before application to free-ranging animals, inferences about feeding and nutritional states based on 13Cbreath should be validated with controlled experiments using captive individuals. Here, we report 13Cbreath values for 4 captive brown bears (Ursus arctos) under different conditions: the bears were 1st given a diet containing carbohydrate, lipid, and protein; they were then switched to a carbohydrate-free diet consisting of salmon and fish oil; and finally they were placed on a fast leading to winter hibernation. Following the switch to the carbohydrate-free diet, values of 13Cbreath and 13Cplasma suggested that although oxidation included a substantial portion of dietary proteins, dietary lipids were preferentially utilized. After the onset of fasting, 13Cbreath values did not change, possibly because of selective mobilization of endogenous fatty acids accreted by bears while consuming the experimental diet. Examination of our data suggests that because CO2 production and exhalation are influenced differentially by fasting state and diet composition, additional measurements such as respiratory exchange ratio should be used to aid interpretation of carbon isotope analysis of breath. © 2012 American Society of Mammalogists.