Targeted 13C enrichment of lipid and protein pools in the body reveals circadian changes in oxidative fuel mixture during prolonged fasting: A case study using Japanese quail

Many animals undergo extended periods of fasting. During these fasts, animals oxidize a ratio of macronutrients dependent on the nutritional, energetic, and hydric requirements of the fasting period. In this study, we use Japanese quail (Coturnix coturnix japonica), a bird with natural intermediate fasting periods, to examine macronutrient use during a 6 d fast. We raised groups of quail on isotopically labeled materials (13C-1-leucine, 13C-U-glucose, or 13C-1-palmitic acid) with the intent of labeling specific macronutrient/tissue pools in each treatment, and then traced their use as fuels by measuring the δ13C values of breath CO2. Based on changes in δ13C values during the fast, it appears that the carbohydrate label,13C-U-glucose, was largely incorporated into the lipid pool and thus breath samples ultimately reflected lipid use rather than carbohydrate use. In the lipid treatment, the 13C-1-palmitic acid faithfully labeled the lipid pool and was reflected in the kinetics δ13C values in breath CO2 during the fast. Endogenous lipid oxidation peaked after 24 h of fasting and remained constantly elevated thereafter. The protein label,13C-1-leucine, showed clear diurnal periods of protein sparing and degradation, with maximal rates of protein oxidation occurring at night and the lowest rates occurring during the day time. This stable isotope tracer method provides a noninvasive approach to study the nutrient dynamics of fasting animals and should provide new insights into how different types of animals use specific nutrient pools during fasting and possibly other non-steady physiological states.