Macronutrient Utilization After Short-term Fasting in Older and Younger Men and Women, a Pilot Study.

Background/Objective: While older people are more prone than younger people to periods of involuntary fasting, systematic assessment of all the three major sources of macronutrient mobilization and oxidation in the same individual older participants during short-term periods of fasting has not been previously reported. Because aging is associated with many metabolic, hormonal, and body composition changes, older humans may have different kinetics of utilization of macronutrient stores during fasting than younger ones. This pilot study aimed to generate exploratory data to test this hypothesis. Methods/Subjects: We examined four groups of five participants each in this study, women and men, and older and younger subjects. We measured body composition by dual-energy X-ray absorptiometry (DEXA) and studied the effects of a 12-hour and a 36-hour fast on protein mobilization, lipolysis, and glucose output (substrate rates of appearance, using stable isotope tracers), as well as macronutrient oxidation. Results: The older participants had a greater percent body fat. Respiratory exchange ratios (RER) decreased with the longer fast. In a linear mixed model analysis of the metabolic data, age was not significant as a fixed effect when added to the model, except for glycerol rate of appearance and leucine oxidation rate. Conclusions: The effects of age and sex on mobilization and oxidation of macronutrient stores, as assessed with stable isotope tracers and indirect calorimetry, were small compared to the large overall effect of a 36-hour fast, suggesting that the macronutrient metabolic switching of older people with fasting is similar to that of younger people.