Abstract
The relationship between overnight postabsorptive (fasting) respiratory exchange ratio (RER) and plasma FFA concentrations was addressed using data from three separate protocols, each of which involved careful control of the antecedent diet. Protocol 1 examined the relationship between fasting RER and the previous daytime RER. In Protocol 2 fasting, RER and plasma palmitate concentrations were measured in 29 women and 31 men (body mass index <30 kg.m(-2)). Protocol 3 analyzed data from Nielsen et al. (Nielsen, S., Z. K. Guo, J. B. Albu, S. Klein, P. C. O'Brien, M. D. Jensen. 2003. Energy expenditure, sex and endogenous fuel availability in humans. J. Clin. Invest. 111: 981-988.) to understand how fasting RER and palmitate concentrations relate within individuals during four consecutive measurements. The results were as follows: 1) Fasting RER was correlated (r = 0.74, P < 0.001) with the previous day's average RER, and less so with RER variability. 2) Fasting RER was correlated (r = -0.39, P = 0.007) with fasting plasma palmitate concentrations. 3) The pattern of the RER/palmitate relationship was similar within individuals and between individuals; a negative slope was observed significantly more often than a positive slope (chi(2) test; P < 0.001). Our findings suggest that, despite a fixed food quotient, the slight departures from energy equilibrium in a controlled General Clinical Research Center environment can effect plasma FFA concentrations. We suggest that including indirect calorimetry as part of FFA metabolism studies may aid in data interpretation.
Highlights
The relationship between overnight postabsorptive respiratory exchange ratio (RER) and plasma FFA concentrations was addressed using data from three separate protocols, each of which involved careful control of the antecedent diet
Despite a fixed food quotient, the slight departures from energy equilibrium in a controlled General Clinical Research Center environment can effect plasma FFA concentrations
We suggest that including indirect calorimetry as part of FFA metabolism studies may aid in data interpretation.—Jensen, M
Summary
The relationship between overnight postabsorptive (fasting) respiratory exchange ratio (RER) and plasma FFA concentrations was addressed using data from three separate protocols, each of which involved careful control of the antecedent diet. Energy deficits typically result in mobilization and oxidation of body fat [3], which is reflected by a lower respiratory exchange ratio (RER) This is relevant because even in the absence of intentional weight loss efforts, using indirect calorimetry to assess interindividual differences in substrate oxidation is complicated by difficulties in achieving energy balance [4, 5]. The metabolic kitchen in the Mayo General Clinical Research Center (GCRC) has been able to closely match energy intake to energy expenditure measured using double-labeled water [8] Given these efforts, we hoped that volunteers participating in our protocols, having consumed a diet with the same food quotient (FQ), would have similar daytime RER values, because if energy balance is achieved, FQ should equal the 24 h respiratory quotient.
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