Relationship between postabsorptive respiratory exchange ratio and plasma free fatty acid concentrations

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 (χ2 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. 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 (χ2 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. Whole-body substrate oxidation can be measured using indirect calorimetry combined with urinary nitrogen excretion rates (1Frayn K.N. Calculation of substrate oxidation rates in vivo from gaseous exchange.J. Appl. Physiol. 1983; 55: 628-634Crossref PubMed Scopus (1800) Google Scholar). Factors known to rapidly affect substrate oxidation include diet composition (2Hill J.O. Peters J.C. Reed G.W. Schlundt D.G. Sharp T. Greene H.L. Nutrient balance in humans: effects of diet composition.Am. J. Clin. Nutr. 1991; 54: 10-17Crossref PubMed Scopus (151) Google Scholar) and energy balance. Energy deficits typically result in mobilization and oxidation of body fat (3Vazquez J.A. Kazi U. Lipolysis and gluconeogenesis from glycerol during weight reduction with very-low-calorie diets.Metabolism. 1994; 43: 1293-1299Abstract Full Text PDF PubMed Scopus (31) Google Scholar), 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 (4Grunwald G.K. Melanson E.L. Forster J.E. Seagle H.M. Sharp T.A. Hill J.O. Comparison of methods for achieving 24-hour energy balance in a whole-room indirect calorimeter.Obes. Res. 2003; 11: 752-759Crossref PubMed Scopus (23) Google Scholar, 5Schrauwen P. van Marken Lichtenbelt W.D. Westerterp K.R. Energy balance in a respiratory chamber: individual adjustment of energy intake to energy expenditure.Int. J. Obes. Relat. Metab. Disord. 1997; 21: 769-774Crossref PubMed Scopus (29) Google Scholar). Despite careful attention to detail by controlling diet and making prestudy measures of metabolic rate, it is not uncommon to find 300 kcal differences between energy intake and expenditure as measured by room calorimetry (4Grunwald G.K. Melanson E.L. Forster J.E. Seagle H.M. Sharp T.A. Hill J.O. Comparison of methods for achieving 24-hour energy balance in a whole-room indirect calorimeter.Obes. Res. 2003; 11: 752-759Crossref PubMed Scopus (23) Google Scholar, 5Schrauwen P. van Marken Lichtenbelt W.D. Westerterp K.R. Energy balance in a respiratory chamber: individual adjustment of energy intake to energy expenditure.Int. J. Obes. Relat. Metab. Disord. 1997; 21: 769-774Crossref PubMed Scopus (29) Google Scholar). We have attempted to minimize these issues by feeding volunteers participating in our studies of fatty acid metabolism a diet with fixed macronutrient distribution for 3–10 days prior to research studies (6Nielsen S. Guo Z. Albu J.B. Klein S. O'Brien P.C. Jensen M.D. Energy expenditure, sex, and endogenous fuel availability in humans.J. Clin. Invest. 2003; 111: 981-988Crossref PubMed Scopus (110) Google Scholar, 7Romanski S.A. Nelson R. Jensen M.D. Meal fatty acid uptake in human adipose tissue: technical and experimental design issues.Am. J. Physiol. Endocrinol. Metab. 2000; 279: E447-E454Crossref PubMed Google Scholar). In general, 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 (8Levine J.A. Eberhardt N.L. Jensen M.D. Role of non-exercise activity thermogenesis (NEAT) in resistance to fat gain in humans.Science. 1999; 283: 212-214Crossref PubMed Scopus (781) Google Scholar). 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. Unfortunately, we still observe significant interindividual variability in overnight postabsorptive RER, which could be driven by constitutional factors (increased constitutive lipolysis driving fat oxidation) or environmental factors, such as energy balance. We believe that understanding the relative contribution of the environment to this measurement is important for our studies of lipolysis because we found a significant association between RER and overnight postabsorptive FFA concentrations (9Shadid S. Kanaley J.A. Sheehan M.T. Jensen M.D. Basal and insulin-regulated free fatty acid and glucose metabolism in humans.Am. J. Physiol. Endocrinol. Metab. 2007; 292: E1770-E1774Crossref PubMed Scopus (42) Google Scholar) and because FFA concentrations are highly predictive of FFA flux (6Nielsen S. Guo Z. Albu J.B. Klein S. O'Brien P.C. Jensen M.D. Energy expenditure, sex, and endogenous fuel availability in humans.J. Clin. Invest. 2003; 111: 981-988Crossref PubMed Scopus (110) Google Scholar). The purpose of this report is to share our experience with factors that relate to inter- and intraindividual variability in postabsorptive RER and FFA concentrations. We also provide data on the relationship between these two metabolic variables in the hope that this will aid future investigations of FFA metabolism. Data from a number of research projects, both published (6Nielsen S. Guo Z. Albu J.B. Klein S. O'Brien P.C. Jensen M.D. Energy expenditure, sex, and endogenous fuel availability in humans.J. Clin. Invest. 2003; 111: 981-988Crossref PubMed Scopus (110) Google Scholar, 7Romanski S.A. Nelson R. Jensen M.D. Meal fatty acid uptake in human adipose tissue: technical and experimental design issues.Am. J. Physiol. Endocrinol. Metab. 2000; 279: E447-E454Crossref PubMed Google Scholar) and unpublished, were used to address our hypotheses. All protocols were approved by the Mayo Clinic Institutional Review Board, and all participants provided informed written consent. These data are independent of our previous finding of a negative relationship between overnight postabsorptive RER and plasma FFA concentrations (9Shadid S. Kanaley J.A. Sheehan M.T. Jensen M.D. Basal and insulin-regulated free fatty acid and glucose metabolism in humans.Am. J. Physiol. Endocrinol. Metab. 2007; 292: E1770-E1774Crossref PubMed Scopus (42) Google Scholar). Table 1 provides the age, body mass index (BMI), and percentage of body fat data for volunteers from each protocol.TABLE 1General subject characteristicsMenWomenProtocol 1Protocol 2Protocol 3Protocol 1Protocol 2Protocol 3Age29 ± 724 ± 330 ± 728 ± 822 ± 331 ± 8BMI23.2 ± 2.124.9 ± 2.826.9 ± 4.320.6 ± 1.724.1 ± 2.826.5 ± 5.7Body fat (%)17.3 ± 4.522.3 ± 7.024.0 ± 7.428.6 ± 5.834.4 ± 6.538.5 ± 8.2Values are mean ± SD. Open table in a new tab Values are mean ± SD. For Protocol 1, data from 51 volunteers (25 women and 26 men) participating in studies of dietary fatty acid metabolism were used. The general to the studies is as S.A. Nelson R. Jensen M.D. Meal fatty acid uptake in human adipose tissue: technical and experimental design issues.Am. J. Physiol. Endocrinol. Metab. 2000; 279: E447-E454Crossref PubMed Google Scholar). The participants were all For Protocol 29 women and 31 men studies of overnight postabsorptive FFA metabolism to for sex differences in FFA The RER and palmitate data have not been For Protocol volunteers (25 women and men) in a how body and energy expenditure relate to FFA and glucose flux (2Hill J.O. Peters J.C. Reed G.W. Schlundt D.G. Sharp T. Greene H.L. Nutrient balance in humans: effects of diet composition.Am. J. Clin. Nutr. 1991; 54: 10-17Crossref PubMed Scopus (151) Google Scholar). The average FFA data from four have been of this a of kg·m−2). or known to or metabolism were In to the volunteers were to be weight for 2 prior to the and and studies were to be for each and the variables in our are in this For design of to S. Guo Z. Albu J.B. Klein S. O'Brien P.C. Jensen M.D. Energy expenditure, sex, and endogenous fuel availability in humans.J. Clin. Invest. 2003; 111: 981-988Crossref PubMed Scopus (110) Google and 7Romanski S.A. Nelson R. Jensen M.D. Meal fatty acid uptake in human adipose tissue: technical and experimental design issues.Am. J. Physiol. Endocrinol. Metab. 2000; 279: E447-E454Crossref PubMed Google We examined overnight postabsorptive RER is to of the previous day's RER, including the of RER to the Nutr. PubMed Scopus Google the average RER. The should in be for all volunteers energy diet with the same be in the data the volunteers to have postabsorptive indirect calorimetry on two consecutive In we that data be from of indirect calorimetry measures on the prior to the overnight postabsorptive indirect All consumed a diet and provided by the for days the energy for weight were the energy expenditure by activity for each was using the J.A. of Basal in of Scholar). The activity reflected the individual activity by the The volunteers were and the weight by the of food provided was by in to with as if weight to the volunteers more than the would this and food intake even if the weight loss was body weight by if the volunteers were to all food the was by For the studies with days of prestudy diet the were if we are able to weight within these The volunteers were to the the prior to the and consumed the were provided during the and The was a that provided of energy on indirect calorimetry that and and of the energy of The volunteers were from the metabolic kitchen and h that provided and of energy as and The energy of these was to each energy on the weight diet for the the the volunteers were for the so that and could be calorimetry was from h for h and the These studies were to the of fat adipose using a the S.A. Nelson R. Jensen M.D. Meal fatty acid uptake in human adipose tissue: technical and experimental design issues.Am. J. Physiol. Endocrinol. Metab. 2000; 279: E447-E454Crossref PubMed Google Scholar). These data were used to the relationship between RER and plasma FFA concentrations in men and All volunteers consumed a diet and fat by the Mayo Clinic metabolic kitchen for 3 days the The energy were as in Protocol 3 days of this the volunteers were to the h and a and was during the The the participants from was measured and was for for measurement of plasma and palmitate concentrations. We used these data to the relationship between measures of RER and FFA in a of individuals 2) is also measures are in the same with rates of adipose lipolysis may more fat overnight or with fat oxidation may FFA more to this of these was we would that individuals would have or overnight postabsorptive RER even on a fixed FQ diet. in individual energy balance could be to of energy in energy The individual data from Protocol 3 was analyzed to address this The was days in The volunteers were provided all and by the Mayo Clinic metabolic kitchen for The diet control was the same as that for Protocol during the the volunteers body composition the participants were to the for four consecutive overnight the volunteers consumed h and a each of the four was measured from in the and was for the measurement of plasma and palmitate concentrations as for Protocol The volunteers were and a to to the each for The results for the average FFA and glucose flux have been published (6Nielsen S. Guo Z. Albu J.B. Klein S. O'Brien P.C. Jensen M.D. Energy expenditure, sex, and endogenous fuel availability in humans.J. Clin. Invest. 2003; 111: 981-988Crossref PubMed Scopus (110) Google Scholar). palmitate concentrations were measured using M.D. of for studies of free fatty acid J. Physiol. Google Scholar, Jensen M.D. of a new for of free fatty acid J. Physiol. Google Scholar) or Z. Nielsen S. B. Jensen M.D. fatty acid measured using of Res. 1997; Full Text PDF PubMed Google Scholar). palmitate concentrations for the volunteers for 2 and 3 were of FFA concentrations. was measured by indirect calorimetry The metabolic was each prior to the and control for the and with of the metabolic using The is for measures of and for in the same environment on days using our We the for our 1) each to the we the with a known and if the from the known is the is 2) if the room concentrations that we with we the room the to 3) we and in of our if the is from we with to for we the same for issues that to we which is used for each and these data as a to composition was measured using a of and a the M.D. Kanaley J.A. Reed J.E. of and fat with and J. Clin. Nutr. PubMed Scopus Google Scholar). and concentrations were measured using and plasma was measured using with R. of plasma by with PubMed Scopus Google Scholar). Values are as mean ± the data were or as the data to two of or were used For of of data with on was used. For of or was on the distribution of the The was We also examined body body fat and plasma concentrations of or were to plasma palmitate and RER using both and was used. The measures of RER variability we examined used measures as of RER relative of and the of SD. The data from a of these participants have been published S.A. Nelson R. Jensen M.D. Meal fatty acid uptake in human adipose tissue: technical and experimental design issues.Am. J. Physiol. Endocrinol. Metab. 2000; 279: E447-E454Crossref PubMed Google Scholar). The average age, and percentage of body fat are provided in Table The of overnight postabsorptive RER we observed on the and in the were ± = ± and ± = ± The overnight postabsorptive RER on the was correlated with the average RER from the previous (r = 0.74, P < of RER variability of RER, of were not correlated with the postabsorptive RER or were The of RER to by average daytime was correlated (r = P < with the overnight postabsorptive RER. for data from men and women the same This to that interindividual in overnight postabsorptive RER in the the prior day's energy balance than differences in the to rapidly between and fat oxidation Table 2 provides the of Protocol 2 and women the same and with to body composition as Protocol 2 ± ± ± ± ± ± and of age, and RER are provided with mean ± of palmitate and The P is the for the for differences between the of men Open table in a new tab The and of age, and RER are provided with mean ± of palmitate and The P is the for the for differences between the of men The postabsorptive RER from despite to energy balance and despite a diet with macronutrient composition to all lower plasma palmitate concentrations < 0.001) and RER = than RER was correlated with plasma palmitate concentrations (r = −0.39, P = and this relationship was not or to body plasma or concentrations in this of men and The relationship was also significant for women (r = −0.39, P = and men (r = P = This is similar to we observed in a and women and men (9Shadid S. Kanaley J.A. Sheehan M.T. Jensen M.D. Basal and insulin-regulated free fatty acid and glucose metabolism in humans.Am. J. Physiol. Endocrinol. Metab. 2007; 292: E1770-E1774Crossref PubMed Scopus (42) Google Scholar) not address the interindividual variability in RER and the relationship between RER and plasma FFA concentrations are The of participants in Protocol 3 have been published (6Nielsen S. Guo Z. Albu J.B. Klein S. O'Brien P.C. Jensen M.D. Energy expenditure, sex, and endogenous fuel availability in humans.J. Clin. Invest. 2003; 111: 981-988Crossref PubMed Scopus (110) Google Scholar). were significant differences in plasma palmitate concentrations or RER between women and palmitate concentrations were correlated with RER for all four measurement days The from to on the The for the four separate with the and P values, are provided in These were not or to body body fat plasma or plasma palmitate concentrations were correlated with plasma concentrations as (6Nielsen S. Guo Z. Albu J.B. Klein S. O'Brien P.C. Jensen M.D. Energy expenditure, sex, and endogenous fuel availability in humans.J. Clin. Invest. 2003; 111: 981-988Crossref PubMed Scopus (110) Google for palmitate RER for each of the four separate days for Protocol 3 are if in RER and palmitate concentrations similar within individual as between we using the days of data from each of the The using each data are in the four data for each the of was of the were significantly from were The relationship between RER and plasma palmitate a negative slope significantly more often than a positive slope (χ2 test; P < 0.001). The average were not significantly between men and the of the relationship between RER and palmitate concentrations were significantly more negative in men than women = We the data from all of the volunteers in these three protocols to of overnight postabsorptive RER to or body composition between Table 3 the body and fasting plasma data we by of RER. is between differences in of the by RER = = = ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± mass ± ± ± fat ± ± ± fat ± ± ± fat ± ± ± fat ± ± ± ± ± ± and to of RER. were significant differences in of the between in the three RER Open table in a new tab and to of RER. were significant differences in of the between in the three RER of the within and between individual variability in overnight postabsorptive FFA concentrations be for by factors, such as and (6Nielsen S. Guo Z. Albu J.B. Klein S. O'Brien P.C. Jensen M.D. Energy expenditure, sex, and endogenous fuel availability in humans.J. Clin. Invest. 2003; 111: 981-988Crossref PubMed Scopus (110) Google Scholar, M.D. M. of lipolysis in and PubMed Scopus Google Scholar). We found a between RER and plasma FFA concentrations (9Shadid S. Kanaley J.A. Sheehan M.T. Jensen M.D. Basal and insulin-regulated free fatty acid and glucose metabolism in humans.Am. J. Physiol. Endocrinol. Metab. 2007; 292: E1770-E1774Crossref PubMed Scopus (42) Google Scholar) to this finding and understand this relationship is constitutional We this to understand the factors that relate to inter- and intraindividual variability in overnight postabsorptive RER and FFA concentrations. The data to address this association is to constitutional and individual factors the of research We interindividual variability in postabsorptive RER even our research volunteers consumed a fixed weight diet for days prior to the or as as technical with the indirect calorimetry can differences in RER, we believe that the of fasting RER with FFA concentrations and the previous day's RER in our volunteers these as the for our energy and body composition balance been achieved, the average RER should the and interindividual in fasting RER would result from in the to between and fatty acid interindividual differences in fasting RER was to the previous day's average RER than RER variability We this as that, (4Grunwald G.K. Melanson E.L. Forster J.E. Seagle H.M. Sharp T.A. Hill J.O. Comparison of methods for achieving 24-hour energy balance in a whole-room indirect calorimeter.Obes. Res. 2003; 11: 752-759Crossref PubMed Scopus (23) Google Scholar, 5Schrauwen P. van Marken Lichtenbelt W.D. Westerterp K.R. Energy balance in a respiratory chamber: individual adjustment of energy intake to energy expenditure.Int. J. Obes. Relat. Metab. Disord. 1997; 21: 769-774Crossref PubMed Scopus (29) Google Scholar), our to energy balance are with the that deficits or in energy balance fat or oxidation and weight PubMed Scopus Google Scholar), these findings the can have on metabolic Given days to a controlled our metabolic kitchen is able to energy balance within kcal of average energy expenditure as measured by double-labeled water (8Levine J.A. Eberhardt N.L. Jensen M.D. Role of non-exercise activity thermogenesis (NEAT) in resistance to fat gain in humans.Science. 1999; 283: 212-214Crossref PubMed Scopus (781) Google Scholar). The to energy balance could be by of or more of energy effect of or activity energy measured in our volunteers with that using the we found this the by average of < for the volunteers in Protocol We have that the effect of can from (8Levine J.A. Eberhardt N.L. Jensen M.D. Role of non-exercise activity thermogenesis (NEAT) in resistance to fat gain in humans.Science. 1999; 283: 212-214Crossref PubMed Scopus (781) Google Scholar). a for each we energy or of to in of these factors are to the data from Protocol intraindividual RER variability was observed the of four consecutive The variability in activity J.A. The of in human weight gain and PubMed Scopus Google Scholar) in the absence of in food intake to for our on the relationship of postabsorptive RER with the energy balance are and Westerterp Westerterp K.R. respiratory quotient and food in and men and J. Clin. Nutr. 2000; 54: PubMed Scopus Google Scholar) have a relationship between postabsorptive RER and weight within the prior We the negative relationship between overnight postabsorptive RER and FFA concentrations (9Shadid S. Kanaley J.A. Sheehan M.T. Jensen M.D. Basal and insulin-regulated free fatty acid and glucose metabolism in humans.Am. J. Physiol. Endocrinol. Metab. 2007; 292: E1770-E1774Crossref PubMed Scopus (42) Google Scholar) in a new data The average fasting RER was significantly less in women than in men participating in this plasma FFA concentrations were in women than we not measured RER, we have that sex was the for the differences in FFA between women and The for this between men and women in this to the that 3 days was not to weight Our often provide less than the diet of our which results in weight loss to negative balance. the of controlled our was able to to energy intake was to energy expenditure, with 3 days were more on the of food was The women participating in Protocol 2 were more than to the were weight data to that the energy intake was to have been more of a to women in Protocol data from Protocol RER and FFA concentrations were not between women and men (6Nielsen S. Guo Z. Albu J.B. Klein S. O'Brien P.C. Jensen M.D. Energy expenditure, sex, and endogenous fuel availability in humans.J. Clin. Invest. 2003; 111: 981-988Crossref PubMed Scopus (110) Google Scholar), the of RER to differences in FFA concentrations. The negative relationship between RER and FFA even within individuals days (6Nielsen S. Guo Z. Albu J.B. Klein S. O'Brien P.C. Jensen M.D. Energy expenditure, sex, and endogenous fuel availability in humans.J. Clin. Invest. 2003; 111: 981-988Crossref PubMed Scopus (110) Google Scholar) that in energy balance a to variability in overnight postabsorptive fatty acid oxidation (RER) and plasma FFA concentrations. the of such as we have in this report (6Nielsen S. Guo Z. Albu J.B. Klein S. O'Brien P.C. Jensen M.D. Energy expenditure, sex, and endogenous fuel availability in humans.J. Clin. Invest. 2003; 111: 981-988Crossref PubMed Scopus (110) Google Scholar, 7Romanski S.A. Nelson R. Jensen M.D. Meal fatty acid uptake in human adipose tissue: technical and experimental design issues.Am. J. Physiol. Endocrinol. Metab. 2000; 279: E447-E454Crossref PubMed Google Scholar), is often a for H.L. and in PubMed Scopus Google Scholar), we data from new studies in our to minimize the of a 1 The of the intraindividual relationship between palmitate and RER 3) is by the that for of volunteers was the slope significantly from four data subject are is to The of intraindividual the of negative In the of the of these individual more The of between palmitate concentrations and the postabsorptive RER as as similar of individual should not be These two would be to in the of Protocol 3 by The of our findings are that despite of and for research results in the studies of FFA metabolism and substrate oxidation in can be a controlled diet is a for studies of FFA the in fasting plasma palmitate concentrations we observed dietary control (6Nielsen S. Guo Z. Albu J.B. Klein S. O'Brien P.C. Jensen M.D. Energy expenditure, sex, and endogenous fuel availability in humans.J. Clin. Invest. 2003; 111: 981-988Crossref PubMed Scopus (110) Google Scholar) is less than that which we observed were dietary control M.D. M. of lipolysis in and PubMed Scopus Google Scholar). In we found that despite to our volunteers a diet in a metabolic was a in RER that was to plasma FFA concentrations both between and within during measurements. in energy balance were the driving of this variability. the of these we suggest that including indirect calorimetry as part of to FFA metabolism will be of even if significant are to minimize in energy balance. The the research volunteers for and the Mayo Clinic respiratory and for technical and with data activity energy expenditure body mass index food quotient General Clinical Research Center energy expenditure respiratory exchange ratio

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