Arch J R S, Hislop D, Wang S J Y, Speakman J R
Clore Laboratory, University of Buckingham, Buckingham, UK.
Int J Obes (Lond). 2006 Sep;30(9):1322-31. doi: 10.1038/sj.ijo.0803280. Epub 2006 Jun 27.
Indirect calorimetry is increasingly used to investigate why compounds or genetic manipulations affect body weight or composition in small animals. This review introduces the principles of indirect (primarily open-circuit) calorimetry and explains some common misunderstandings. It is not widely understood that in open-circuit systems in which carbon dioxide (CO2) is not removed from the air leaving the respiratory chamber, measurement of airflow out of the chamber and its oxygen (O2) content paradoxically allows a more reliable estimate of energy expenditure (EE) than of O2 consumption. If the CO2 content of the exiting air is also measured, both O2 consumption and CO2 production, and hence respiratory quotient (RQ), can be calculated. Respiratory quotient coupled with nitrogen excretion allows the calculation of the relative combustion of the macronutrients only if measurements are over a period where interconversions of macronutrients that alter their pool sizes can be ignored. Changes in rates of O2 consumption and CO2 production are not instantly reflected in changes in the concentrations of O2 and CO2 in the air leaving the respiratory chamber. Consequently, unless air-flow is high and chamber size is small, or rates of change of O2 and CO2 concentrations are included in the calculations, maxima and minima are underestimated and will appear later than their real times. It is widely appreciated that bigger animals with more body tissue will expend more energy than smaller animals. A major issue is how to compare animals correcting for such differences in body size. Comparison of the EE or O2 consumption per gram body weight of lean and obese animals is misleading because tissues vary in their energy requirements or in how they influence EE in other ways. Moreover, the contribution of fat to EE is lower than that of lean tissue. Use of metabolic mass for normalisation, based on interspecific scaling exponents (0.75 or 0.66), is similarly flawed. It is best to use analysis of covariance to determine the relationship of EE to body mass or fat-free mass within each group, and then test whether this relationship differs between groups.
间接量热法越来越多地用于研究化合物或基因操作为何会影响小动物的体重或组成。本综述介绍了间接（主要是开路）量热法的原理，并解释了一些常见的误解。人们普遍不太了解的是，在开路系统中，如果不从离开呼吸室的空气中去除二氧化碳（CO2），测量离开呼吸室的气流及其氧气（O2）含量，反常地会比测量氧气消耗更可靠地估计能量消耗（EE）。如果还测量离开空气的CO2含量，就可以计算氧气消耗和二氧化碳产生量，从而得出呼吸商（RQ）。只有在测量期间可以忽略改变其库大小的常量营养素相互转化的情况下，呼吸商与氮排泄相结合才能计算常量营养素的相对燃烧情况。氧气消耗率和二氧化碳产生率的变化不会立即反映在离开呼吸室的空气中O2和CO2浓度的变化中。因此，除非气流大且室体小，或者在计算中包括O2和CO2浓度的变化率，否则最大值和最小值会被低估，并且出现时间会比实际时间晚。人们普遍认识到，身体组织更多的较大动物比较小的动物消耗更多能量。一个主要问题是如何校正动物体型差异来进行比较。比较瘦和肥胖动物每克体重的EE或O2消耗量会产生误导，因为不同组织的能量需求不同，或者它们以其他方式影响EE的方式也不同。此外，脂肪对EE的贡献低于瘦组织。基于种间标度指数（0.75或0.66）使用代谢体重进行标准化同样存在缺陷。最好使用协方差分析来确定每组中EE与体重或去脂体重之间的关系，然后测试组间这种关系是否不同。
