Ambulatory Techniques for Measurement of Energy Expenditure
MEASURING ENERGY EXPENDITURE AND fuel utilization during field (ambulatory) operations is critical to the military mission. Chapters 12 through 14 present techniques currently in use in the field, while Chapter 15 explores a laboratory-based technique.
The doubly labeled water method, the subject of Chapter 12, estimates energy expenditure and total body water by measuring production of singly labeled carbon dioxide and water after administration of water doubly labeled with stable isotopes of hydrogen and oxygen. After subjects digest the labeled water, samples of urine or saliva are taken and the respiratory quotient (carbon dioxide produced/oxygen consumed) is determined, which allows for a noninvasive, accurate estimate of energy expenditure in the field. Care must be taken to ensure the accuracy of analysis, and in longer field studies, consideration must be given to the variances in the natural abundance of the stable isotope of water due to geography.
In Chapter 13, three portable systems for determining minute ventilation and oxygen uptake are profiled. All have been validated in the laboratory, but only one has been validated in the field. The systems are costly and require trained personnel, but it is easy to learn the technique and analyze the data.
As a tool for the immediate assessment of physical activity and determination of energy needs, the ambulatory foot contact monitor is used to estimate the metabolic cost of locomotion, as described in Chapter 14. Estimations are based on the ratio of total body weight to foot contact time during each stride through an electronic device implanted in a shoe. While the technology is simple, easy to use, and inexpensive, it is unable as yet to determine total energy expenditure, needs to incorporate body weight, and does not account for up-and downhill movement.
The author of Chapter 15 details the use of near-infrared spectrometry as a method for the measurement of a single constituent within a complex organic mixture both in vitro and in vivo. After scanning a mixture over a range of wavelengths, the peaks in the resultant absorption patterns are analyzed using Beer's law or regression analysis to determine the concentration for the constituent of interest. One of the main uses of this technique is for monitoring blood metabolites noninvasively, but improvements in technology need to be made before its potential in the field can be tested.