A Portable Unit for Anthropological Research

This article describes a lightweight, portable unit which can be used by anthropologists in field studies to obtain data on human caloric expenditures. Human energetics data are in growing demand and the unit here described permits efficient collection and rapid analysis of caloric expenditures in even the most remote research situations. The entire unit consisting of a backpack respirometer and a fuel-cell powered oxygen analyzer with the spares and accessories can be fitted into a small carrying kit at a total weight of under 25 pounds. The use of the unit and the analysis of the data are also discussed. With the growth of anthropological interest in ecology and nutrition, there has been an increasing concern with human energy utilization. The caloric values of human energetics equations---food, activity, and the products of activity---have become desired data, and yet, lacking adequate reference standards for most of the world's populations, anthropologists have been unable to move ahead quickly with their efforts to build or test models of human energetics. Most efforts have been confined to estimating the caloric values of diets and foods produced. Anthropological use of estimates of caloric outputs are far fewer in number, and nearly all studies to date have relied upon calculations of activity-specific energy expenditures from other populations as reference standards (e.g., Rappaport 1967, Lee 1969, Gross and Underwood 197l). Besides the critical importance of energy expenditure data for understanding human activity in context, such data are urgently needed to better assess human food needs (Durnin, Edholm, Miller, and Waterlow 1973). It is now feasible to measure energy expenditures in field situations, and the remainder of this article describes a portable unity which can be used in anthropological research. There are three basic steps in calculating energy expenditures, (1) measurement of the quantity of air used in a known period of time, (2) determination of the quantity of oxygen consumed in that volume of air, and (3) conversion of that volume of consumed oxygen to kilocalories. The first two steps require instrumentation (a gas metering device and a gas analysis apparatus) and the third can be accomplished much more efficiently with the aid of a calculator. The portability of the equipment is thus a decisive factor in conducting field studies. Physiologists and nutritionists in their many decades of research on human pulmonary function and energy metabolism have developed increasingly more convenient methods for the measurement of gas volumes, but until recently the available methods for gas analyses could be done only in a laboratory. The laboratory procedures have been well-described by Consolazio, Johnson, and Pecora (1963), and the history of physiological research on human energetics through the mid1960s has been clearly summarized by Durnin and Passmore (1967). A Portable Unit for Field Studies In field research, the volume of air used by a person can be conveniently measured with a Max Planck Respiration Gas Meter. This small, compact, lightweight backpack respirometer was developed by physiologists at the Max Planck Institute for Work Physiology, Dortmund, West Germany. It combines a gas volume meter and a sampling device for continuous sampling of each breath of expired air. The presently available version (Model 59) comes equipped with a rubber mouthpiece, a low resistance two-way respiratory valve, a corrugated rubber connecting tube, a rubber nose clip, a two-liter rubber bladder for collection of the sampled expired air, and a pair of shoulder straps.** This instrument weighs only 8 1/2 pounds and is 4 3/4" x 7 3/4" x 10 5/8" in size. The shoulder straps are adjustable so that it can be comfortably worn without restricting arm or trunk movements. It has been shown that the energy costs of wearing it do not differ significantly from those without it (Consolazio, Johnson, and Percora, 1963, pp. 46-47). The gas meter has an on/off tap for starting and stopping the metering and sampling, and it has a sampling selection tap to permit either 0.3% or 0.6% sampling. With a two-liter rubber bladder, and given that respiration rates for about 6 to 50 liters per minute are likely to be observed depending upon the activity and size of the person being tested, sample at the 0.3% or 0.6% rate can be done for periods of up to 13 to 110 minutes or 6 to 55 minutes, respectively. The exact time periods of respiration measurements can be recorded with a good stopwatch, preferably one capable of 30 minute timings. Breathing through the mouthpiece while wearing a nose clip can be learned quickly, and most individuals rapidly adjust to using the respirometer. The advantages of this unit over the Douglas bag method and the use of meteorological balloons are several. The respirometer is lighter and has been found to be less cumbersome (Durnin and Passmore 1967, pp. 20-21). The rubber sampling bladder can be directly fitted to the oxygen analyzer, thereby eliminating the transfer of expired air samples in glass gas-tight syringes. Also, the respirometer permits the continuous metering of activities of even considerable duration while the rubber sampling bladders are changed or removed as is necessary. In contrast, the fixed volumes of the Douglas bag and weather balloons (maximum capacities, 200 and 120 liters, respectively***) restrict their use to relatively shorter periods of time. The development of portable battery and fuel-cell powered oxygen analyzers makes it possible to complete oxygen analyses of expired air samples in the field. The Teledyne Oxygen Monitor (Model 331-B)**** enables simple, direct analysis of the concentration of oxygen over the 0-25% range with a sensitivity of 0.125%. This completely portable unit is powered by a micro-fuel-cell with a twelve-month life which is housed in a cylindrical probe assembly connected to the meter. It operates at temperatures between 32 and 125 degrees Fahrenheit, is 3 3/4" x 4 3/4" x 8 1/4" in size, and weighs just under two pounds. The two-liter sampling bladder can be fitted directly to the probe assembly by a short piece of plastic tubing (3/16" I.D., 5/16" O.D.) attached to the inlet nipple of the flow-through adapter cap on the probe. The entry of ambient air into the probe is prevented by placing in water the loose end of another piece of plastic tubing connected to the outlet nipple of the flow-through adapter cap. Oxygen is measured by the micro-fuel-cell which consumes oxygen from the atmosphere surround the cell under the cap in the probe, and the oxygen consumed generates a proportional electric current to drive the micro-ampere meter. A small but sufficient volume of sampled air flowing into the probe will flush out the remaining dead air. A stabilized reading of the oxygen percentage can be made within 30 seconds of introducing the new sample. Four or five same from each bladder should be tested to check the constancy of the readings. The meter can be calibrated against atmospheric air, but in field situations the continued accuracy of the analyzer can be tested against samples of a precisely known concentration (<25%) of bottled oxygen with a nitrogen or carbon dioxide background. For a tested sample, the difference between 20.93% (atmospheric air) and the oxygen concentration of the expired air represents the percentage of oxygen consumed. Once the expired air volume and oxygen consumption are known, the caloric expenditure of an individual can be most conveniently derived using the Weir formula (Durnin and Passmore 1967, p. 18). This formula, which follows, expresses the principle that 4.92 kilocalories of energy are liberated from each liter of oxygen utilized by the body. 4.92 X (corrected vol. of ) X (20.93% oxygen%) (air consumed/min) ( in sample ) kcal/min = __________________________________________