LEADING ARTICLE SportsMed. 19(3): 166-172. 1995 0112-1642/95/ooo3-o166/S00.5O/0

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The Use of Doubly Labelled Water in Quantifying Energy Expenditure During Prolonged Activity Personal Observations

Joel M. Stager, Alice Lindeman and Jeff Edwards Department of Kinesiology, Indiana University, Bloomington, Indiana, USA

All living organisms require the acquisition of energy in a biologically utili sable form if they are to survive, grow and mUltiply. The relationship be­tween the acquisition of energy or energy intake and the energy usage or energy expenditure of the organism is referred to as energy balance.

When energy intake and energy expenditure are more or less equal, body weight is maintained and the individual is considered to be in energy bal­ance. This is true for all organisms, including hu­mans. If either energy intake or energy expenditure is significantly altered, some appropriate compen­sation must be made in one or other variable for energy stores to remain constant. If adequate ad­justments are not made, weight will increase or de­cline: the states of either positive- or negative­energy balance, respectively.

Energy intake, the amount of food consumed in a given period, is theoretically easy to assess through careful quantification of food intake.11] The energy content of various foodstuffs is known with some small degree of error largely because of compositional differences in similar foods .121 How­ever, error in the quantification of food intake is often introduced when the diet is analysed. Regard­less of level of skill, individuals can to some degree underestimate or overestimate the quantity of food consumed. Food intake may be unconsciously under­estimated by forgetting to include some items, in­accurately estimating portions of food, consciously

altering food intake, or choosing to record only those foods which are considered appropriate,l31 These errors may lead to biased results, but can be controlled to some degree by measuring respira­tory quotient, through assurance of confidentiality, training individuals to accurately assess and record foods consumed, and careful scrutiny and clarifi­cation of the recorded intake.14.51

Other methods of measuring intake may be more accurate, in that food consumption is control­led or how it is recorded is controlled. To date, however, self-reporting is the least reactive, most realistic and noninvasive method of gathering nu­trition information in a free-living population. Through estimates of energy intake and the as­sumption that the body is in energy balance, energy expenditure has been indirectly assessed by these means.

1. Energy Expenditure

In contrast to the routine measurement of energy intake, one of the more elusive goals of human biol­ogists has been the development of a valid tech­nique for the measurement of free-living energy expenditure.l61 While certain errors are inherent in the measurement of energy intake, nevertheless, relatively reliable energy-intake values can be ob­tained from individuals in the free-living state. The measurement of energy expenditure has been a much more complex problem. Various approaches

Quantifying Energy Expenditure Using DLW

have been taken to obtain energy expenditure esti­mates. Since heat is liberated during the conver­sion of food stuffs to energy, one approach has been the development of calorimeters large enough to house study participants for extended periods of time.!7] Unfortunately, the values obtained do not necessarily reflect true free-living conditions. Por­table systems which rely on indirect means of quantifying activity have been developed. Esti­mates based on heart rate monitors or motion de­tectors have been attempted. Again, the employ­ment of these devices tend to affect the very measure they attempt to quantify. Activity records and questionnaires can provide rough energy expen­diture estimates. These, however, require substan­tial cooperation and participation by the indivduals being studied. Thus, despite the application of these approaches, many questions remain concern­ing the caloric expenditure of diverse and distinct groups. How much energy does an athlete expend during training? What are the caloric needs of chil­dren? Does obesity result from caloric imbalance? How do the energy needs of the elderly differ from those of the young? Conventional methodology for assessing either the quantity of daily activity or ultimately the state of energy balance has been too restrictive or unacceptably inaccurate to allow these questions to be properly addressed.

2. Doubly Labelled water

One of the more recent advances in the study of energy balance has been employment of the doubly labelled water (DLW) technique as a method to quantify energy expenditure. The use of stable isotopes to measure energy expenditure was first proposed and used in animals by Lifson et al. in 1955.!8] It was not until 1982 that this technique was employed in humans.!9] The delay was primar­ily due to the high cost of the isotope, its limited availability and the relative imprecision of mass spectrometry at that time. Recent improvements in these aspects have made the DLW method a viable approach with considerable potential.

Briefly, the DLW method is based on the con­cept that oxygen (160) in expired carbon dioxide is

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167

in isotopic equilibrium with the oxygen in body water. A dose of water containing the isotope deu­terium (2H2) and the stable oxygen isotope 180 (as 2H2180) is ingested and allowed to equilibrate with the total body water. Over time, water lost from the body during normal physiological activity con­tains the 2 isotopic labels in proportion to their concentrations in the body water. However, in ad­dition to the loss of 180 as water, this isotope is also lost in expired air as carbon dioxide (C02)' There­fore, the difference in the rate of loss between the 2 labels is related to the rate of C02 production.! 10] Using a measured or assumed respiratory quotient, along with the estimated C02 production rate, oxy­gen consumption and energy expenditure can be calculated.

The technique is safe because the isotopes em­ployed are naturally occurring rather than radio­active. Owing to the enhanced sensitivity of the mass spectrometers available today, the necessary loading doses are small, elevating the naturally ex­isting levels only minimally. The protocol in using DLW to measure energy expenditure requires that participants provide an initial body fluid (urine, blood or saliva) sample before ingestion and an­other sample 3 hours or so after ingestion.! II] Sub­sequent to these initial measures, the participants go about their normal routine with little to no inter­ruption of life activities other than daily collection of serial fluid (urine, blood or saliva) samples. Otherwise there is no investigator interaction, and therefore little to no participant reaction.

Recent reports by several investigators have validated the use ofDLW in humans during periods of sedentary and high activity.!12] In comparison with respirometry or direct calorimetry, no system­atic bias has been observed. It has been reported that DLW is as accurate as gas analysis or respirometry, with precision being 2 to 8% depend­ing upon the amount of isotope ingested and the isotope turnover rate.[13] As a result of our own validation experiments, we support the general conclusion that the DLW method is an effective index of energy expenditure in free-living hu­mans.[14]

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168

3. The Female Endurance Athlete

One of the initial applications of the DLW method has relevance to the concerns of the female athlete.[IS] It has been well documented that many women athletes are lower in body fat than their sedentary contemporaries. Average body fat levels for collegiate women endurance runners are com­monly reported to range from 10 to 13%, while values for inactive women of a similar age vary from 20 to 25%)16] While at one time it was thought that this low body fatness was responsible for menstrual­cycle irregularities, low fat mass is no longer ac­cepted as causal to athletically induced amenor­rhoea[17,18] Nevertheless, the mechanism(s) behind the cessation of menstrual cycles are incompletely understood. [19] The 2 most obvious causes relate to the extensive activity regimens in which these women engage and the underlying or predispos­ing genetic traits. Because these women's body­weights are stable, however, it is suggested that they are in energy balance. Estimates of energy in­take must therefore reflect energy expenditure.

Despite this, anecdotal as well as scientific re­ports have indicated that many female endurance athletes may be in chronic caloric deficit with no apparent weight loss, suggesting a substantially greater metabolic efficiency than that which is gen­erally accepted for the average individuaJ.f20] Re­ported energy intake for women endurance runners ranges from 1200 to 2200 kcal, which appear im­possibly low given the extent and rigour of their daily training regimen)21,22] The questions we ad­dressed were: What is the source of the purported discrepancy in energy balance for women endur­ance runners? Is it due to a systematic bias in re­ported food intake? Have energy expenditure esti­mates been too high or food intake artificially low?

Results which were obtained from DLW and food diaries confirmed earlier reports on an appar­ent mismatch between energy intake and energy expenditures in a group of 9 distance runners as noted in table I.

There was an average 32% discrepancy between energy intake and energy expenditure. Yet, the bodyweight of the athletes did not change during

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Mean age (y)

Daily running distance (km)

Weight (kg)

19

6.5 ± 0.9

55,3±6,2

Stager et al.

Body fat (%) 13.0 ± 3.4%

Energy intake (kcal/day) 2037 ± 280

Energy output (kcaVday) 2990 ± 415

Correlations: Energy intake to energy expenditure: r - -<l.83; bodyweight to energy expenditure: r = +0.82; energy intake to bodyweight: r = -D.74.

the 7 days of the study)IS) These results are confus­ing, given the laws of thermodynamics as we cur­rently understand them.

Two additional relationships provide clues to the proper interpretation of these findings. First, we obtained a positive correlation (r = 0.82) between energy expenditure and bodyweight)IS) This is rea­sonable when given that, if all participants were running similar distances, those that weighed the most would be expected to expend the most energy, with all other factors being equal. Training logs kept by the participants indicated that their exercise bouts were similar. The high correlation lent fur­ther support to the validity of the DLW energy ex­penditure estimates. In contrast, the negative cor­relation (r = -0.74) observed between the energy intake and bodyweight translates to the heavier women eating less and the lighter women eating comparatively more. Our interpretation of these findings is that the energy imbalance observed in this population was probably due to measurement error associated with energy intake estimates rather than any significant error in energy expenditure. Underreporting ranged from 4 to 58%, with the heavier women showing the greater discrepancy between input and expenditure)IS] This is consis­tent with reports from other studies based upon DLW estimates of energy expenditure, specifically showing a tendency to underreport food intake by between 20 and 30%, particularly in the over­weight population)9)

4. High Altitude

A second area of interest in which we recently employed the DLW technique to obtain energy

Sports Med, 19 (3) 1995

Quantifying Energy Expenditure Using DLW

balance information concerns the weight loss ob­served during high altitude expeditions.l231 The ob­servation that weight loss occurs at high altitude is not new. Investigators have reported losses of up to 25% of bodyweight during stays at altitude of only a few weeks' durationP41 It has not been de­termined, however, to what extent this weight loss is due to a deficit in energy intake. Almost nothing is known about the energy requirements during ac­tivities of this sort. Estimated energy requirements from 3200 to 6000 kcal/day have been derived from food records and weight changes. Expe­ditions are extremely physically demanding, re­quiring daily 6- to 8-hour ascents carrying packs weighing between 20 and 30kg (50 and 70Ib). Ob­viously, food intake is limited by the weight of the food that climbers are capable of carrying.

We utilised the DLW method to provide esti­mates of energy expenditure during a mountaineer­ing expedition on Mt McKinley in Alaska. At 6190m (20 320ft), Mt McKinley is the tallest peak on the North American continent. Referred to more correctly as Denali, the native Alaskan word mean­ing the high one, it is one of the tallest mountains on Earth from base to summit. Because it is situ­ated so close to the Arctic circle, the atmosphere is thinner still, making it some 450 to 600m (1500 to 2000ft) higher from the physiological perspec­tive than geologically equivalent peaks. Also, due to its northern latitude, some of the most horren­dous weather routinely occurs on DenaliP51

To assess energy expenditure on a mountain so­journ, DLW was administered to 6 climbers at base camp [2l90m (7200ft)] on the day preceding the c1imb.l261 During conditioning for the climb, climb­ers maintained food and activity records to deter­mine baseline energy needs. During the climb, daily urine samples were provided for DLW anal­ysis and food· intake was recorded. Because the menu was preprogrammed, very similar for each climber and only available in preset portions, it was felt that the results of the food analysis were highly accurate. Table II shows the pertinent data for the climbers before and during the ascent.

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169

Table II. Assessment of energy expenditure during a mountain climbl26]

Mean age (y)

Body fat (%)

Energy intake (kcal/day)

before climb

during climb

Energy expenditure during climb (kcal/day)

Bodyweight (kg)

before climb

after climb

36

13.8%±4.3

2950 ±729

3147 ± 952

5586 ± 282

78.4 ± 0.2

76.1 ±0.8

Results indicated a substantial energy-intake deficit averaging 2439 kcal per day. The energy intake was not even adequate to maintain body­weight while conditioning for the climb, let alone meet the energy needs for the arduous ascent. While not nearly as large as the losses observed by others, this energy deficit did result in sizable weight loss over a short period: 3.2 ± 1.8% of total body weight. When the weight loss over the testing period was considered, nearly all (97%) of the loss could be attributed directly to intake deficit.

Results suggested that the caloric need of climbers is greater than previously recommended. Furthermore, our initial findings suggested that the weight loss at high altitude was not necessarily as­sociated with physiological changes and/or altered gastrointestinal absorption, in that it appeared to be simply due to the inadequate amount of food eaten by the climbers. More recent studies confirm these initial results by showing that climbers can virtu­ally eliminate the weight loss by maintaining ade­quate energy intake. l271

5. Ultraendurance Cycling

A third example of our use of the DLW method concerns one of the truly extreme athletic events, the Race Across America (RAAM). This 5000km (3000-mile) cycling event represents sustained ac­tivity beyond that generally considered possible. The race begins on the Pacific coast and continues virtually nonstop until riders arrive at the Atlantic Ocean. Few rules govern the racers' activity other than that they must begin together, follow a moni­tored, prescribed path, be attended by escort vehi-

Sports Med. 19 (3) 1995

170

cles and finish within 48 hours of the lead cyclist of the same sex. It is not unusual for riders to stay on the bike for as long as 48 hours at a time! Riders may cover more than 400 miles a day and sleep/rest less than 2 hours a day over an 8- to 10-day period.

In addition to obtaining some basic under­standing of how these cyclists complete this ardu­ous task, our interest in this event was related to obtaining a value for the energetic ceiling in hu­mans. The recommended average daily intake for a reference man [70kg (l501b)) ranges from 2400 kcal on a very sedentary day to 3900 on a very active dayPSl Obviously, this variation in require­ment is primarily caused by changes in activity pat­tern. Is there a limit as to how much energy we can expend? The suggestion has been made that the ability to increase expenditure and sustain it at a very high rate over a prolonged time may be lim­ited. Cross-species comparisons have shown sur­prising consistency; sustainable energy expendi­ture may be approximately 5 to 6 times the average sedentary rate: this would be interpreted to repre­sent an upper limit of 12000 kcal for a 70kg manP61 But can this quantity be consumed day af­ter day to meet energy requirements? Interestingly, but not too surprising, one critical limiting factor that has been hypothesised relates to energy intake. It has been proposed that the gut and the processes associated with assimilating nutrients from food­stuffs may not be capable of handling and absorb­ing food at rates higher than 5 or 6 times the aver­ageJ291 It is difficult to identify which parameter, intake or expenditure, determines which. Is energy intake and nutrient assimilation limited because energy expenditure is no greater and thus intake need is no greater? Or is the ceiling for energy ex­penditure limited by the ability to assimilate and metabolise food stuffs?[301

To prepare and qualify for RAAM, riders must compete in 24-hour rides. Food records were kept during these rides to estimate how much energy and fluids would be needed during the actual 5000km competition (table III).l3 11

Because of the demand for carbohydrates and fluids during endurance events, the participants

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Stager et al.

Table III. Energy intake during training. qualifying races and competitionl31 ]

Age(y)

Weight (kg)

Body fat (%)

Energy intake (kcal/day)

training

24h qualifying races

RAAM

39

79 7.6

4743

10343

8429

Abbreviation: RAAM = Race Across America.

concentrated mostly on glucose-electrolyte bever­ages to meet their needs. This translated into almost 18kg (401b) of food and fluids per day, including 3.9 gallons of fluid. Most of the energy was in the form ofliquids. The participants averaged 347 kcal and 677ml of fluid per hour throughout the race. This energy intake represents 3.5 times the re­quired energy intake for sedentary humans. Al­though bodyweight was maintained throughout the race, it appeared that a significant amount of fluid was retained; the participants appeared much leaner at the end of the race. Considerable weight loss occurred in the 72 hours following the race. Fluid and energy intake varied with the number of miles cycled per day. Fatigue, gastric distress and sleep deprivation affected all of these variables. How­ever, the question still is, did the participant eat more on the days on which he rode farther, or did he ride farther because he was able to eat more?

6. Limitations to DLW Use

While the data from these research projects are exciting and insightful, widespread application of the DLW technique for the measurement of energy expenditure is not likely for some time. There are a number of barriers which need to be removed for this technique to become routine.

The first limitation is the availability of the iso­tope. Currently, there are few sources worldwide, with only one in the United States. As a result, it is common for there to be as much a year between the placement of an order for the isotope ISO and its arrival. Clearly, funding schedules for projects and timetables for completion of projects make these ex­cessive delays unacceptable in many situations.

Sports Med. 19 (3) 1995

Quantifying Energy Expenditure Using DLW

The second limitation concerns isotope cost.

While the cost of the deuterium is trivial, the cost

for a single dose of 180 for a typical adult subject

is approximately $US600 to $US800. This single

dose will provide data for 5 to 10 days of energy

expenditure. Thus, at the moment, the use of DLW

in large population studies or those requiring mul­tiple dosages is economically impossible. It was

thought that increased interest in stable isotope us­

age would stimulate more production and result in

an increase in the number of commercial produc­ers, ultimately helping to reduce the cost. How­

ever, this has not been the case.

The third primary limitation relates to the nec­essary technology and analytical process by which the concentrations of the isotopes are determined. The technology involved relates primarily to the

mass spectrometer. An initial investment of several

hundreds of thousands of dollars is required to pur­

chase and maintain a mass spectrometer facility.

Technical support is necessary in order to maintain

the equipment and assure the reliability and valid­ity of the values obtained. The process by which the body fluid sample is converted into a sample that the mass spectrometer can handle is also technically complicated and requires specialised equipment. Obviously, these are limitations that cannot be easily surmounted unless collaboration with colleagues in organisations with this equip­ment can be arranged.

The questions that remain are: Does DLW rep­

resent the long sought Holy Grail of energy-expen­

diture techniques? Is this the procedure which will

provide the answers that transform the current understanding in numerous fields of inquiry? At

present, the answer appears to be a qualified 'yes'. DLW has been employed in some important and novel but limited ways. Until analytical techniques are simplified, isotopic costs are reduced and the isotopes become more available, the DLW tech­nique will be utilised sparingly at best, and will be limited to only a few laboratories in the world.

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171

Acknowledgements

Support for the work was kindly obtained from Ross Laboratories, USA Track and Field, NIH BRSG, Lipton's and the National Institute for Fitness and Sport.

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Stager et al.

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Correspondence and reprints: Dr Joel M. Stager, Department of KineSiology, Indiana University, Bloomington, IN 47405, USA.

Sports Med. 19 (3) 1995