Review Articles IPediaiiic Exercise Science. 1994, 6.11 -30tt' 1994 Human Kinetics Publishers, Inc.

Application of the Doubiy Labeied WaterTechnique for Studying Total Energy

Expenditure in Young Chiidren: A Review

Michael i. Goran

The doubly labeled water tecbnique represents an unobtrusive and non-invasive means lo measure total daily energy expenditure in free-living humansubjects who are unaware that energy expenditure is being measured. Whencombined with measurement of resting energy expenditure, the doublylabeled water technique can also be used to estimate energy expenditurerelated to physical activity. The relatively recent availability of the doublylabeled water technique in humans has led to several advances in the funda-mental understanding of whole body energy metabolism in several importantareas. The purpose of this paper is to review the areas in which the doublylabeled water technique has specifically advanced our understanding ofwhole-body energy metabolism in young children.

Description of the Doubly Labeled Water Technique

The doubly labeled water technique is the first truly noninvasive means to accu-rately measure total daily energy expenditure in free-living humans. The techniquewas first introduced by Lifson et al. (37) in the 1950s as an i.sotopic techniquefor measuring carbon dioxide production rate in small animals. Unfortunately,it was not possible to apply the technique to humans becau.se the dose requiredwas cost prohibitive, given the relatively poor sensitivity of isotope ratio massspectrometry at that time. It was not for another 20 years that Lifson et al. (38)described the feasibility of applying the technique to humans, an application thatwas later recognized by Schoeller et al. (65, 67).

The doubly labeled water technique is based on the kinetics of two stableIsotopes of water, -H.O (deuterium labeled water) and H;"*O (oxygen-18 labeledwater). These stable i.sotopes are naturally occurring compounds without anyknown toxicity or side effects at the low doses used (33). The preferred routeof dosing is by mouth, but under some situations water can be given intravenously.The isotopes mix in the body water pool, reaching equilibrium at approximately4 hours after dosing. Deuterium labeled water is lost from the body in the usualroutes of water loss (urine, sweat, evaporative losses). The biological half life

Michael I. Goran is with the Division of Endocrinology, Metabolism and Nutrition.Department of Medicine and the Sims Obesity/Nutrition Research Center, College ofMedicine at the University of Vermont, Burlington, VT 05405.

11

12— Gorati

of deuterium labeled water in a typical 5-year-otd child is approximately 7 days(22). Oxygen-18 labeled water is lost from the body at a slightly faster rate sincethis isotope is also lo.st via carbon dioxide production through the carbonicanhydrase equilibrium (37). in addition to all routes of water loss. The differencein the rate of loss between these two isotopes is therefore a function of the rateat which the body produces carbon dioxide. Several equations are available forcalculating CO: production from doubly labeled water, and these are reviewedelsewhere (49. 69).

The doubly labeled water technique has been validated in humans in severallaboratories around the world by comparison with indirect calorimetry in bothadults (8. 54, 65, 67. 68, 71) and infants (32. 57). These studies, which arereviewed elsewhere (48, 56. 61, 63, 64), generally show the technique to beaccurate to within 5-10%, relative to data derived by indirect calorimetry forsubjects living in metabolic chambers. Speakman et al. (69) recently providedrevised equations for calculating energy expenditure from doubly labeled waterthat have been shown to improve the accuracy of previously performed validationstudies to approximately 3%.

The theoretical precision of the doubly labeled water technique has beenexamined by several investigators (7, 20, 66). These studies suggest that thetechnique has a theoretical precision of 3-5%. However, the experimental reliabil-ity for triplicate measures of total energy expenditure in young free-living males(under conditions of constant caloric intake) was shown to be ±12%, due tofluctuations in physical activity levels within individuals over time (20). Undermore controlled, sedentary living conditions, the experimental reproducibility ofthe technique was approximately ±8%. which is closer to theoretical estimates(27). It is therefore difficult to assess the actual experimental reliability of thedoubly labeled water technique under the conditions in which it is typicallyemployed because of inherent variation in physical activity related energy expen-diture.

There are several assumptions of the doubly labeled water technique thathave been described in other papers (37. 38, 39, 45). Briefly, these assumptionsinclude the usual assumptions inherent in any tracer study: (a) The pool sizedoes not change (i.e.. no expansion or contraction of total body water during thestudy period); (b) no exogenous addition of extra isotope during the study (i.e..the level of the naturally occurring stable isotopes in the diet remain constant);(c) the rate of water loss and carbon dioxide is constant from day to day; (d) thestable isotopes of water ('H;O and H:'*'O) have the same chemical and physicalproperties as that found in the most abundant form of water (HnO); and (e) thestable isotope 'HjO is lost only through loss of body water, and H ' O is lostonly via water loss and carbon dioxide production. It is well known that some ofthese assumptions are violated in human studies, but several modeling refinementshave been introduced to take these into account (44. 49. 65, 69). Although doublylabeled water has been compared to long-term indirect calorimetry data in infants(32, 57), adults (8. 54, 65, 67. 68, 71), and many animals (45), the techniqueha.s never been specifically cross-calibrated in young children. However, thereare no obvious reasons why the technique should not be valid in young children.

There are several advantages of the doubly labeled water technique thatmake it a powerful technique. First, the technique is noninvasive and thereforeallows for unobtrusive measures of energy expenditure in children who are living

Doubly Labeled Water — 13

in their normal environment and are unaware, as are their parents, that energyexpenditure is being measured. Second, measurements are performed over exten-sive time periods, typically 1 to 2 weeks. Third, by combining the doubly labeledwater technique with conventional indirect calorimetry. It is possible to measurethe individual components of daily energy expenditure (26). Thus, by measuringtotal energy expenditure (TEE), using doubly labeled water, and resting energyexpenditure (REE), using indirect calorimetry, the energy expenditure of physicalactivity (EEPA) can be estimated by subtraction after adjusting for the energycost of meal induced thermogenesis (usually 10% of total energy expenditure)using the equation EEPA = 0.9 TEE - REE (26).

The utility of the doubly labeled water technique for measuring the dailyenergy cost of physical activity is of particular importance because accuratequantification of this component of daily energy expenditure has traditionallyproven difficult, especially under free-living conditions. Other methods that havepreviously been used to estimate the daily energy cost of physical activity {e.g.,activity diaries, motion sensors, heart rate monitoring) have been difficult toapply because either they require a high degree of dependence on the researchsubject, or the intrusive nature of the technique applied does not exclude thepossibility of change in usual activity during the measurement period.

The disadvantages ot the doubly labeled water technique include the ex-pense (currently approximately $300 for a study in a 5-year-old child) andavailability of the stable isotope H2"*0, and the reliance on sophisticated isotoperatio mass spectrometry analysis of samples. Thus, the technique does not lenditself to large-scale epidemiologic studies. In addition, the doubly labeled watertechnique is a direct measure of CO; production, and additional informationon macronutrient oxidation during the study period is required to convert CO;production to O consumption before energy expenditure can be calculated withthe Weir equation (10). However, under the usual conditions of approximateenergy balance, the food quotient of the diet is used to derive O; consumptionfrom CO; production. The food quotient of the diet can be obtained from publishedpopulation-specific data (15, 75) or calculated from the relative macronutrientcomposition of the diet using the equations of Black et al. (4). Even in thecomplete absence of information on the food quotient of the diet for the populationunder study, the maximum error in deriving energy expenditure from carbondioxide production rate is 3-5% (15).

The energy expenditure data that has subsequently been gathered in free-living humans using doubly labeled water has proved to be invaluable in extendingthe understanding of the regulation of human energy metabolism as it relates toobesity (2, 50, 70), growth (5), aging (26), pregnancy (18, 29, 35), energyrequirements (20, 22, 26, 52, 58), as well as understanding the adaptation tooverfeeding (34, 60), underfeeding (30), and exercise (3, 25). Most of thesestudies have either been in adults or infants and have been reviewed elsewhere(48. 56, 61, 63, 64). The relatively few studies that have heen pertbrmed inyoung children are reviewed below.

Measuring Total Energy Expenditure in Young ChildrenTotai Energy Expenditure in Healthy ChiidrenSeveral laboratories have reported measurements of total energy expenditure inyoung, healthy, free-living children (17, 22, 40, 42). These studies have been

14 — Goran

Table 1 Comparison of Published Data on Tola! Energy Expenditure (TEE)Using Doubly Labeled Water in Young, Healthy, Free-Living. Cauca.sian Children

Study

Goran et al. (22)Goran et al. (22)Fontvieille et al.Fontvieille et al.Livingstone et a

Livingstone et a.

(17)(17).(42)

.(42)

Subjects

16 boys from Burlington, VT14 girls from Burlington, VT15 boys from Phoenix. AZ13 girls from Phoenix. AZ6 boys from Belfast,

Northern Ireland6 girls from Belfast,

Northern Ireland

Age

4-64-65-65-65

5

Weight(kg)

M

20.321.021.118.917.9

18.1

SD

4.34.73.92.52.5

2.2

TEEM

,440,309.468,297,626

^87

SD

271304263207240

266

performed in Phoenix, Arizona (17), Belfast, Northern Ireland (40, 42), andBuriington, Vennont (22). Despite marked differences in geographic locations,the data are very similar among these three laboratories (Table I). In studiesconducted during the summer months In Phoenix, Arizona, Eontvieille et al. (17)reported total energy expenditure in Caucasian children (mean age = 5.5 years;mean weight = 20.1 kg) similar to that found in age (5 ± 1 years) and weight(20.6 ± 4.4 kg) matched children living in Buriington, Vermont. The total energyexpenditure data were similar for both boys (1468 ± 263 kcal/day in Phoenix,compared to 1440 ± 271 kcal/day in Burlington) and girls (1297 ± 207 kcal/day in Phoenix, compared to 1309 ± 304 kcal/day in Budington). In addition,Livingstone et al. reported total energy expenditure (by doubly labeled water)of 1456 ± 301 kcal/day in a group of 5-year-old children (mean weight = 18.0± 2.2 kg) living in Northem Ireland (42). In absolute terms the total energyexpenditure of these children living under varied climatic conditions are remark-ably similar. However, in order to make a more thorough comparison of the totalenergy expenditure values among these various studies, an analysis of covarianceof the raw data is required in order to normalize for individual differences inbody composition.

Energy Expenditure In Children Recovering From Burn Injury

The metabolic response during recovery from bum injury includes an increasein resting energy expenditure, although this is not necessarily a function of theextent of bum (21). The nutritional care of patients recovering from hum injuryis a crucial part of their therapy since both underfeeding and overfeeding canlead to metabolic complications (72, 73). The widely used formulae to predictenergy needs in bum patients are not based on measurement of energy expenditure,and they estimate that most patients require 2 to 2.5 times their estimated basalenergy requirements throughout recovery (21). Using the doubly labeled watertechnique, total energy expenditure was 1606 ± 697 kcal/day during late conva-

Doubly Labeled Water — 15

lescence in children recovering from bum injury, equivalent to 1.33 ± 0.27 timespredicted basal energy expenditure and 1.18 ± 0.17 times nonfasting restingenergy expenditure, as measured by respiratory gas analysis (24). Total energyexpenditure in 6 of these younger bum patients (age 5-7 years; 22.0 ± 2.1 kg)was 1463 ± 269 kcal/day. comparable to that found in studies in healthy, free-living children {Table 1).

These data suggest that energy requirements of bum patients are not elevatedrelative to healthy, free-living children. This surprising finding is explained bythe fact that resting energy expenditure is not as elevated in bum patients aspreviously speculated and is not a function of bum size or time after the injury(21), probably due to improvements in wound care that reduce heat loss. Inaddition, energy requirements in patients recovering from bum injury are reducedbecause of the sedentary nature of their hospitalization. The studies of childrenrecovering from bum injury led to the recommendation that optimal predictionsof total energy requirements can be obtained by supplying calories at a rate of1.2 times the individually measured rate of resting energy expenditure, or 1.55times the predicted basal energy expenditure (21). This level of energy require-ment is considerably lower than even the most conservative estimates that suggestcalories should be supplied at a rate of twice the predicted basal requirements(9, 43).

Comparing Energy Expenditure of Chiidren and Aduits

One question of interest is whetber total energy expenditure in young childrenis comparable to adults after normalization for body size. For comparing restingmetabolic rate in subjects of differing body sizes, Ravussin et al. (53) suggestedusing a regression-based approach with fat-free mass as a covariate in place ofa ratio in which resting metabolic rate is divided by fat-free mass. The regression-based approach takes into account the fact that the relationship between restingmetabolic rate and fat-free mass bas a nonzero intercept (53). Data are normalizedwith the regression-based approach by subtracting resting metabolic rate thatwould be predicted by fat-free mass (based on the defined regression equation)from the measured value, thus deriving a residual value for each individual.Normalized, or adjusted, resting metabolic rate is then derived by adding theresidual value to the group mean value (53).

For comparing total energy expenditure data between individuals, the tradi-tional approach has been to use a ratio in which total energy expenditure isdivided by resting metabolic rate, generating an "activity factor." However, byperforming a meta-analysis of the relationship between total energy expenditureand resting energy expenditure, we have shown that a regression-based approach,rather than a ratio-based approach, should similarly be used for normalizing totalenergy expenditure data (6). In reviewing 13 studies in adults from variouslaboratories, the mean regression equation relating total energy expenditure tobody mass is TEE = 24.7 BM + 947 kcal/day. where TEE = total energy expendi-ture in kcal/day and BM = body mass in kg (6).

Applying this regression equation to tbe average 5-year-old child weighing20 kg predicts a total energy expenditure of 1456 kcal/day, similar to the valuesmeasured in several studies (Table 1). This comparison suggests that youngchildren have a total energy expenditure similar to that of adults, after adjusting

16 —Goran

for their smaller body mass. Comparison of subjects differing in body sizecannot be made by simply dividing energy expenditure by body weight sincethe regression between these two variables has a nonzero intercept (6). !f onewere to compare the total energy expenditure:body mass ratio between childrenand adults, one would come to the false conclusion that total energy expenditureis approximately twice as high in children (approximately 70 kcalAg) comparedto adults (approximately 38 kcal/kg). Comparing the regression equation betweentotal energy expenditure and body mass in children (22) with that pooled from13 studies in adults (6) shows a greater regression coefficient in children (53kcal/day/kg body mass in children vs. 24.7 kcal/day/kg in aduits) and a lowerintercept in children (278 kcal/day in children vs. 947 kcal/day in adults), sug-gesting thai across a wide age range, the relationship between total energyexpenditure and body mass is nonlinear.

Applications of the Doubly Labeled Water Techniquein Children

Assessing the Validity of Energy Intake Techniques

Several studies have used the doubly labeled water technique to examine thevalidity of energy intake methodologies. These studies are based on the conceptthat for subjects in energy balance, as indicated by stable body weight and bodycomposition, energy intake should be equivalent to total energy expenditure asmeasured by the doubiy labeled water technique. This concept holds true forstudies carried out over extended time periods (i.e., weeks) due to day-to-dayvariability in food intake. Various studies using this approach in young (I. 62),elderly (26), and obese (36) adults have led to the conclusion that self-reponedmeasurements of energy intake underestimate habitual intake.

The methodological limitations of measuring energy intake are of furtherconcem in studies of young children in which information on intake is dependenton recollection of school teachers and parents. Livingstone et al. (42) haveperformed an extensive and rigorous comparison of several energy intake method-ologies in children by comparison with energy expenditure measured using thedoubly labeled water technique. These investigators compared measurements oftotal energy expenditure over l()-14 days with energy intake measured by thetechniques of 7-day weighted dietary records and reported dietary history inweight stable children. On a group mean basis, energy intake by weighted dietaryrecords was found to be in good agreement with measurements of total energyexpenditure (±3-8%) in 7- and 9-year-oId children, although the weighted dietaryrecords lacked individual precision (±15-25%). The technique of assessing ititakeby diet history was found to overestimate total energy expenditure by 14 ± 19%,II ± 19% and 11 ± 23% in groups of 3-year-old, 5-year-old, and 7-year-oldchildren, respectively, and again there was large variation between individualsin the discrepancy between total energy expenditure and energy intake.

In addition, data from our laboratory in Vermont was used to compareestimates of energy intake obtained by interviewing mothers using a semiquantita-tive food frequency questionnaire with measures of free-living total energy expen-diture using doubly labeled water in young children (N = 45; 23 boys 22 girls;36 Caucasian, 9 Mohawk Indian; 4.2-6.9 years; 20.2 ± 4.0 kg body weight).

Doubly Labeled Water — 17

Energy intake by the food frequency questionnaire (2,145 ± 535 kcal/day) wassignificantly higher than total energy expenditure by doubly labeled water(1,403 ± 276 kcal/day; p < .001). The overestimation of energy intake was 809± 586 kcal/day and was not significantly influenced by gender, race, or age. Thediscrepancy between total energy expenditure and reported energy intake wasnot significantly correlated with child or maternal anthropometries or body com-position. These data suggest that the semiquantative food frequency questionnairesignificantly overestimates energy intake in children by approximately 60% andthat the magnitude of this overestimation is not a function of body compositionin the children or their mothers who filled out the questionnaire.

Collectively, these data suggest that measuring food intake should not berelied upon for individual estimates of energy intake in children. Altematively,as will he discussed below, it is becoming increasingly apparent that measurementof total energy expenditure may serve as a useful marker of energy intake.

Assessing Existing Information on Energy Requirements

One of the major applications of the doubly labeled water technique is to assessenergy requirements (20, 22, 26. 58). The use of the technique in this context isbased on the premise that the energy requirements for maintaining a state ofwhole body energy balance must be equivalent to total daily energy expenditure.Thus, measurement of total daily energy expenditure with the doubly labeledwater technique acts as a proxy indicator of energy requirements for maintenanceof body energy stores. In addition, any new formulations for energy requirementsthat are derived from measurement of energy expenditure will have to be increasedto take into account the energy storage that occurs during growth. Our bestestimate for energy deposition during childhood growth is based on the assump-tions that (a) the average child gains 2.7 kg per year and (b) 75% of body weightgain is fat-free mass (l.I kcal/g, since about 73% of fat-free mass is water) and25% is fat mass (9 kcal/g). With these figures we compute that energy depositionis approximately 23 kcal/day in a growing child.

Energy requirements have traditionally been examined in the context ofmalnourishment or to assess population energy requirements in developing coun-tries (74). Clearly, there is also a need to provide well-founded recommendationsfor dietary energy, not only in developing countries but also in developed countrieswhere ovemourishment (i.e., obesity) is a growing concem. Information on energyrequirements to support healthy childhood growth is essential to control theincreasing incidence rates and prevalence of childhood obesity (11, 12). Paradoxi-cally, the incidence of obesity in children is rapidly increasing despite a generalrise in the awareness of health and fitness in our society. This "fattening" ofyoung Americans is probably due in part to a general increase in sedentaryactivities in children, such as increased television viewing (12, 13) in the absenceof appropriate guidelines for the energy required to promote healthy growth.

Current recommendations for dietary energy are laid out in a documentpublished by the Worid Health Organization (WHO) in 1985 (74). These recom-mendations are based on energy intake data collected in 12,500 children indeveloped countries and are presented as a function of body weight, classifiedaccording to age and sex (74). These tables suggest that the energy requirementsof a 4-5-year-old boy and girl are 95 and 92 kcal/kg body weight, respectively.

18 — Goran

Table 2 Comparison of Recommended Energy Intake With MeasuredTotal Energy Expenditure in Caucasian Children Studied in Burlington, VT

Variable

REE, kcal/dayTEE, kcal/dayTEE - REE, kcal/dayREQ, kcal/dayREQ - TEE, kcal/day

Boys(n = 16)

M

1,1341,440

3061,872

432

SD

140271184394218

Girls(H = 14)

M

1.0871,309

223t,832

523

SD

105304222372176

Boys

M

1,1111,379

2671,853

475

+ Girls= 30)

SD

124290203378202

Note. REE = average of two measures of resting energy expenditure while subjectsview television or video cartoons 2-3 hours after consuming breakfast at home; TEE= total, free-living energy expenditure averaged over 14 days by doubly labeled water;REQ = published requirement.s for energy intake as laid out by the World HealthOrganization (74).

Thus, for an average 20 kg child the recommended energy requirement is 1,850-1,900 kcal/day, which is clearly much greater than the measured value in youngchildren in three independent laboratories (Table 2).

In our recent study my colleagues and I compared measured rates of totalenergy expenditure with estimated energy requirements (22). Mean total energyexpenditure in thirty 4—6-year-old children (20.6 ± 4.4 kg) was 1,379 kcal/day,compared to the average value of energy requirements of 1,853 ± 378 kcal/dayfor these children, according to the 1985 report (74). On average the estimatedenergy requirement was 475 ± 202 kcal/day greater than the measured totalenergy expenditure, and the discrepancy on an individual basis is shown in Figure1. The lower than expected values for total energy expenditure in children wasdue primarily to low activity-related energy expenditure, which we estimated tobe approximately 267 kcal/day (Table 2), representing only 17 ± 11% of totaldaily energy expenditure. This study represents the first report of the caloric costof activity-related energy expenditure in young, free-living children.

The lower than expected energy cost of daily activities in children isprobably explained by the fact that although children appear to be active, theydo not perform high intensity exercise for long periods of time (14, 31), and in6-7-year-old children, heart rate i.s less than 120 bpm (approximately 50% aboveresting heart rate) for 75% of the day (9). In addition, the actual energy cost ofplaying has been shown to be less than a slow walk (46). Therefore, whereaschildren may appear to be active, the total caloric cost of these activities isactually smaller because these activities are low intensity and are not sustainedover extended periods of time.

The lower than expected total energy expenditure in young children, relativeto recommended intakes, extends similar findings in children under 3 years ofage (52). Prentice et al. (52) summarized data comparing measurements of total

Doubly Labeled Water — 19

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800 -

600 -

400 -

200 -

0 -I

r -1 r - i|-j

kXX

X\X

\\XX

\\X\\^

p

1k\\V

vX

\\\\\XX

\\\\VPI -|

1PI1

k\\\\\\\\\\\\\\X

V\X

XX

V

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1PIPI

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1Figure 1 — Discrepancy between current recommendations for energy intake andtotal energy expenditure in 30 children. Recommended intake is the energy require-ments as recommended by (he FAO/DNU/WHO report (74); total energy expenditurewas measured under free-living conditions over 14 days with doubly labeled waterHatched bars are boys, and open bars are girls.

energy expenditure in children under 3 years of age with recommended intakes.These data demonstrate that requirements derived from measurement of totalenergy expenditure (adjusted for growth) are 11%' lower than recommendedintakes in infants less than 12 months of age. In infants between 1 and 3 years,the discrepancy is more striking: The requirements derived from expenditure(adjusted for growth) are approximately 20% lower than the recommended in-takes. For example, in 3-year-old children total energy expenditure by doublylabeled water averages 1,210 kcal/day, whereas the recommended intake for thesechildren is 1,470 kcal/day (52).

Collectively, the reduced energy expenditure in young children highlightsseveral limitations of existing tables for estimating energy requirements in chil-dren. First, the current recommendations are based on measurements of energyIntake, a technique that has recently been subject to much scrutiny, as alreadydiscussed. Second, exi.sting recommendations should not be applied to individualsbecause these recommendations were developed for populations and do not takeinto account the heterogeneity of the population, specifically with respect tophysical activity, an important modulator of energy needs. Finally, the old databases may not be relevant to the modified lifestyle of the 1990s, particularly dueto sociological changes in physical activity patterns over the last few decadesdue to increased reliance on labor-saving devices (e.g., central heating, cars) in

20 — Goran

adults (51). In children, a similar phenomenon has been suggested with a proposeddecline in recreational activities associated with increased television viewing (12,13).

Identifying Alternative Approachesfor Estimating Energy Requirements

Traditionally, energy needs are detemiined as a function of resting metabolicrate using a factorial approach that uses multiples of resting energy expenditure(74). This approach, however, assumes an inappropriate mathematical model fortotal energy expenditure in which resting metabolic rate is multiplied by a constantactivity factor (i.e., a single compartment model) although it is known that totalenergy expenditure has at least three compartments (resting, activity, and themiicresponse to feeding). Using resting metabolic rate to estimate daily energy expen-diture (and thus requirements) lacks precision because of wide interindividualvariation in physical activity level.

In our laboratory, resting metabolic rate only explains 43% of individualvariation in total energy expenditure in young children (22), 40% in young aduits(20), and 42% in elderly adults (26). This finding has been shown in severalother studies that my colleagues and 1 have previously reviewed (6). The lackof association between total energy expenditure and resting energy expenditureis explained hy the fact that the daily energy cost of physical activity—whichincludes the energy cost of all physical movement. Including spontaneous physicalactivity (fidgeting)—is highly variable from one subject to another and withinsubjects over time (20). The daily energy cost of physical activity is thereforenot a function of resting energy expenditure, which is assumed when using thefactorial-based approach to estimate total energy expenditure from resting energyexpenditure. Thus, other factors in addition to resting energy expenditure needto be identified to further explain individual variation in total energy expenditure,and thus more accurately predict energy requirements on an individual basis.

In our recent studies of 30 Caucasian children in Burlington, Vemiont, mycolleagues and I used the data collected to develop an altemative predictivemodel of total energy expenditure (as measured by doubly labeled water). Regres-sion analysis was performed with total energy expenditure as the dependentvariable and other potential markers (body composition, heart rate, age, restingenergy expenditure, and anthropometric data) as the independent variables (22).Total energy expenditure was most significantly related to fat free mass (r = .86;p < .001; Figure 2), body weight (r = .83; p < .001), and resting energy expenditure(/• = .80; p < .001). After adjusting for fat-free mass, total energy expenditureremained significantly correlated with heart rate (partial r = .54; p ~ .002; Figure3).

In this study we measured heart rate for 1 min every 30 min during the 2-to 3-hour period that the children attended the Clinical Research Center fortesting. Collectively, 86% of inlerindividual variation in total energy expenditurewas accounted for by three easily measured variables: (a) fat-free mass as mea-sured by bioelectrical impedance analysis using new equations specificallydeveloped in young children (23), (b) random heart rate, and (c) resting energyexpenditure. Interestingly, we did not detect gender as a significant factor ex-plaining inlerindividual variation in total energy expenditure, even though this

Doubly Labeled Water — 21

UJ

u

2200 -1

2000 -

1B00 -

1600

1400 -

1200 -

1000 -

800 -

10 15 20 25

FAT FREE MASS, kg

Figure 2 — Correlation between total energy expenditure (TEE) and fat free mass(FFM) in boys and girls. Filled circles are boys, and open circles are girls. Theregression line for boys and girls combined in TEE = (94.1 - FFM) - 146 kcal/day,

is considered an important factor in existing recommendations for energy intake(74). The analysis suggested the following preliminary model as a possiblealtemative method to estimate total energy expenditure and thus energy require-ments in young children: TEE = 66 FFM + l lHR + 0.74 REE - 1,482 kcal/day. where R' = 0.86, standard error of the estimate = ±115 kcal/day, TEE =total energy expenditure, FFM = fat-free mass measured by bioelectrical imped-ance using equations developed specifically in young children (23). HR = arandom measurement of heart rate during testing, REE = resting energy expendi-ture measured under the standardized conditions previously described (22).

Examining the Adaptation to Exercise Intervention

The prescription of exercise in combination with diet restriction is becoming apopular tool for weight-control therapy. The rationale behind this concept is thatprescribed physical activity should theoretically elevate daily energy expenditure.However, the idea of increasing daily energy expenditure by exercise interventionis not as straightforward as it seems since exereise intervention may affect othercomponents of daily energy expenditure. For example my colleagues and Irecently examined the effects of exercise intervention in healthy elderly subjectson the various components of daily energy expenditure (25). After 8 weeks ofendurance training three times per week, there was no significant increase intotal daily energy expenditure as assessed by the doubly labeled water technique,despite a 10% elevation in resting metabolic rate measured 36 hours after theprevious exercise session and despite the fact that daily energy expenditure was

22 — Goran

2000 -|

^ 1800 -

Ui

1600 -

0)

to

<

1400 -

1200 -

1000 -80 9 0 100 110

Random heart rate (bpm)Figure 3 — Correlation between total energy expenditure (TEE) adjusted for fatfree mass (FFM) and random heart rate. TEE adjusted Tor FFM using a regressionbased approach, partiai r =: .54, p < .002.

increased by an average of 150 kcal/day due to the exercise intervention. Thefailure to increase total energy expenditure in the face of increased resting meta-bolic rate and the increased energy cost of training was accounted for by acompensatory decline in physical activity during the remainder of the day byapproximately 200 kcal/day, thus negating the caloric benefits of the exercise.

There has been one similar study that has examined the impact of trainingon energy expenditure and spontaneous activity in children. Blaak et al. (3)examined the effect of 4 weeks of training (five 1-hour sessions per week ofcycling at 50-60% of peak VO;) in a group of 10 obese boys (10-11 years,32.4% body fat). There was no change in body composition or sleeping metabolicrate in response to this short-term training program. However total energy expen-diture using doubly labeled water was significantly increased by 12% duringtraining (2,476 ± 93 kcal/day before training, 2,778 + 96 kcal/day during train-ing). The authors estimated that about half of the increase in total energy expendi-ture was attributable to the direct energy cost of the training program. Sincethere was no change in sleeping or resting metabolic rate, the remaining increasein energy expenditure (approximately 150 kcal/day) was explained by an increasein physical activity over and above that expended as part of the training program.However, sleeping and resting metabolic rate may have been underestimatedduring the exercise treatment phase, since they were measured 24--48 hours afterthe last exercise session and, therefore, did not include the well-known residualeffects of exercise on resting and sleeping metabolic rate (47).

From these studies it is clear that changes in other components of dailyenergy expenditure play a key role in the overall energetic adaptation to energy

Doubly Labeled Water — 23

intervention and thus may help explain the varied responses seen among energyintervention studies (47). Thus, various energy prescriptions will need to beexamined for their utility in increasing the various components of daily energyexpenditure so that an optimal design can be identified.

Future Areas of Research

Developing Altemative TechniquesSuitable for Epidemiological Application

The power of the doubly labeled water technique is that it provides accurate andprecise measures of free-living energy expenditure. However, as outlined above,the technique does not lend itself to epidemiological studies. Thus, altemativemethods to measure free-living energy expenditure in large numbers of subjectswould be advantageous for large-scale epidemiological studies. This rationalehas already led to various studies that have attempted to cross-calibrate thedoubly labeled water technique with other more readily available techniques formeasuring total energy expenditure in children.

Livingstone et al. (40) compared the technique of heart rate monitoringover 2-3 days for assessing free-living energy expenditure with the doubly labeledwater technique over 10-15 days in children living in Belfast, Northern Ireland.The mean difference between heart rate monitoring and the doubly labeled watertechnique was -4% and -9% in 7-year-old and 9-year-old children, respectively.However the relative difference between the two techniques varied from -17%to +19%, and therefore, the technique of heart rate monitoring lacked precisionon an individual basis. The overall concordance between heart rate monitoringand the doubly labeled water technique in the absence of individual precision issimilar to that seen in adults using similar techniques from the same laboratory(41).

In addition, Emons et al. (16) compared the technique of heart rate monitor-ing with measurements of energy expenditure in 19 children aged 7-11 years.Heart rate monitoring was compared with energy expenditure measured over a24-hour period in which the children lived in a metabolic chamber and over a2-week period using doubly labeled water. In this study, heart rate monitoringoverestimated energy expenditure measured in the metabolic chamber by 10%and overestimated energy expenditure measured with the doubly labeled watertechnique by 12%. Thus, heart rate monitoring with application of individualcalibration is just as good a technique in children as in adults for measurementof actual energy expenditure, although the technique lacks individual precisionin all age groups. The strength of heart rate monitoring however is that it providesthe unique infonnation of physical activity pattems.

The difficulty in estimating energy expenditure from heart rate is that therelationship between heart rate and energy expenditure breaks down at lowerlevels of energy output. Therefore, since 85% of total energy expenditure inyoung children is resting energy expenditure (22), it is not surprising that heartrate monitoring lacks individual precision fore.stimating total energy expenditurein young children. As outlined above, my colleagues and I used a regression-based approach to develop an altemative method for estimating total energyexpenditure based on biological markers of the individual components of total

24 — Goran

energy expenditure. This new approach combines measurement of heart rate withother biological markers of energy expenditure. This approach serves as aninteresting altemative method for estimating total energy expenditure in individu-als compared to other traditional altemative methods such as FLEX heart ratemonitoring. The advantage of the aitemative regression approach is that themajority of individual variation in total energy expenditure is explained by falfree mass alone. Furthermore, it is difficult to estimate lower levels of energyoutput since the relationship between energy expenditure and heart rate is notlinear at lower levels of energy output.

Examining the Relationship of Total Energy Expenditure to Obesity

One potential area of research interest is examining whether the documentedreduced level of energy expenditure in young children places children at greaterrisk of obesity. In the already-obese state there is no difference in total energyexpenditure between lean and obese adolescents (2) or between lean and obeseadults (50, 70) after adjusting for the larger body size of the obese. However,several studies provide indirect evidence that reduced energy expenditure in thepreobese state is involved in the etiology of obesity in infants (59) and youngchildren (28), similar to existing evidence in obesity-prone Pima Indian adults(55).

In 1976, Griffiths and Payne (28) used heart rate monitoring and indirectcalorimetry to show that children at high risk of becoming obese (at least oneobese parent) had lower total energy expenditure (1,174 ± 297 kcal/day vs. 1,508± 352 kcal/day./? < .01) and lower resting energy expenditure (999 ± 146 kcal/day vs. 1,183 ± 184 kcal/day, p < .05) than did children al low risk of becomingobese {two lean parents). There are however a number of methodological concemswith this study, including the reliability of the heart rate method for assessingenergy expenditure, although as previously discussed, this technique is valid ona group mean basis. More importantly, it is not known whether the children withlower energy expenditure actually became obese in later years.

Roberts et al. (59) used the doubly labeled water technique to comparetotal energy expenditure in infants (aged 3 months) bom to either lean or over-weight mothers. Total energy expenditure was 20% lower in infants bom tooverweight mothers compared to infants bom to lean mothers (61.2 ± 6.5 kcal/kg/day vs. 77.2 ± 3.4 kcal/kg/day, p < .05). Moreover, the infants wiih lowerenergy expenditure at 3 months gained more weight during the first year of life.The evidence for the role of energy expenditure in the development of obesityin children is therefore not yet convincing, and other environmental factors,particularly dietary factors, should also be considered.

Thus, longitudinal studies in chiidren who are not yet obese, but who areat risk of becoming obese, are essential to reveal the sequence of biologicalevents leading to the onset of obesity. These types of studies would serve thepurpose of determining whether any of the components of daily energy expendi-ture serve as a biological marker to identify children at most risk of developingobesity. If reduced energy expenditure is implicated in the development of obesityin children, then raising energy expenditure via physical activity programs maybe a useful preventive tool. However, various exercise prescriptions will need

Doubly Labeled Water — 25

to be examined to examine their utility in increasing the various components ofdaily energy expenditure so that an optimal design can be identified.

Measuring Energy Requirements in Young Chiidren

Additional measurements of total energy expenditure In well-characterized sub-jects in different populations are needed to further identify factors causing individ-ual variation in total energy expenditure and. thus, energy requirements. Inaddition, further studies are required to cross-validate any new prediction equa-tions. Also, results from one single laboratory should not be u.sed in isolationfor the development of new equations for predicting energy requirements inchildren. The development of new prediction equations should be based onpooling data from several laboratories to enable a larger and more heterogenoussample size. It will then be imperative to cross-validate new equations in indepen-dent data sets to demonstrate their predictive capacity.

Summary and Conclusions

Several studies in healthy young children are consistent in showing that total,free-living energy expenditure is 25% lower than previously assumed values.Therefore, in young children, energy requirements are approximately 400 kcal/day below current estimates, which are based on previous energy intake datagathered over 20 years ago. This suggests that either energy expenditure in youngchildren has declined over the last 20 years or that previous information ondietary intake in children may have been in error due to methodological limitationswith measuring energy intake in children. Nevertheless, it is clearly emergingthat alternative formulations to estimate energy requirements should be sought.

If the new equations are to have any practical value for tailoring individualprescriptions of energy intake, with the aim of preventing the development ofobesity in young children, these should take into account the heterogeneity ofthe population, particularly with regard to the factors that contribute to individualdifferences in energy requirements. Studies show that fat-free mass and physicalactivity are two of the most important variables leading to differences in energyexpenditure and, thus, energy requirements in young children. The doubly labeledwater technique has been used to examine the validity of energy intake techniquesin children. These studies suggest that rigorous measurement of energy intakeby 7-day weighted records lacks precision on an individual basis, although groupmean data are accurate. Heart rate monitoring with application of individualcalibration to oxygen consumption is a readily available alternative to the doublelabeled water technique in children, though again, the technique lacks individualprecision. Combining measurements of body composition with shorter periodsof heart rate recording may provide an alternative model for predicting totalenergy expenditure in young children that can readily be applied to large-scaleepideniiological studies. Body composition is useful for this purpose since itexplains the majority of inter-individual variation in total energy expenditure.

There is currently no direct evidence linking reduced energy expenditureto the development of obesity in young children, and longitudinal studies overperiods of years are required to assess this effect. If reduced energy expenditureis involved in the etiology of obesity, then raising energy expenditure via physical

26 — Gorati

activity may help in the prevention of obesity, particularly in children who areat most risk if they can be identified early enough. However, it will be crucialto identify an optimal exercise intervention for children that is both practical andsafe and that enhances the various components of total energy expenditure withoutnegating the caloric benefits of exercise.

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A cknowiedgmen ts

Dr. Goran is supported by a Clinical Research Grant from The American DiabetesAssociation. The United States Department of Agriculture (92-01048). The NationalInstitute of Child Health and Human Development (R55 HD28720). and in part by TheSims Obesity/Nutrition Research Center and General Clinical Research Center (NIHRR-109).