effect of energy expenditure on postprandial triacylglycerol in adolescent boys

ORIGINAL ARTICLE

Effect of energy expenditure on postprandial triacylglycerolin adolescent boys

Keith Tolfrey • Conor Bentley • Mary Goad •

Joanna Varley • Sebastian Willis • Laura Barrett

Received: 21 December 2010 / Accepted: 18 March 2011 / Published online: 6 April 2011

� Springer-Verlag 2011

Abstract This study examined the effects of 30 and

60 min of moderate intensity exercise on postprandial

triacylglycerol concentration [TAG] in 11 healthy, 13-year-

old boys. The boys completed three counterbalanced

conditions. On day 1, they either rested (CON), or jogged

for 30 min (EX30) or 60 min (EX60) at 55% peak _VO2:

Following a 12-h fast, on day 2 a capillary blood sample

was taken for fasting [TAG] before a high fat milkshake

was consumed. Further blood samples were taken every

hour over a 6-h postprandial rest period for [TAG]. The

estimated energy expenditure for EX30 (982 kJ) was

doubled in EX60 (1967 kJ). Differences in fasting [TAG]

between the conditions were small (ES = 0.23, P = 0.35).

Differences in postprandial TAG over time between

conditions were identified (ES = 0.41, P = 0.03). Mean

[TAG] was lower in EX60 than CON (-33 to -3%,

P = 0.03) with a similar strong trend for EX30 (-29 to

1%, P = 0.06); EX60 and EX30 were not different from

each other (-21 to 14%, P = 0.62). The total area under

the [TAG] versus time curve was different between con-

ditions (ES = 0.42, P = 0.03). Again, EX60 was lower

than CON (-31 to -2%, P = 0.02) with a strong trend for

EX30 (-31 to 4%, P = 0.06); EX30 and EX60 were not

different from each other (-17 to 16%, P = 0.58). This

study shows for the first time that both 30 and 60 min of

jogging, with energy expenditures equivalent to 982 and

1,967 kJ, attenuates postprandial [TAG] in adolescent

boys, but not in a dose-dependent manner.

Keywords TAG � Intermittent exercise � 30 min � PPL

Introduction

The evidence supporting the paediatric origins of athero-

sclerosis are compelling (Malcom et al. 2009) and

long-standing (Zeek 1930). This has prompted calls for

preventive steps to be taken during childhood (Froberg and

Andersen 2005). Postprandial plasma triacylglycerol con-

centration ([TAG]) has a strong independent relationship

with cardiovascular events in adult population studies

(Bansal et al. 2007; Nordestgaard et al. 2007; Sarwar et al.

2010). In contrast, similar independent associations with

fasting [TAG] have not been reported so consistently

(Sarwar et al. 2007), highlighting the importance of the

postprandial period. There is a dearth of research on the

postprandial lipaemic response to exercise in young people.

Consequently, the identification of effective intervention

strategies to reduce postprandial [TAG] in young people is

critical.

Several reviews of studies with adults have indicated that

aerobic exercise-induced energy expenditure (EE) reduces

postprandial [TAG] (Hardman 1998; Petitt and Cureton

2003; Katsanos 2006). Two recent separate studies with

lean and obese adults, respectively, found that exercise EE

of only 1.10 and 0.87 MJ (*262 and 208 kcal) were

required to reduce [TAG] significantly compared with a

non-exercise control condition (Miyashita et al. 2008;

Miyashita 2008). Moreover, the efficacy was achieved

equally by accumulation or continuous exercise EE models.

In stark contrast, similarly designed exercise interventions

Communicated by Klaas R Westerterp.

K. Tolfrey (&) � C. Bentley � M. Goad � J. Varley �S. Willis � L. Barrett

Paediatric Exercise Science Research Group, School of Sport,

Exercise and Health Sciences, Loughborough University,

Loughborough, Leicestershire LE11 3TU, UK

e-mail: [email protected]

123

Eur J Appl Physiol (2012) 112:23–31

DOI 10.1007/s00421-011-1936-x

have been restricted to only three studies with adolescents

boys as far as we are aware (Barrett et al. 2007; Tolfrey

et al. 2008; MacEneaney et al. 2009). Each of these studies

reported attenuated postprandial [TAG] following the

exercise interventions. The minimum estimated exercise EE

across the conditions used in these studies was *1.5 MJ

(*366 kcal; Tolfrey et al. 2008). The overweight, late-

adolescent boys in the MacEneaney et al. (2009) study

experienced a similar reduction (*20%) in postprandial

[TAG] as normal weight boys.

Previously, we compared 60-min bouts of moderate and

vigorous intensity intermittent exercise, but did not identify

a dose-dependent effect with both bouts leading to similar

reductions (*22%) compared with a non-exercise control

(Tolfrey et al. 2008). We chose 60 min of exercise because

international guidelines for recommended daily physical

activity for adolescents tend to range from 60 to 90 min,

depending on the participant characteristics and country of

origin (e.g. Department of Health, Physical Activity,

Health Improvement and Prevention 2004; Janssen and

LeBlanc 2010). In light of previous findings, the purpose of

the current study was to reduce the exercise time to only

30 min in an attempt to identify the lowest dose of mod-

erate exercise intensity EE that could still reduce post-

prandial [TAG] in healthy, though not endurance trained,

adolescent boys. This reduction in exercise time represents

a significant and important advance in research with young

people in this area.

Methods

Participants

Eleven adolescent boys volunteered for this study after

giving their written assent; written informed consent was

also provided by a parent for each boy. The university

ethical advisory committee approved all of the study pro-

cedures. A general health questionnaire showed that they

were all in good health, not taking any substances known to

influence fat or carbohydrate metabolism, and physically

active through participation in sports, but not engaged in

specific endurance training. Baseline participant charac-

teristics are shown in Table 1.

Anthropometry and physical maturation

Body mass (Seca, Hamburg, Germany) and stature (Holtain,

Crosswell, UK) were measured with the boys wearing run-

ning shorts, socks, and T-shirt. Triceps and subscapular

skinfold thickness was measured using Harpenden callipers

(John Bull, St. Albans, UK) on the right hand side of the

body with the median of three measurements calculated as

the fold thickness. Triceps and subscapular skinfold thick-

nesses were used to estimate percent body fat (BF%) using

maturation, race, and sex-specific equations (Slaughter et al.

1988). A self-assessment of secondary sexual characteristics

by the boys was used to estimate physical maturity. The

boys used drawings of the five stages of genitalia and pubic

hair development to provide this information (Morris and

Udry 1980). The parents were asked to assist the boys with

this assessment by (1) discussing the schematic illustrations

with them, and (2) comparing their son’s genital and pubic

hair development with the schematics and accompanying

written descriptions.

Preliminary exercise measurements

Before the main trials, the boys were habituated to exercise

on the treadmill (Technogym Runrace, Gambettola, Italy)

set at 1% gradient. Next, 4 3 4 min incremental exercise

stages were used to identify the steady-state relationship

between treadmill speed, oxygen uptake ( _VO2), and heart

rate. The starting speed was 6.5 with 0.5 km h-1 incre-

ments up to 8.0 km h-1. Heart rate (HR) was monitored

continuously via radio telemetry (Polar PE4000, Kempele,

Finland) whenever the boys exercised on the treadmill, and

ratings of perceived exertion (RPE) were measured using

the 6–20 scale (Borg 1974) in the final 15 s of each exer-

cise bout. Expired gas samples were collected into 100-L

Douglas bags (Cranlea and Company, Birmingham, UK)

during the final minute of each progressive bout. Oxygen

and carbon dioxide concentrations in each Douglas bag

were analysed using a paramagnetic oxygen analyser and

an infrared carbon dioxide analyser (Servomex 1400,

Sussex, UK) calibrated against gases of known concen-

tration before and after each series of four bags. The vol-

ume of expired gas was determined using a dry gas meter

(Harvard, Kent, UK). For each sample, oxygen uptake

( _VO2), expired carbon dioxide ( _VCO2), minute ventilation

( _VE), and respiratory exchange ratio were calculated. These

Table 1 Physical and physiological characteristics

Age (years) 13.3 (0.8)

Genital developmenta 3 2

Body mass (kg) 51.3 (14.6)

Stature (m) 1.64 (0.14)

Body mass index (kg-1 m2) 18.6 (2.6)

Percent body fat (%) 14.6 (4.1)

Lean body mass (kg) 43.6 (12.1)

Peak _VO2 (mL kg-1min-1) 53.6 (6)

All values are mean (SD) where n = 11

_VO2—oxygen uptakea Self-assessment—median (interquartile range)

24 Eur J Appl Physiol (2012) 112:23–31

123

submaximal _VO2 data were used subsequently to (1)

choose an appropriate speed for each individual at which

their peak _VO2 could be determined (below) and (2) to

establish the speed required to elicit 55% peak _VO2 for the

two experimental exercise conditions described below.

Peak oxygen uptake (peak _VO2)

After a standardised 10-min rest period, peak _VO2 was

determined with each boy running at a fixed individual

speed (9–11 km h-1), while the treadmill belt was raised

by 1% each minute until volitional exhaustion. Oxygen

uptake, HR, and RPE were measured using the methods

described previously. The boys were asked to run until

volitional exhaustion, which was verified using the fol-

lowing criteria: (1) a plateau in _VO2 (B3%) with an

increase in treadmill gradient; (2) a maximum heart rate

(HRmax) C 95% of age-predicted maximum (220—chro-

nological age); and (3) respiratory exchange ratio C1.10.

Experimental design

A within-measures, counterbalanced crossover design was

used in which the boys completed three separate condi-

tions, each separated by a standardised 14-day period.

A 2-day model was used similar to our previous study

(Tolfrey et al. 2008). A schematic representation of the

design is shown in Fig. 1.

Day 1

On the first day, the boys either (a) rested in the laboratory

for 110 min (CON); (b) completed 60 min of intermittent

treadmill exercise (EX60); or (c) completed 30 min of

intermittent treadmill exercise (EX30). The exercise was

designed to be of moderate intensity and to elicit *55%

peak _VO2. An intermittent exercise model was used because

initial pilot work with the boys indicated that they preferred

to complete it in this manner, and some suggested that they

might not be able to exercise continuously for 60 min. The

boys arrived at the laboratory at 15:30 h on each occasion,

and each condition was completed at 17:30 h on day 1. The

30 and 60 min bouts of exercise were completed in 3 or

6 3 10 min blocks separated by passive rest periods of

equal duration. During each 10-min interval of exercise,

samples of expired gas were collected in the fourth and

tenth minute and analysed using the procedures described

previously to verify the relative exercise intensity. Subse-

quently, assuming that the urinary nitrogen excretion rate

was negligible and that the participants had reached a

physiological steady state, these samples were used to

estimate exercise EE and the oxidation of carbohydrate and

fat (Frayn 1983). The treadmill speed was adjusted peri-

odically throughout each condition in an effort to match the

target exercise intensity (Table 2). Heart rate was recorded

continuously and RPE was recorded during the last 15 s of

each expired air sampling period as described previously.

Day 2

Following a standardised 12-h overnight fast, the boys were

driven to the laboratory. After providing an initial fasting

capillary blood sample at *07:55, a high fat test milkshake

was consumed within 10 min and then six further blood

samples were taken at hourly intervals (Fig. 1). The timing of

the postprandial period commenced when the boys started

consuming the milkshake (08:00) and was standardised so

that it occurred *14.5 h after completion of the treadmill

exercise or rest period the previous day. During this post-

prandial period, the boys were asked to remain seated

throughout whilst they read, played on a non-active computer

games console or watched DVD films. One-and-half litres of

plain water was provided, and the boys were asked to drink

this in small quantities divided equally over the 6 h.

Standardisation of diet, physical activity, and milkshake

With parental assistance, the boys recorded their food

and drink intake and all physical activities in the 48-h

Day 2 Day 1

Evening meal

Milk shake

15:40 to 17:30

Rest (CON)

30 min intermittent exercise (EX30)

60 min intermittent exercise (EX60)

Exercise completed at 17:30

** ↑↑ ↑ ↑ ↑ ↑ ↑ ↑

Evening meal was replicated from 1st condition 07:55 08:00 09:00 10:00 11:00 12:00 13:00 14:00

* Key: ↑ capillary blood sample for

[TAG] and [glucose] ↑ capillary blood sample for [TAG],

[glucose], [haemoglobin], and haematocrit

Fig. 1 Schematic of 2-day

protocol

Eur J Appl Physiol (2012) 112:23–31 25

123

period leading up to day 2 of the first assigned experi-

mental condition (including the evening meal shown in

Fig. 1). This information was used to match their diet

and activity patterns across the three experimental con-

ditions. The boys were reminded verbally of this

requirement to replicate their nutritional intake and

activity just prior to the second and third conditions. In

addition, the boys were asked to minimise their

engagement in physical activity, other than the prescribed

treadmill exercise, in this 48-h period; however, no

measurements were taken to verify this. Before leaving

the laboratory on day 1 of each experimental condition,

the boys were reminded that they could drink plain water

but should not consume any food after 20:00 h that

evening. They were asked to eat a small cereal snack bar

at 19:45 h to standardise the fasting period across par-

ticipants and experimental conditions. They were also

asked to remain as inactive as possible after leaving the

laboratory in an effort to minimise this as an extraneous

factor on measurements during day 2.

The milkshake was a 3:1 mix of vanilla dairy ice

cream and double cream with 10 g of either powdered

strawberry or chocolate flavour added. It provided 1.50 g

of fat (70% of total energy), 1.20 g of carbohydrate

(25%), and 0.21 g of protein (5%) per kilogram of

body mass (80 kJ kg-1). The composition of the milk-

shake consumed differed slightly in the current study to

account for the small changes to the macronutrient

content of the ice cream and double cream by the

manufacturers since our last study (Tolfrey et al. 2008).

None of the boys reported any gastrointestinal problems

when consuming the milkshake or during the 6-h post-

prandial period.

Analytical methods

The fasting and postprandial capillary blood samples were

used to quantify [TAG] and glucose concentration ([glu-

cose]). Haematocrit and haemoglobin concentration were

determined from the fasting and final samples to estimate

change in plasma volume (Dill and Costill 1974). The

whole hand was pre-warmed for 5 min in water heated to

40�C whilst the participant remained seated. The hand was

dried thoroughly and cleaned with a steret before the tip of

the finger was pierced (Unistick 3 Extra, Owen Mumford,

Oxford, UK). After the initial drop had been discarded,

between 300 and 600 lL of whole blood was collected

into potassium-EDTA coated Microvette CB 300 tubes

(Sarstedt Ltd, Leicester, UK) and centrifuged immediately

at 12,800g for 15 min (Eppendorf 5415c, Hamburg,

Germany). Plasma was separated immediately after cen-

trifugation; 20 lL was removed and then diluted 50 times

by the addition of 980 lL of ice-cold saline (0.9%; Barrett

et al. 2007; Tolfrey et al. 2008) to prevent any freeze-

drying effect as a result of storage. This procedure was

repeated so that two aliquots of diluted plasma were stored

at -80�C for a month for subsequent analysis by enzy-

matic, colourimetric methods (Randox Laboratories Ltd,

Crumlin, UK) with the use of a centrifugal analyser (Cobas

Mira Plus, Roche, Basel, Switzerland). The predilution

procedure precluded the dilution step of the assay when

[TAG] and [glucose] were measured, and three times the

sample volume recommended in the Randox kit assay

procedure was used. This resulted in the concentration of

the sample for analysis being the same as that in the ori-

ginal assay procedure (Barrett et al. 2007; Tolfrey et al.

2008). The within-batch coefficients of variation for [TAG]

Table 2 Intermittent treadmill exercise profiles across experimental conditions

EX30 EX60 Mean difference 95% CIa Alpha�

Heart rate (beats min-1) 149 (12) 147 (12) -1 -11 to 7 0.66

Percent peak HR (%) 74 (3) 73 (3) -1 -5 to 4 0.67

Oxygen uptake (L min-1) 1.52 (0.56) 1.53 (0.57) 0.01 -0.05 to 0.06 0.72

Percent peak _VO2 (%) 55 (1) 55 (1) 0 -2 to 3 0.74

Respiratory exchange ratio 0.92 (0.06) 0.89 (0.06) -0.03 -0.07 to 0.02 0.19

Rating of perceived exertion 10 (2) 11 (2) 1 0.1 to 1.9 0.03

Fat oxidation (g) 6.7 (4.5) 16.5 (10.0) 9.7 4.7 to 14.7 0.001

Carbohydrate oxidation (g) 43.5 (18.8) 80.5 (47.9) 37.0 14.5 to 59.4 0.004

Energy expenditure (kJ) 982 (353) 1,967 (789) 984 681 to 1,288 P B 0.0005

All data are mean (SD) where n = 11

EX30 thirty min exercise condition, EX60 sixty min exercise condition, HR heart rate, _VO2 oxygen uptakea 95% Confidence interval of the absolute difference between EX30 and EX60� Paired Student’s t test


123

and [glucose] using the methods described above were 2.9

and 1.8%, respectively. The capillary sampling technique

provides plasma concentrations that are systematically

lower than from venous samples, but the difference is small

(TAUC-TAG unbiased typical error 4.9% [95% CI 3.8 to

7.1%]) and does not affect the interpretation of the study

(unpublished data).

Statistical analyses

The data were stored and analysed using the Statistical

Package for the Social Sciences (SPSS for Windows Ver-

sion 16; SPSS Inc., Chicago, USA). Descriptive statistics

[means (SD)] representing the physical and physiological

characteristics at baseline (Table 1) were calculated for the

11 boys. Normality of the data was confirmed by Shapiro–

Wilk tests, and Mauchly’s test of sphericity was used to

check for homogeneity of variances. A Greenhouse–Geis-

ser correction of the degrees of freedom was used if

sphericity could not be assumed. Differences between the

EX30 and EX60 responses (Table 2) were examined using

Student’s paired t tests and effect sizes (ES; Field 2009).

The total 6 h area under the plasma concentration versus

time curves for TAG (TAUC-TAG) and glucose (TAUC-

glucose) were calculated using the trapezium rule for each

experimental condition. The TAUC and fasting concentra-

tions for TAG and glucose were compared across the

experimental conditions using separate one-way within-

measures ANOVA. Estimated changes in plasma volume

across the conditions were analysed using the same proce-

dure. Separate 3 9 7 (condition by time) within-measures

ANOVA were used to identify differences in [TAG] and

[glucose] over the postprandial period. A priori simple

planned contrasts with CON as the reference category were

used to follow up the effects from the omnibus ANOVAs.

The 95% CI for mean absolute and percentage differences

between conditions were calculated using the t distribution

and n - 1 degrees of freedom. Percentage changes over time

for paired results were calculated after the data had been

transformed using a natural logarithm. The mean percentage

difference was then obtained from 100 (emean difference - 1).

Pearson’s product moment correlations were used to identify

which factors were determinants of exercise-induced chan-

ges in TAUC-TAG and to explain the anticipated inter-

individual variability. All results are presented as mean (SD).

Results

Treadmill exercise responses

On average, the boys jogged at a target intensity of 55%

peak _VO2 in both exercise conditions. Aside from a 1 unit

increase in RPE between EX30 and EX60, the only

meaningful differences were the anticipated increase in EE

(95% CI 82 to 111%) and absolute (g) macronutrient oxi-

dation for EX60 compared with EX30 (Table 2).

Plasma volume and fasting concentrations

Average estimated changes in plasma volume between the

fasting and 6-h samples across each condition were small

(CON 0.1%, EX30 0.8%, EX60 0.6%; ES = 0.06,

P = 0.88), so the raw [TAG] and [glucose] were used in all

subsequent analyses. Differences in fasting [TAG] were

small (ES = 0.23, P = 0.35), whereas differences in

fasting [glucose] were small to moderate (ES = 0.40,

P = 0.04) with a progressive decline across the three

conditions as EE increased (Table 3). A simple planned

contrast indicated that glucose in EX60 was lower than

CON (95% CI -0.73 to -0.11 mmol L-1, P = 0.01).

Postprandial changes in [TAG] and [glucose] over time

Changes in plasma [TAG] over time and across the three

conditions are shown in Fig. 2. [TAG] was lower in both

EX30 and EX60 compared with CON (main effect condi-

tion ES = 0.41, P = 0.03; main effect time ES = 0.49,

P = 0.002, condition 9 time interaction ES = 0.08,

P = 0.65). The percentage differences (95% CI) in [TAG]

for the entire study period were moderate: EX30 versus

CON -16% (-29 to 1%, P = 0.06); EX60 versus CON

-20% (-33 to -3%, P = 0.03); and EX30 versus EX60

-5% (-21 to 14%, P = 0.62). Similarly, moderate chan-

ges in TAUC-TAG were identified (ES = 0.42, P = 0.03);

simple planned contrasts revealed that both EX30 (95% CI

-31 to 4%, P = 0.06) and EX60 (95% CI -31 to -2%,

P = 0.02) were lower than CON, although not different

from each other (95% CI -17 to 16%, P = 0.58) (Table 3).

Individual changes (delta) in TAUC-TAG are provided

in Fig. 3; only participant 1 was a ‘non-responder’ (i.e.

reductions in exercise conditions did not exceed the con-

trol) in both exercise conditions, whereas participants 2 and

3 were ‘non-responders’ in EX30 and, likewise, partici-

pants 4 and 9 in EX60. The Pearson product moment

correlation between EX30 and EX60 for these delta values

was moderate (r = 0.60, P = 0.05). These individual

changes in TAUC-TAG for both EX30 and E60 were not

significantly related to any of the other measured variables

shown in Tables 1 and 2 (P [ 0.05). For EX30, BMI

shared the strongest relationship with delta TAUC-TAG

accounting for *10% of the variance (P = 0.32), whereas

the exercise respiratory exchange ratio explained *24% of

the variance (P = 0.13) in EX60 delta TAUC-TAG.

Scaling the EE and substrate oxidation values using dif-

ferent body size/composition factors, either linearly or


123

allometrically, did not help to explain any more of this

individual variability.

[Glucose] was lower in both EX30 and EX60 compared

with CON (main effect condition ES = 0.40, P = 0.04;

main effect time ES = 0.60, P = 0.009, condition 9 time

interaction ES = 0.15, P = 0.92). The percentage differ-

ences (95% CI) in [glucose] for the entire study period

were small: EX30 versus CON -2% (-4 to 0%, P =

0.01); EX60 versus CON -4% (-7 to 0%, P = 0.05);

EX30 versus EX60 -1% (-5 to 2%, P = 0.52). The dif-

ference in TAUC-glucose was small to moderate (ES =

0.35, P = 0.09) (Table 3).

Discussion

The results of this study show, for the first time in ado-

lescent boys, that a single bout of only 30 min of

moderate intensity jogging completed *14.5 h before

drinking a high fat milkshake was sufficient to reduce

postprandial [TAG] by *16% (-29 to 1%). The gross

exercise EE was only 982 kJ, and the average exercise

RPE of 10 (between very light and fairly light) suggests

that the boys were comfortable throughout the moderate

intensity jog. Doubling the EE by increasing the exercise

time from 30 to 60 min had a small (*4%) additional

effect on the reduction in postprandial [TAG], although

the confidence intervals were indicative of a more robust

effect. Changes in fasting [TAG] were small confirming

that a greater EE may be required to reduce this param-

eter (Magkos 2009).

Our results support previous research showing an exer-

cise effect on postprandial TAG metabolism in adolescents

(Barrett et al. 2007; Tolfrey et al. 2008; MacEneaney et al.

2009). However, all these studies required the participants

to exercise for *60 min with EE ranging from *1.5 to

2.5 MJ, thus matching most international guidelines for

accumulated daily physical activity time in young people

(Department of Health, Physical Activity, Health

Improvement and Prevention 2004; Janssen and LeBlanc

2010). Given many young people may still fall short of this

minimum recommendation (Riddoch et al. 2007), we were

interested in seeing whether a lower dose might still

attenuate postprandial [TAG] as recommended in a recent

review (Katsanos 2006). This lower dose is consistent with

the minimum expenditure in public health guidelines for

adults (Haskell et al. 2007) and has recently been shown to

be efficacious with adults (Miyashita et al. 2008; Miyashita

2008). The lack of any differences in the physiological

responses to the two exercise conditions (Table 2) shows

that we were successful in providing a similar metabolic

challenge, which differed only in the time it was performed

and consequently the amount of energy expended.

Table 3 Fasting (baseline) and total area under the time (TAUC) for plasma triacylglycerol (TAG) and glucose (GLU) concentrations across

experimental conditions

CON EX30 EX60 CON vs. EX30

(95% CI)

CON vs. EX60

(95% CI)

EX30 vs. EX60

(95% CI)

Fast triacylglycerol (mmol L-1)a 0.87 (0.46) 0.77 (0.50) 0.77 (0.45) -0.31 to 0.11 -0.20 to 0.01 -0.17 to 0.18

Fast glucose (mmol L-1)b 4.98 (0.55) 4.84 (0.45) 4.56 (0.47) -0.56 to 0.28 -0.73 to -0.11bi -0.58 to 0.02

TAUC-TAG (mmol L-1 6 h)c 7.06 (3.48) 6.15 (3.63) 5.94 (3.56) -1.86 to 0.05ci -2.01 to -0.22cii -1.04 to 0.62

TAUC-glucose (mmol L-1 6 h)d 29.9 (2.04) 29.1 (1.54) 28.9 (1.53) -1.58 to 0.13 -2.33 to 0.25 -1.25 to 0.88

Data are mean (SD) where n = 11; 95% CI 95% confidence interval of the absolute difference (mmol L-1 and mmol L-1 6 h) between

conditions

CON control condition, EX30 thirty min exercise condition, EX60 sixty min exercise condition, TAG triacylglycerol, TAUC total area under the

concentration versus time curvea ANOVA F(2,20) = 1.1, P = 0.35b ANOVA F(2,20) = 3.8, P = 0.04, simple planned contrast—biP = 0.01 CON vs. EX60c ANOVA F(2,20) = 4.4, P = 0.03, simple planned contrasts—ciP = 0.06 CON vs. EX30; ciiP = 0.02 CON vs. EX60d ANOVA F(2,20) = 2.8, P = 0.09

Fig. 2 Fasting (F) and postprandial plasma triacylglycerol concen-

trations for the control (CON), and 30 min (EX30), and 60 min

(EX60) exercise conditions. Data are mean (SD); n = 11. Blackrectangle is when the milkshake was consumed. Main effect for

condition (P = 0.03); main effect for time (P = 0.002); condition by

time interaction (P = 0.65)


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Exercise EE has been identified previously as the pri-

mary determinant for a reduction in postprandial [TAG]

(Hardman 1998; Katsanos 2006), although once two out-

lying studies were accounted for, the strength of this effect

reported in a meta-analysis (r = -0.35; Petitt and Cureton

2003) was only moderate. Manipulations of exercise

intensity and duration support a dose-dependent response

in adults (Tsetsonis and Hardman 1996; Gill et al. 2002;

Katsanos et al. 2004), although variations in design char-

acteristics preclude the identification of a so-called optimal

or minimal dose. The 12% difference between conditions

in late-adolescent boys was linked tentatively to the greater

EE in the intermittent-games activity (Barrett et al. 2007).

However, the authors acknowledged it was not possible to

estimate exercise EE accurately due to the sprinting com-

ponent of the simulated games activity. Moreover, the

between-groups design in this study may have been a

confounding factor. In stark contrast, our previous study

showed that despite a mean difference of 644 kJ between

the 60-min moderate and vigorous intensity bouts of

treadmill exercise, the reductions in [TAG] were similar at

21 and 18%, respectively (Tolfrey et al. 2008). The current

study design resulted in an even larger EE differential

between the conditions (Table 2). Yet, the results still do

not provide a convincing argument for a dose–response

change in [TAG], or they suggest a threshold attenuation

above which further ‘improvements’ may not be possible.

Despite the large fat load that the milkshake provided,

the postprandial lipaemic effect in the control condition

was blunted compared with other studies of healthy

adolescents or adults consuming similar or less fat for

breakfast (e.g. Pfeiffer et al. 2006; Barrett et al. 2007;

MacEneaney et al. 2009). Consequently, the group mean

reductions in [TAG] in the two exercise conditions may

represent an underestimation of the extent to which the

exercise EE might affect postprandial lipaemia. Alterna-

tively, it might reflect the younger boys in our study

compared to those mentioned above. The mean milkshake

fat consumption of 77 g should lead to exaggerated post-

prandial lipaemia with a tendency for [TAG] to plateau

(Lairon et al. 2007). Furthermore, this quantity may result

in less clear dose dependence than with more moderate

(30–50 g) fat loads (Lairon et al. 2007); however, sys-

tematic evaluation of this in adults supports a linear

increase in plasma [TAG] up to 120 g of fat (Cohen et al.

1988). The dairy milkshake may have also contributed to a

diminished lipaemic response because of the high propor-

tion of short- and medium-chain fatty acids that mostly

enter the liver via the portal system (Barr et al. 1985).

The low EE and perceived exertion associated with the

30-min moderate intensity jog (EX30) may be attractive to

adolescent boys who are too busy with other non-physical

activities to accumulate much more than *1 MJ each day.

The sample size is comparable with previous exercise

studies with adolescents (Barrett et al. 2007; Tolfrey et al.

2008; MacEneaney et al. 2009), and we acknowledged that

it is a major commitment for the participants to complete a

study of this nature. Furthermore, although the effect sizes

for the changes in [TAG] are modest, the confidence

intervals suggest that main between-condition comparisons

are quite robust. The EX30 resulted in a reduction in

TAUC-TAG for the majority of the boys in the study

(Fig. 3), but for two boys lipaemia was greater in the

exercise condition(s) than the control. Although a number

of control measures were implemented to reduce between-

condition variability beyond the manipulated differences in

EE, compliance cannot be guaranteed and deviations that

cannot be discounted are likely to have a significant impact

2.1

0.70.1

-0.9 -1.1 -1.2 -1.3-1.5

-1.9 -1.9

-3.1

0.8

-1.2 -1.3

0.2

-2.3

-1.0

-1.4

-0.6 -0.2

-1.2

-4.2

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

1 2 3 4 5 6 7 8 9 10 11

Del

ta T

AU

C-T

AG

(Exe

rcis

e m

inus

Con

trol

) (m

mol

/L 6

h)

Individual Participants

EX30 minus CON

EX60 minus CON

Fig. 3 Individual changes in

postprandial TAG responses

(TAUC) between the two

exercise conditions and control

(EX30 - CON, P = 0.061;

EX60 - CON, P = 0.019).

Order of participants (left to

right) according to size of the

EX30-induced change in

TAUC-TAG. Negative values

correspond to an attenuation in

TAUC-TAG in the exercise

condition compared with CON


123

(Magkos 2009). Moreover, considerable day-to-day vari-

ability in fasting [TAG] has been shown that may also be a

factor (Tolfrey et al. 1999). Regardless of the research

design and rigorous standardisation/control, it is not known

if any of the reported changes in [TAG] are relevant from a

clinical perspective, because this is not a clinical end point.

However, the clinical relevance of non-fasting TAG as an

independent risk factor for future cardiovascular disease is

clear (Bansal et al. 2007; Nordestgaard et al. 2007; Sarwar

et al. 2010), and numerous longitudinal cohort studies that

have included a measure of TAG have identified an asso-

ciation with future cardiovascular disease (e.g. Cook et al.

2009).

The mechanisms underlying the changes in postprandial

[TAG] in the exercise conditions cannot be determined

directly from our study and, to our knowledge, have not

been measured in adolescents to date. However, two sepa-

rate studies with adults found that individuals who experi-

enced an increase in either muscle (Herd et al. 2001) or

plasma (Gill et al. 2003) lipoprotein lipase (LPL) activity

16–24 h after a single bout of moderate exercise had the

greatest attenuation in fasting and postprandial [TAG]

concentrations. The authors concluded that LPL activity

probably contributed to the exercise-induced reductions in

fasting and postprandial plasma [TAG] through increased

hydrolysis, but it was unlikely to be acting on its own (Herd

et al. 2001; Gill et al. 2003). Whether this applies to the

boys in our study is not clear because of possible matura-

tional effects and the relatively low EE (Ferguson et al.

1998). An exercise-induced reduction in hepatic very low

density lipoprotein (VLDL)-TAG synthesis and secretion

has been implicated (Gill et al. 2007; Magkos 2009), but the

clearance rate of VLDL-TAG appears to increase only

above a threshold of *2 MJ in adults that is considerably

higher than EX30 although similar to EX60 in our study and

has been questioned following moderate intensity exercise

(Gill et al. 2007; Magkos 2009). Although the exercise-

induced changes in TAUC-TAG following both EX30 and

EX60 varied considerably across the 11 boys, it was clear

that the majority of the boys experienced an exercise-

induced reduction in both conditions compared with the

non-exercise control (CON). It was not possible to identify

a clear determinant of this inter-individual variation within

our study; differences in exercise substrate metabolism and

energy expenditure between the individual boys did not

appear to be significant contributory factors. A limitation of

the indirect calorimetry estimations is that we assumed that

the urinary nitrogen excretion rate was negligible and did

not account for non-respiratory carbon dioxide excretion

that may have resulted in an underestimation of fat oxida-

tion in some individuals (Rowlands 2005). However, a

study with adults, where a lipolysis inhibitor (acipimox)

was used to reduce exercise lipid metabolism, showed that

reductions in postprandial lipaemia were independent of

specific substrate metabolism during exercise (Malkova

et al. 1999)—we are not aware of a similar study with

adolescents. In a recent study designed to examine hetero-

geneity in the effects of exercise on postprandial [TAG] in

adults with type 2 diabetes, Gill et al. (2007) found that

changes in 3-hydroxybutyrate (3-OHB) explained almost

half of the variance in TAUC-TAG. The participants with

the largest increase in 3-OHB, a marker of hepatic fatty acid

oxidation, experienced the greatest exercise-induced

reductions in postprandial TAG (Gill et al. 2007). The

capillary blood sampling technique used in our study did

not permit us to measure 3-OHB, but it is possible that this

mechanism might explain some of the heterogeneity found

in this group of boys. Moreover, this would be compatible

with an emerging body of evidence suggesting that mod-

erate exercise-induced reductions in postprandial TAG

were mediated by changes in hepatic fatty acid flux rather

than changes in LPL activity (Gill et al. 2007).

In conclusion, we believe that this is the first study to

find that \1 MJ of moderate intensity treadmill exercise

was sufficient to reduce postprandial [TAG] in healthy

adolescent boys. Furthermore, although doubling the gross

exercise energy expenditure resulted in small further

attenuation in the postprandial [TAG], it could not be

described as a dose–response change. This might suggest

that an energy expenditure threshold exists in adolescent

boys beyond which further reductions are unlikely to be

demonstrated. However, further work is required to

examine this systematically.

Acknowledgments We thank all of the participants for their dedi-

cation to the study and their parents/carers for supporting them

throughout. We also thank Mr Graham Bett, Mr Ian Smith and Mrs

Sophie Diaper from Woodbrook Vale High School, Loughborough

for their support with the study. The research was not supported

directly by any funding other than that available internally through

Loughborough University.

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effect of energy expenditure on postprandial triacylglycerol in adolescent boys

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