Although the process of atherosclerosis is initiated during childhood, the clinical symptoms of athero sclerotic disease are not evident typically until adulthood (25). Elevated plasma triacylglycerol concentrations ([TAG]) are associated with an increased risk of atherosclerosis (38). The magnitude and duration of the rise in [TAG] in the postprandial period is positively related to the progression of atherosclerosis (4). Furthermore, prospective cohort studies have demonstrated recently that nonfasting (postprandial) [TAG] independently predicts future cardiovascular events in men and women (1,27). Indeed, accumulating evidence shows that postprandial [TAG] may better predict cardiovascular disease risk than traditional fasting measures (1). Because most waking hours are postprandial, a combination of low physical activity and diets high in fat or carbohydrate may increase the risk of future cardiovascular disease with prolonged elevated [TAG] (28). Consequently, interventions that modulate postprandial lipemia by improving TAG metabolism should begin ideally during childhood and adolescence (25).
Many studies have shown that acute aerobic exercise performed up to 18 h before a high fat meal attenuates postprandial [TAG] in adults, resulting typically in a 15%–25% reduction in the total area under the [TAG] versus time curve (28). In contrast, only five studies that we are aware of have examined the influence of a single bout of moderate to vigorous-intensity exercise on postprandial [TAG] in adolescent boys (2,21,30,34,36). In each of these studies, the postprandial TAG response was lower after exercise compared with a nonexercise control condition. Exercise-induced energy expenditure (EE) appears to reduce postprandial [TAG] in a dose-dependent manner in adults (12,37). In contrast, this has not been shown in studies in boys (34,36). In the most recent of these studies, both 30 and 60 min of intermittent treadmill exercise at 55% of peak V˙O2 were shown to be similarly efficacious at reducing mean postprandial [TAG] in 13-yr-old boys (34). Although gross EE in the 60-min exercise condition (1967 kJ) was approximately double that for 30 min (982 kJ), the extra attenuation of postprandial [TAG] was only 4%, leading the authors to conclude a dose response was not evident.
Although accumulating evidence indicates that an exercise-induced reduction in postprandial [TAG] occurs in boys, it has not been possible to find any similar studies with girls. Studies have consistently demonstrated greater postprandial [TAG] in men than women (7,16), which may reflect between sex differences in abdominal visceral adipose tissue, enhanced skeletal muscle clearance of circulating TAG and greater suppression of upper-body subcutaneous adipose tissue lipolysis in women (7,15,18). Although the majority of exercise postprandial studies are with males, Gill et al. (13) compared 30 men and 43 women and reported that the exercise-induced attenuation in the postprandial TAG response was not different (23.5% vs 19.8%, respectively). However, it is not known whether exercise reduces postprandial [TAG] in girls, suggesting this is a serious gap in our understanding of this important marker of future atherogenic risk in young people.
International physical activity recommendations for children and adolescents generally range from 60 to 90 min·d−1 (8,17). Evidence suggests that girls are less likely than boys to meet these recommendations (29). Furthermore, the decline in physical activity with age is more marked among young females (26). Clearly, in order for exercise intervention strategies to be effective in the long term, the recommended exercise programs must be considered acceptable and sustainable by the target group. This highlights the need to identify the lowest dose of exercise EE that can reduce postprandial [TAG] in girls.
Therefore, the aim of this study was to examine the effect of 30 and 60 min of moderate-intensity exercise on postprandial [TAG] in healthy, but not endurance trained, girls for the first time.
After approval from the University Ethical Advisory Committee, 18 girls volunteered for the study and provided their written assent. Written informed consent was also obtained from a parent or guardian. All participants were physically active generally, in good overall health, not taking any medications or dietary supplements known to affect lipid or carbohydrate metabolism, and had no contradictions to exercise participation. Physical and physiological characteristics are presented in Table 1.
Anthropometry and physical maturation.
All anthropometric measurements were conducted with participants wearing shorts, T-shirt, and socks. Stature was measured to the nearest 0.01 m using a fixed stadiometer (Holtain, Crosswell, UK). Body mass was quantified to the nearest 0.1 kg using a balance beam scale (Avery, Birmingham, UK). Body mass index was calculated as body mass (kg) divided by stature (m) squared. Skinfold thickness was measured to the nearest 0.2 mm using Harpenden calipers (John Bull, St. Albans, UK), with measurements taken on the right-hand side of the body at two sites (triceps and subscapular). The median of three measurements at each site was used to estimate percent body fat (31). Lean body mass was estimated as follows:
Participants provided a self-assessment of their level of physical maturity using drawings depicting the five stages of breast and pubic hair development (32). The scale ranges from 1, indicating prepubescent, to 5, indicating full sexual maturation, and participants were required to select the stage most closely resembling their current level of sexual development. We did not control for the menstrual cycle because menstruation had not started for 11 of the 18 girls, and 4 of the remaining 7 girls reported that their menstrual cycle was sporadic and irregular. This variability is a limitation of working with a mixed maturation sample.
Preliminary exercise measurements.
During the first visit to the laboratory, participants were familiarized with walking and running on the treadmill (Technogym Runrace, Gambettola, Italy) before completing two preliminary exercise measurements. The first test involved a 16-min incremental treadmill protocol divided into 4 × 4 min stages. The treadmill started at a speed of 4 km·h−1 and increased 1 km·h−1 at the start of each subsequent stage, with the gradient set at 1% throughout (19). This enabled the individual steady-state relationship between treadmill speed, oxygen uptake (V˙O2), and heart rate to be established. Participants were given a standardized 10-min passive rest period before completing the second exercise measurement to determine peak V˙O2. The girls ran at a fixed individual speed (6.8–9.5 km·h−1), identified from the incremental treadmill protocol, while the treadmill gradient was increased 1% each minute until volitional exhaustion. Throughout both tests, heart rate was monitored continuously via short-range telemetry (PE4000; Polar-Electro, Kempele, Finland), and Borg’s ratings of perceived exertion were recorded at standardized intervals (5). Expired air samples were monitored continuously using an online breath-by-breath gas analysis system (Cosmed K4b2, Rome, Italy), which was fully calibrated according to the manufacturer’s instructions before each use. Participants wore an appropriate size facemask (Hans Rudolf, Shawnee, KS), which was checked for leaks and connected to the online system via a flowmeter before each exercise test began. At least two of the following criteria were satisfied by all participants during the peak V˙O2 test to confirm maximal effort was achieved: a plateau in V˙O2 (≤3%) with an increase in treadmill gradient, a peak heart rate ≥95% of age-predicted maximum (220 − chronological age), and a respiratory exchange ratio ≥1.10. An average of the breath-by-breath V˙O2 data were taken every 10 s, and peak V˙O2 was defined as the highest 30-s rolling average. Data from the incremental and peak V˙O2 protocols were used to determine the treadmill speed required to elicit ∼55% peak V˙O2 during the main experimental exercise conditions.
Similar to previous studies with boys (2,30,34,36), a within-measures, counterbalanced crossover design was adopted whereby participants completed three, 2-d experimental conditions: resting control (CON), 30 min of intermittent treadmill exercise (EX30), and 60 min of intermittent treadmill exercise (EX60). The conditions were separated by a standardized period of 14 d. The study design is shown schematically in Figure 1.
The girls arrived at the laboratory at 1530 h, and all measures were completed by 1730 h. Body mass was recorded at the start of each experimental condition to standardize the test breakfast provided on day 2 (see next section). During CON, participants rested for 110 min in the laboratory. During the exercise conditions, participants completed either 30 min (EX30) or 60 min (EX60) of moderate-intensity intermittent treadmill exercise designed to elicit ∼55% peak V˙O2. The exercise conditions were completed in intervals of either 3 × 10 min or 6 × 10 min with a standardized 10-min period of passive rest between each interval. Oxygen uptake and heart rate were monitored continuously throughout each 10-min exercise interval as described previously, and the treadmill speed was adjusted accordingly to ensure the target exercise intensity was achieved. Participants provided RPE at standardized intervals as described previously. The exercise EE and the oxidation rate of carbohydrate and fat were estimated via indirect calorimetry (10), assuming that participants reached a physiological steady state and that the urinary nitrogen excretion rate was negligible.
After a 12-h overnight fast, the girls arrived at the laboratory at ∼0745 h, and a fasting capillary blood sample was taken after a 10-min seated rest. A high-fat milkshake was consumed for breakfast within 10 min, marking the start of the postprandial period (0800 h), and six further capillary blood samples were taken at hourly intervals throughout the 6-h postprandial period (Fig. 1). Participants rested throughout the postprandial period and were able to read, watch DVD films, and play nonactive computer games. Participants consumed water ad libitum during the first condition and replicated the ingested volume during the subsequent conditions.
Standardization of diet, physical activity, and test milkshake.
Participants recorded their dietary intake, and all physical activity categorized according to intensity level in a detailed diary during the 48-h period preceding day 2 of the first experimental condition. They were also asked to minimize their physical activity during this period and to replicate this diet and physical activity pattern before the second and the third experimental conditions, which was confirmed verbally by the lead investigator. The overnight fasting period was standardized by asking participants to consume a small carbohydrate snack at 1945 h on day 1 of each experimental condition (Kellogg’s® Nutri-Grain Strawberry, 502 kJ, 3 g fat, 24 g carbohydrate, 2 g protein). After 2000 h, the participants were allowed to drink plain water but were asked not to consume any other drinks or food before arriving at the laboratory on day 2.
The milkshake provided for breakfast on day 2 contained vanilla dairy ice cream and double cream, at a ratio of 3:1, with 10 g of either powdered strawberry or chocolate flavor added to ensure the milkshake was palatable. Participants consumed the same milkshake flavor on all visits for consistency. The milkshake quantity was adjusted relative to body mass so that it provided 1.50 g of fat (70% of total energy), 1.20 g of carbohydrate (25%), 0.21 g of protein (5%), and 80 kJ·kg−1 body mass (34).
After the hand was prewarmed in water heated to 40°C for 5 min, the fingertip was pierced (Unistick 3 Extra, Owen Mumford, UK), and between 300 and 600 µL of whole capillary blood was collected into potassium-EDTA coated Microvette CB 300 tubes (Sarstedt Ltd., Leicester, UK). The whole blood samples were centrifuged immediately for 15 min at 12,800g (Eppendorf 5415c, Hamburg, Germany). An automatic pipette was used to dispense 20 µL of plasma into a 1.5-mL Eppendorf tube (Fisher Scientific Ltd., Loughborough, UK), which was then diluted 50 times by adding 980 µL of ice-cold saline (0.9% NaCl) (2) (Hamilton Microlab 500 series, Reno, NV). This procedure was repeated so that two aliquots of diluted plasma were stored at –80°C for up to 2 months before subsequent analysis (34). All samples were analyzed for [TAG] and glucose concentration ([glucose]) by enzymatic, colorimetric methods (Randox Laboratories Ltd., Crumlin, UK) using a centrifugal analyzer (Cobas Mira Plus; Roche, Basel, Switzerland). To account for the predilution of the plasma samples, the dilution step of the assay was eliminated, and three times the sample volume stated in the Randox kit assay procedure was used to ensure the concentration of the sample for analysis was the same as the original assay procedure (2,34,36). The within-batch coefficients of variation for [TAG] and [glucose] were 3.4% and 2.2%, respectively. Hemoglobin concentration and hematocrit were also quantified in the fasting and postprandial samples to estimate the acute change in plasma volume (9). Hemoglobin was measured in duplicate by hemophotometry (HemoCue AB, Ängelholm, Sweden), and hematocrit was assessed in duplicate using a microhematocrit centrifuge and reader (Haematospin 1300 Microcentrifuge; Hawksley & Sons Ltd., Sussex, UK).
Data were stored and analyzed using the IBM SPSS statistics software for Windows version 19 (IBM Corporation, Armonk, NY). Descriptive statistics outlining the physical and physiological characteristics of participants were determined. Normality of the data was confirmed by Shapiro–Wilk tests, and the homogeneity of variances was checked by Mauchly’s test of sphericity, with a Greenhouse–Geisser correction of the degrees of freedom applied if sphericity was violated. Student’s paired t-tests were used to identify differences between the EX30 and EX60 responses. 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. Incremental versions of these were also calculated after accounting for respective fasting concentrations (iAUC-TAG and iAUC-glucose) across the conditions. The AUC responses, fasting blood concentrations, and estimated changes in plasma volume were compared using separate one-way within-measures ANOVA. Differences in postprandial [TAG] and [glucose] were identified using separate 3 × 7 (condition by time) within-measures ANOVA. The 95% confidence intervals (95% CI) for the mean absolute pairwise differences between experimental conditions were calculated using the t-distribution and degrees of freedom (n – 1), and absolute standardized effect sizes (ES) are provided to supplement the findings. In the absence of a clinical anchor, an ES of 0.2 was considered the minimum important difference for all outcome measures, 0.5 moderate and 0.8 large (6). Bivariate correlations identifying possible determinants of the exercise-induced changes in TAUC-TAG were quantified using linear regression. The interpretation of the data will be based on 95% CI and ES.
Responses to treadmill exercise.
The treadmill responses for EX30 and EX60 are shown in Table 2. The girls exercised at an average intensity of 55% and 56% in EX30 and EX60, respectively (95% CI = –0.2% to 2.0%, ES = 0.30). As expected, the exercise EE (95% CI = 686 to 832 kJ, ES = 3.59) and absolute fat (95% CI = 8 to 13 g, ES = 2.42) and carbohydrate oxidation (95% CI = 16 to 28 g, ES = 1.95) were greater for EX60 compared with EX30. The only other difference in the exercise responses was a two-unit increase in RPE for EX60 compared with EX30 (95% CI = 1–2, ES = 0.69).
Plasma volume changes and fasting [TAG] and [glucose].
Average changes in plasma volume between the fasting and the 6-h postprandial samples were small between the conditions (CON = –1.1%, EX30 = –2.4%, EX60 = –1.8%, ES = 0.10–0.18). Therefore, the raw plasma [TAG] and [glucose] were not adjusted before statistical analyses. The fasting plasma [TAG] and [glucose] for each condition are displayed in Table 3. Fasting plasma [TAG] was lower after EX60 compared with CON (95% CI = –0.36 to 0.04 mmol·L−1, ES = 0.41) and EX30 (95% CI = –0.47 to 0.04 mmol·L−1, ES = 0.46), with no difference observed between EX30 and CON (95% CI = –0.22 to 0.34 mmol·L−1, ES = 0.13). However, all group mean concentrations were low (≤0.90 mmol·L−1). Differences in fasting plasma [glucose] between the three conditions were trivial (ES = 0.02–0.21).
Plasma [TAG] and [glucose] in the postprandial period.
Plasma [TAG] responses over the postprandial period for each experimental condition are shown in Figure 2. Differences in postprandial plasma [TAG] across the conditions were evident (main effect condition ES = 0.36, main effect time ES = 0.39, condition–time interaction ES = 0.10). The TAUC-TAG (Table 3) was lower after EX60 compared with CON (95% CI = –2.66 to –0.04 mmol·L−1 6 h, ES = 0.40) and EX30 (95% CI = –2.11 to 0.15 mmol·L−1 6 h, ES = 0.30); EX30 and CON were similar (95% CI = –1.44 to 0.71 mmol·L−1 6 h, ES = 0.10). Individual changes (delta) in TAUC-TAG between the exercise conditions and CON are shown in Figure 3. The reductions in TAUC-TAG after EX30 and EX60 were greater than changes in CON for 10 (∼56%) and 12 (67%) girls, respectively. Of the 12 girls that responded to EX60, nine also experienced a lower postprandial TAG response after EX30 compared with CON (data not shown). The Pearson’s product moment correlation for the individual changes in TAUC-TAG between EX30 and EX60 revealed a moderate effect (r = 0.57). The measured physical and physiological characteristics (Table 1) and exercise responses (Table 2) were not meaningfully correlated with the delta TAUC-TAG values for EX30 and EX60. The differences in iAUC-TAG between EX30 and the other two conditions were trivial (ES ≤ 0.15), and only a small effect was seen between EX60 and CON (ES = 0.30), although the 95% CI suggested this was unlikely to be meaningful (−1.26 to 0.33 mmol·L−1 6 h).
There was no difference in postprandial plasma [glucose] across the conditions (main effect condition ES = 0.06, main effect time ES = 0.50, condition–time interaction ES = 0.09). Similarly, there was little difference in either TAUC-glucose (ES = 0.05–0.13) or iAUC-glucose (ES = 0.08–0.22) between the conditions.
The main finding of the present study was that 60, but not 30 min, of moderate-intensity treadmill exercise reduced postprandial [TAG] in healthy girls. Therefore, provided the EE is sufficient, performing moderate-intensity exercise may represent an effective intervention to improve this aspect of cardiovascular health from a young age.
In the present study, a small to moderate reduction in fasting [TAG] was evident in EX60 compared with both CON and EX30. Although this largely supports previous findings in boys where the exercise gross EE was between 1.0 and 2.2 MJ (2,34,36), fasting [TAG] are highly variable in children (35), and postprandial [TAG] are more predictive of cardiovascular disease risk than traditional fasting concentrations (1).
It is well established that a single session of moderate-intensity exercise reduces postprandial [TAG] in adults (28). Furthermore, accumulating evidence demonstrates that moderate to vigorous-intensity exercise promotes favorable changes in postprandial [TAG] in boys (2,21,30,34,36), but no studies are available with girls. The reduction in postprandial [TAG] after EX60 supports previous studies in boys adopting a similar duration of moderate-intensity exercise and inducing an exercise gross EE between 1.5 and 2.5 MJ (2,21,30,34,36). The magnitude of change in postprandial lipemia after ∼60 min moderate-intensity exercise is highly variable in these studies, shown by the range of estimated ES between 0.32 and 0.86 (2,21,30,34,36). Nevertheless, the moderate reduction seen in the present study is comparable with the average attenuation reported in the other studies with boys. In contrast to the present study, despite a similar doubling of the exercise gross EE in the 60-min condition (1967 kJ) compared with the 30-min condition (982 kJ), the reduction in postprandial [TAG] was similar after both exercise conditions in healthy boys (34). The lower exercise gross EE in this study is likely to reflect sex-based differences in body composition and peak V˙O2 (33). Although the TAG-lowering effect of moderate-intensity exercise may be determined by the exercise EE in adults (12,37), this relationship has not been supported in healthy boys (34,36). In the present study, the gross EE in the 60-min condition was approximately double that that in the 30-min condition; however, only EX60 reduced postprandial [TAG]. A dose response is not supported directly by these results, but given some of the girls experienced a reduction in EX30 TAUC compared with CON, it cannot by discounted completely. Further examination of the data did not reveal a “threshold” exercise EE suggested in our previous study (34) and elsewhere in the literature (24).
The 60-min exercise condition meets the minimum recommended level of physical activity to promote and maintain health in children and adolescents (8,17). Although EX30 did not alter the postprandial TAG response systematically, the lowest dose of moderate-intensity exercise EE required to reduce postprandial lipemia consistently in girls is likely to be between 777 and 1536 kJ, which is lower than the ∼2-MJ threshold suggested in adults (24). Although it is not possible to determine the clinical significance of our findings, the origin of atherosclerosis in childhood is well established (25), and cardiovascular risk factors are known to track from childhood to adulthood (3). It has been suggested that postprandial [TAG] <2.3 mmol·L−1 in young people represents a healthy response (20), which is true of 95% of the postprandial TAG samples in the present study. Therefore, the lower postprandial [TAG] after EX60 in girls with a predominantly healthy postprandial TAG profile is promising and supports the contention that interventions aimed at reducing cardiovascular risk factors should be initiated early in life (25).
Several studies have demonstrated that premenopausal women experience a lower postprandial TAG response than men (7,16). Although the mechanisms responsible for this sex difference are not well established, it has been suggested that greater skeletal muscle uptake and retention of plasma [TAG], lower abdominal visceral adipose tissue lipolysis, and greater suppression of upper-body subcutaneous adipose tissue lipolysis in women may be responsible (7,15,18). However, it appears that men and women experience similar acute exercise-induced reductions in postprandial [TAG] (23.5% vs 19.8%, respectively) (13). The moderate reduction in postprandial [TAG] after EX60 in the present study is comparable with the magnitude of change evident in similar studies with boys (2,21,30,34,36). Therefore, provided adequate energy is expended, healthy girls may acquire potential metabolic health benefits in postprandial TAG metabolism after moderate-intensity exercise.
Considerable interindividual variability was present in the postprandial TAG responses after the exercise conditions (Fig. 3), with “responders” and “nonresponders” identified in both exercise conditions. Similar heterogeneity in the exercise-induced changes in postprandial [TAG] has been reported with boys (34), and consistent with the present study, the determinants of this variation could not be identified. In adults, changes in postprandial 3-hydroxybutyrate, a marker of hepatic fatty acid metabolism, accounted for almost half the variance evident in TAUC-TAG (11), and it is possible that this marker may explain some of the heterogeneity evident in our sample. Interestingly, a positive association was identified between the delta TAUC-TAG for EX30 and EX60, supporting previous findings with boys (34). This suggests that, at least at an individual level, girls demonstrating an attenuated postprandial TAG response after EX60 may also experience a reduction in postprandial [TAG] after EX30. It should be noted that the majority of girls in the current study self-classified themselves as mid to late pubertal. Although we found no effect of maturity status on any of the outcome measures or the degree of heterogeneity evident in postprandial [TAG], a possible maturation effect cannot be ruled out entirely.
The mechanisms underpinning the reduction in postprandial [TAG] after EX60 cannot be determined from our findings. However, in adults, it is proposed that a greater removal rate of circulating TAG facilitated by higher skeletal muscle lipoprotein lipase activity may be responsible (14), although other mechanisms were also suggested. An alternative mechanism implicates the diminished appearance of VLDL-TAG in the circulation (23), possibly due to the secretion of fewer, TAG-richer VLDL particles, which are likely to have a greater affinity for lipoprotein lipase (22). Indirect evidence for altered hepatic VLDL secretion can be derived from a recent study, reporting that the exercise-induced reduction in postprandial [TAG] is associated with the elevation in the concentration of postprandial 3-hydroxybutyrate (11). This suggests that exercise may result in a shift toward hepatic fatty acid oxidation and away from fatty acid re-esterification and VLDL-TAG synthesis (11).
In conclusion, 60 min of moderate-intensity exercise, with a gross EE of 1536 kJ performed ∼14.5 h before a high fat breakfast, reduced postprandial [TAG] in healthy girls. Performing 30 min of moderate-intensity exercise, inducing half the exercise gross EE (777 kJ), was insufficient to reduce postprandial [TAG] in this population.
The authors thank Alsager High School, Outwoods Edge Primary School, and Woodbrook Vale High School in Loughborough for their support and the participants and their parents for their commitment throughout this research.
The authors declare no conflict of interest.
No funding was received for this research, other than that available internally through Loughborough University.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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Keywords:© 2014 American College of Sports Medicine
ADOLESCENT; FEMALE; TREADMILL; TRIACYLGLYCEROL CONCENTRATION