Exercise Energy Expenditure and Postprandial Lipemia in Girls

Keith Tolfrey; Alex Engstrom; Caoileann Murphy; Alice Thackray; Robert Weaver; Laura A. Barrett

Disclosures

Med Sci Sports Exerc. 2014;46(2):239-246. 

In This Article

Discussion

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 V̇O2.[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.

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