The objective of the present study was to examine the dose-response relationship between MVPA and dyslipidemia in youth. A curvilinear dose-response relationship was observed with high-risk HDL cholesterol and triglyceride values such that most of the effect was observed within the first 15 min to 30 min per day of MVPA. There was no clear relationship between MVPA and high-risk LDL cholesterol values.
The observation of decreased odds for high-risk HDL cholesterol and triglycerides with increasing MVPA is consistent with previous observational studies in youth (4
). Intervention studies have also demonstrated improvements in HDL cholesterol (8
) and trigycerides (23
) with increases in exercise in youth. Research examining the relationship between physical activity and LDL cholesterol in both adults and youth is contradictory (24
). Within the adult population, endurance athletes have LDL cholesterol levels that are only 8% to 10% lower than their sedentary counterparts (25
). Conversely, HDL cholesterol levels are 40% to 50% higher in highly active adults compared with inactive adults (25
). Thus, the observation in the present study that MVPA was related to high-risk HDL cholesterol and triglyceride values, but not high-risk LDL cholesterol values, is consistent with existing knowledge.
Kelley and Kelley (26
) completed a meta-analysis that included 12 randomized controlled trials investigating the effect of exercise training on plasma lipid levels in youth eight to 16 years of age. They concluded that aerobic exercise training has no effect on non-HDL cholesterol (calculated as total cholesterol minus HDL cholesterol) or HDL cholesterol. The lack of a significant effect of exercise training on plasma lipids in this meta-analysis may be partially attributable to the inclusion criteria. Specifically, this meta-analysis only included studies that recruited youth with normal preintervention lipid levels. Previous findings in adults have suggested that physical activity interventions are most effective at improving plasma lipids in those with abnormal lipid values before intervention (27
). Our study focused on predicting high-risk plasma lipid values and, with this analytical approach, we were able to demonstrate relationships of MVPA with HDL cholesterol and triglycerides in youth.
We are aware of only two previous studies (7
) that have examined the dose-response relationship between physical activity and plasma lipids in children and youth. Tolfrey et al (16
) published the results of a 12-week intervention study that was conducted using a small sample (n=36) of 10- to 12-year-old adolescents. Participants were prescribed either low-volume (approximately 60 min/week) or moderate-volume (approximately 78 min/week) exercise programs. Not surprisingly, given the small total volume of exercise prescribed and the small difference (approximately 18 min per week) in exercise volume between the groups, the researchers failed to find a dose-response or threshold effect of physical activity on total cholesterol, LDL cholesterol, HDL cholesterol or triglycerides. The second study (7
) was a cross-sectional study of 2358 young Finnish people nine to 24 years of age. Participants were divided into tertiles based on their level of self-reported MVPA. Plasma triglyceride and HDL cholesterol levels improved across physical activity tertiles in this study. Although the results suggested that a dose-response relationship between MVPA and plasma lipids exists, the authors were not able to explore the pattern of the dose-response relationship (eg, linear or curvilinear). Only three physical activity categories were compared, and each of these categories included a wide range of physical activity scores. The present study examined MVPA on a continuum, which enabled us to better characterize the dose-response relationships.
The shape of the dose-response curves between MVPA and high-risk HDL cholesterol and triglyceride values offers encouraging news for clinicians and other health care practitioners who are concerned about promoting physical activity in their young patients. The majority of the benefits of MVPA on lipids were observed within the first 15 min to 30 min per day. This is far less than the 60 min of daily MVPA that have been recommended by many countries and organizations (9
), and the 90 min of MVPA that is recommended in Canada’s physical activity guidelines for children and youth (33
). While it would be ideal to reach MVPA levels that conform to these guidelines, clinicians and health care practitioners need to recognize that getting a very sedentary young person to engage in even minimal levels of MVPA may offer tremendous benefits for their cardiovascular health. In the present study, which used objective measures of MVPA, almost one-half of the youth studied accumulated 15 min or less of MVPA per average day; one-quarter accumulated 7 min per day or less. This implies that a large percentage of youth are extremely sedentary and need to engage in more MVPA to promote good health.
To our knowledge, the present study is the first to use a representative study sample and/or an objective measure of physical activity to examine the relationship between MVPA and plasma lipids in youth. Accelerometers provide a reliable and objective measure of physical activity in a ‘real-life’ setting. A limitation of previous observational studies is that information regarding the level and intensity of physical activity was often obtained through subjective, self-report questionnaires (7
). It is well documented that most children and youth over-report their time spent engaging in physical activity (34
).Over-reporting of physical activity may have masked the true relationship between physical activity and lipid/lipoprotein level in the past. The statistical modelling approach used in the current study allowed us to use the model that best represented the relationship between MVPA and high-risk plasma lipid values, thereby producing dose-response curves that best represent the data.
As with any study, the present study had a number of limitations. The primary study limitation was that it was observational and cross-sectional in nature. This limits our ability to make any causal inferences about the relationship between MVPA and plasma lipids. Furthermore, although accelerometers are objective in nature and have been validated in previous population research (35
), they are not a perfect measure of physical activity. Accelerometers are insensitive measure devices when body movement in the hip region is independent of physical activity intensity (such as weight lifting or bicycling) (36
). The accelerometry measurement period ranged from four to seven days for each participant, and although this measurement period has a high test-retest reliability (12
), it may not accurately reflect habitual activity levels. Thus, limiting our physical activity measurement period to four to seven days may have contributed to measurement error for physical activity. This measurement error would have resulted in biased estimates of the associations between activity and lipid values. Finally, it is important to note that the present study only examined the relationship between MVPA and the traditional plasma lipids. We did not consider emerging lipid and lipoprotein risk factors, such as apolipoprotein B and small, dense LDL particles. Nonetheless, the available information suggests that the relationship between physical activity and apolipoprotein B in adolescents is similar to that between physical activity and LDL cholesterol (37