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Unit for Preventive Nutrition, Department of Biosciences and Nutrition at Novum, Karolinska Institute, Huddinge, SwedenImmunonutrition Research Group, Department of Metabolism and Nutrition, Institute of Refrigeration, Institute of Food Science, Technology and Nutrition (ICTAN), Spanish National Research Council, Madrid, SpainDepartment of Physical Education, Sport and Human Movement, Faculty of Teacher Training and Education, The Autonomous University of Madrid, Madrid, Spain
Clinical Investigation Center, National Institute for Medical Research (INSERM), Regional University Hospital Center (CHRU) of Lille, and Faculty of Medicine, Institute of Preventive Medicine and Therapeutic Research, University Lille, Lille, France
Immunonutrition Research Group, Department of Metabolism and Nutrition, Institute of Refrigeration, Institute of Food Science, Technology and Nutrition (ICTAN), Spanish National Research Council, Madrid, Spain
It is unclear how much physical activity is necessary to prevent obesity during adolescence.
The purpose of this study was to examine whether the current physical activity guidelines for youth are associated with a lower risk of excess of body fat in European adolescents.
A sample of 2094 adolescents from the HELENA cross-sectional study was selected. Levels of moderate, vigorous, and moderate to vigorous physical activity (MVPA) were assessed using accelerometers. BMI and percentage of body fat (skinfolds) were calculated and used to categorize the adolescents as normal-weight/normal-fat, overweight/overfat, and obese. Data were collected during 2006–2007. Receiver operating characteristic (ROC) analysis, performed in 2009, was used to determine thresholds that best discriminate between weight and body fat categories.
ROC analysis showed that ≥18 minutes/day in vigorous physical activity and ≥55 minutes/day in MVPA significantly discriminated between normal-weight and overweight+obesity categories. Moreover, ≥9 minutes/day of vigorous physical activity and ≥49 minutes/day of MVPA discriminated between normal-fat and overfat/obese adolescents. Adolescents who did not meet the current physical activity guidelines for youth of 60 minutes/day in MVPA increased the risk of having overweight+obesity (OR=1.24, 95% CI=1.01, 1.534) and overfat+obesity (OR=1.79, 95% CI=1.33, 2.42). ORs increased when adolescents did not meet the guidelines of at least 15 minutes/day in vigorous physical activity.
Current physical activity guidelines seem to be appropriate to prevent excess of body fat in European adolescents. However, participation in vigorous physical activity might have additional importance in preventing obesity.
Many years have passed since the first recommendation of physical activity for children and adolescents stated by the American College of Sports Medicine in 1988
During these years, findings from cross-sectional, longitudinal, and experimental studies have contributed to the development of different physical activity guidelines. Several experimental studies helped to examine the dose–response effect of physical activity on pediatric obesity.
and it is stated that adolescents should participate in at least 20 minutes/day of MVPA.
The aim of the present study was to examine whether the current physical activity guidelines are associated with a low risk of having excess of body fat in European adolescents participating in the Healthy Lifestyle in Europe by Nutrition in Adolescence Cross-Sectional Study (HELENA-CSS).
Design and Participants
The HELENA-CSS is a multicenter investigation carried out in 10 European cities: Athens (Greece); Dortmund (Germany); Ghent (Belgium); Heraklion (Greece); Lille (France); Pécs (Hungary); Rome (Italy); Stockholm (Sweden); Vienna (Austria); and Zaragoza (Spain). The main aim of the HELENA-CSS was to obtain reliable and comparable data on a broad battery of relevant nutrition- and health-related parameters.
In brief, a multiple-stage cluster random sample of adolescents, stratified for geographic location, age, and SES, was obtained, striving for representativeness on the level of these cities. Data collection from the HELENA-CSS took place in 2006−2007.
A total of 3865 adolescents were recruited at schools and met the general HELENA-CCS inclusion criteria: aged 12.5–17.5 years, not participating simultaneously in another clinical trial, free of any acute infection 1 week before the inclusion, and having information on weight and height.
Adolescents and their parents or guardians were informed of the nature and characteristics of the study, and all provided a written informed consent. Ethics committees from each country approved the HELENA-CSS protocols.
Measurements were performed with the adolescent barefoot and in underwear. Body weight was measured to the nearest 0.1 kg with an electronic scale and height was measured with a telescopic stadiometer to the nearest 0.1 cm.
In addition, age- and gender-specific 85th and 95th percentile of %BF in the study sample were used to define overfat and obese adolescents.
Physical Activity Measurements
Levels of physical activity were objectively measured using the ActiGraph GT1M accelerometer. This accelerometer is a compact, small, lightweight, and uniaxial monitor designed to detect vertical acceleration movements. This accelerometer has been widely validated in youth.
Adolescents were instructed to place the accelerometer at the lower back using an elastic waistband. They were also instructed to wear the accelerometer at all times for 7 consecutive days and to remove it only during water-based activities and sleeping. The interval of time (epoch) was set at 15 seconds in concordance with consensus recommendations for assessing physical activity in these ages.
The data were downloaded onto a computer using the manufacturer's software and later were analyzed by a software based on Visual Basic. The analyzer excluded from the analysis bouts of 20 continuous minutes of zeros output, considering these periods as nonwearing time. Inclusion criteria of at least 3 days of valid recording and a minimum of 8 hours' registration per day were necessary to be included in the study.
The time spent (minutes/day) in moderate and vigorous physical activity was calculated based on cutoffs of 500 counts per 15 seconds and 1000 counts per 15 seconds, respectively. Further, MVPA was calculated using the cutoff point of ≥500 counts per 15 seconds that is equivalent to walking at 3 km/hour.
A sample of 2199 adolescents had valid data on physical activity and BMI. Both samples were equivalent to the full HELENA-CSS sample according to age, gender distribution, and anthropometric measures. Further, because the aim of this study was to study the excess of body fat, those adolescents classified as underweight
were not included (n=105), even though there were no differences in levels of physical activity between underweight and normal-weight adolescents (all p>0.05). Thus, the present study sample comprised 2094 adolescents (973 boys and 1121 girls), and a total of 1994 (902 boys and 1092 girls) had valid data on skinfold thicknesses.
Study sample characteristics are presented as mean (SD), unless otherwise stated. Differences between genders were analyzed by ANOVA for continuous variables and by the chi-square test for nominal variables. Receiver operating characteristic (ROC) curves
were used to calculate the optimal physical activity cutoff points for moderate physical activity, vigorous physical activity, and MVPA that best discriminate between the normal-weight/normal-fat and overweight+obesity/overfat+obesity categories. Additional analyses were performed to discriminate between normal-weight/normal-fat and obesity using both classifications.
An ROC curve provides the whole spectrum of specificity/sensitivity values for all the possible cutoffs. Sensitivity was considered as the probability to correctly identify an overweight/overfat or obese adolescent (true-positive proportion). Specificity was considered to be the probability of correctly identifying a normal-weight/normal-fat adolescent. The false-positive proportion were those normal-weight/normal-fat adolescents who were incorrectly identified as being overweight/overfat or obese.
To identify the best cutoff, the distance between the ideal test and each sensitivity and 100-specificity pair was calculated with the MedCalc statistical software, version 10.4.5). The area under ROC curve (AUC) and 95% CIs were calculated using the same software. The AUC represents the ability of the test to correctly classify adolescents according to their weight/body-fat category. Values of AUC range from 0.5 (noninformative test) to 1.0 (ideal test). The cutoffs, AUC, and the sensitivity/specificity pairs were also calculated for the total sample and for each gender.
Finally, the percentages of adolescents meeting different definitions of physical activity recommendations for youth (20, 60, and 90 minutes/day in MVPA, 60 minutes/day in MVPA with 30 minutes/day in vigorous physical activity, and the recommendation obtained in this study from the ROC analysis) were calculated. Further, the relationship between not meeting the physical activity recommendations and an unhealthy weight or body fat status was examined by binary logistic regression analysis. These analyses were conducted using SPSS, version 15.0, for Windows, and the level of significance was set at <0.05. All the analyses were performed in 2009.
Characteristics of the study sample are shown in Table 1. ROC analysis using BMI categories (Table 2) showed that moderate physical activity does not discriminate among weight status categories, whereas vigorous physical activity significantly discriminated between normal-weight and overweight+obesity, and between normal-weight and obesity. The significant cutoff points of vigorous physical activity associated with normal-weight were 18 minutes/day for all, 18–19 minutes/day in boys, and 10 minutes/day in girls. The optimal cutoffs for time spent in MVPA were also significant between normal-weight and overweight+obesity and between normal-weight and obesity. These cutoffs were with 55–71 minutes/day in MVPA for all, 56 minutes/day of MVPA for boys, and 49–51 minutes/day of MVPA for girls.
Table 1Physical characteristics, body fat, and levels of physical activity in European adolescents
Receiver operating characteristic analysis using %BF categories (Table 3) showed that a cutoff point of 29 minutes/day in moderate physical activity discriminated between the normal-fat and excess-of-body-fat categories in girls. Vigorous physical activity and MVPA cutoffs significantly discriminated between the normal-fat and excess-of-body-fat categories. The vigorous physical activity cutoffs were 11–12 minutes/day for all, 17–18 minutes/day for boys, and 9–10 minutes/day for girls. The MVPA cutoffs were 49–52 minutes/day for the total sample, 47–55 for boys, and 41–42 for girls. The higher AUC values were always found when comparing the normal-weight/normal-fat and obesity categories.
Table 3Physical activity cutoff points to identify the risk of an excess of body fat according to body fat categories
From the results obtained with the ROC analyses, cutoff points for MVPA could be slightly rounded to 60 minutes/day for boys and girls. However, the large differences between boys and girls in cutoff points for vigorous physical activity deserve emphasis. Therefore, three recommendations were proposed, based on the time spent in vigorous-intensity activity: (1) a combined recommendation for boys and girls of 60 minutes in MVPA with at least 15 minutes in vigorous physical activity; (2) a recommendation for boys of 60 minutes in MVPA with at least 20 minutes/day of vigorous physical activity; (3) a recommendation for girls of 60 minutes of MVPA with at least 10 minutes of vigorous physical activity. The percentages of adolescents meeting the physical activity recommendations by gender, weight, and body fat categories are displayed in Table 4.
Table 4Percentage of adolescents meeting physical activity recommendations for youth stratified in body fat categories
Figure 1a shows the OR of having overweight+obesity and overfat+obesity if the adolescent did not meet the physical activity recommendations for the total sample and by gender. In the whole sample, only those adolescents who did not meet the 60 minutes in MVPA recommendation and the recommendation of 60 minutes in MVPA with at least 15 minutes in vigorous physical activity had an increased risk of being in both the overweight+obesity (OR=1.24, 95% CI=1.01, 1.53, and OR=1.36, 95% CI=1.10, 1.70, respectively) and the overfat+obesity (OR=1.79, 95% CIs=1.33, 2.42, and OR=1.85, 95% CI=1.40, 2.45, respectively) categories. Similarly in boys, only the 60 minutes in MVPA recommendation and the recommendation of 60 minutes in MVPA with at least 20 minutes in vigorous physical activity obtained significant ORs of having both overweight+obesity and overfat+obesity. In girls, none recommendation obtained significant ORs for overweight+obesity. Despite this, the 60 minutes in MVPA recommendation and the recommendation of 60 minutes in MVPA with at least 10 minutes in vigorous physical activity obtained significant ORs of overfat+obesity in girls. All analyses were repeated by analyzing the risk of having obesity in normal-weight/normal-fat adolescents and results were similar (Figure 1b).
The main result of the current study suggests that physical activity recommendations for youth that advise 60 minutes/day in MVPA are associated with reduced risk of overweight and obesity in European adolescents. Adolescents who did not meet 60 minutes/day in MVPA with at least 10–20 minutes/day in vigorous physical activity might have a higher risk of having an excess of body fat than adolescents who meet the daily overall 60 minutes in MVPA.
The USDHHS together with several medical institutions recently launched the Physical Activity Guidelines for Children and Adolescents, where it is mentioned that this population group should participate in 60 minutes or more of physical activity daily.
This recommendation continues along the same lines as previous public health recommendations for this population. However, a final note in the USDHHS recommendation suggests that children and adolescents should do vigorous physical activity at least 3 days per week but none volume is stated.
The current results concur with these recommendations because it was also shown that between 10 and 20 minutes/day in vigorous physical activity might have a major benefit in reducing body fat in this life period. Only the Health Canada recommendation of 90 minutes/day in MVPA included a specific recommendation for vigorous physical activity (i.e., 30 minutes in vigorous physical activity).
was achieved for most of adolescents independently of gender and body fat. According to the current results, the 20 and 90 minutes/day in MVPA thresholds do not seem to be adequate.
A threshold of 60 minutes/day in MVPA might discriminate between normal-weight and overweight+obesity adolescents as well as between those in the normal-fat versus overfat+obesity category. Nevertheless, approximately 20 minutes/day in vigorous physical activity in boys, 10 minutes/day in girls, and 15 minutes/day combined for both genders discriminated more accurately between normal-weight/normal-fat and unhealthier weight and body fat categories. The key role of vigorous physical activity on overweight and obesity has been highlighted in a number of cross-sectional and experimental studies. For example, an earlier study
evaluated the effect of a vigorous physical activity training in 38 nonobese adolescents (18 control) and the results showed reductions in subcutaneous adipose tissue measured by magnetic resonance. Another study
examined the associations of objectively assessed physical activity with body fat measured by dual-energy x-ray absorptiometry (DXA), and only objectively assessed vigorous physical activity explained a significant proportion of the variance in body fat after controlling for potential confounders. Similarly, one study
found that insufficient vigorous physical activity estimated by accelerometry was the only risk factor for higher weight in 878 adolescents. Likewise, the authors have found similar results in previous reports.
These and the current findings highlight the role of daily vigorous physical activity to prevent overweight and obesity during adolescence.
When the thresholds were examined by intensities, in general, moderate physical activity cutoff points did not significantly discriminate among weight and body fat categories, whereas vigorous physical activity always found significant cutoffs. Interestingly, MVPA cutoffs also discriminated between weight status and body fat categories. Taken together, the current results suggest that MVPA is negatively associated with an excess of body fat as a result of the vigorous physical activity included into the MVPA threshold. Likewise, in a previous study with a subsample of 365 Spanish adolescents from the HELENA-CSS, where body fat was measured by DXA, BodPod, and anthropometry, MVPA and especially vigorous physical activity were negatively associated with indicators of body fat.
Subsequently, the MVPA cutoff points found in the current study seem to represent the necessary time in MVPA to guarantee enough time in vigorous physical activity, in order to avoid excess of body fat in adolescents. Following this rationale, it is recommended that the guideline of 60 minutes in MVPA be maintained and that a specific period of vigorous physical activity be added as part of that time period instead of adding more overall time spent in physical activity (e.g., 60 minutes in MVPA plus 15 minutes in vigorous physical activity). Thus, an after-school intervention in black girls provided 80 minutes in MVPA in order to ensure enough vigorous physical activity, and the greatest decreases were found in those girls who maintained the highest heart rates during the training.
Differences observed between boys and girls in vigorous physical activity cutoff points allowed a different recommendation with 20 minutes/day for boys and 10 minutes/day for girls and a combined recommendation of 15 minutes/day for both genders. The obvious biological and physical activity differences between genders across adolescence may need to be reflected in the physical activity guidelines. Several recommendations based on steps measured by pedometer have suggested different thresholds for boys and girls related to a healthy weight status.
Nevertheless, a combined recommendation was included for both genders of 60 minutes/day in MVPA including 15 minutes/day in vigorous physical activity, similar to the recommendation made in a previous study.
found that 45 minutes/day in moderate physical activity and 15 minutes/day in vigorous physical activity were independently associated with reduced body fat and BMI status in children. Unfortunately, the small sample did not allow an analysis by gender, but girls were engaged in less-vigorous physical activity than boys.
In further analyses, an examination was made of the ORs of having overweight+obesity, overfat+obesity, and obesity in adolescents not meeting several physical activity recommendations, including the current proposals obtained by the ROC analysis. These analyses showed that the whole sample that did not meet the recommendation had a significantly greater risk of having an excess of body fat than other recommendations. These results were stronger in boys than in girls, for whom the current proposal obtained similar results compared with the overall 60 minutes/day in MVPA recommendation. All the other physical activity recommendations failed to detect significant risks of having both overweight+obesity and overfat+obesity if the adolescent did not meet such recommendations.
Strengths of the current study include measurements by accelerometer and a relatively large sample of adolescents from 10 European cities. Also, two methods were used to estimate body fat (BMI and %BF by skinfolds). BMI is widely used for the screening of obesity both in children and adolescents; however, BMI seems to be associated with lean mass rather than with fat mass.
These are the reasons why both BMI and %BF variables were included in this study.
The current study has limitations as well. The cross-sectional design limits the possibility to infer directionality. Thus, although reverse causality might be considered, there is certain evidence with regard to the plausible effect of physical activity on body fat in youth.
Another limitation of the study is that both BMI and %BF by skinfolds are indirect methods to assess body fat. In addition, there are inherent limitations when the current physical activity outcomes are compared with those of other studies because of several factors such as type of monitor, length of the epoch, and cutoff points selected.
In summary, the physical activity recommendation of 60 minutes/day in MVPA may be enough to avoid excess of body fat in adolescents. However, vigorous physical activity seems to be a key factor for decreasing the possibility of being obese. Therefore, public health recommendations should include a specific amount of vigorous physical activity for adolescents. In this framework, these results need to be contrasted in experimental designs.
We acknowledge the help of all the adolescents who took part in the Helena study. The writing group takes sole responsibility for the content of this article. The content of this article reflects the views of the authors only, and the European Community is not liable for any use that may be made of the information contained therein. The HELENA study was supported by the European Community 6th RTD Framework Programme (Contract FOOD-CT-2005-007034 ). This study is also being supported by grants from the Spanish Ministry of Education ( EX-2007-1124 , EX-2008-0641 , AP2006-02464 ), the Spanish Ministry of Health (Maternal, Child Health and Development Network , RD08/0072 ), the Swedish Council for Working Life and Social Research , and the Swedish Heart Lung Foundation ( 20090635 ).
No financial disclosures were reported by the authors of this paper.
Drs. Guinhouya, Hubert, and Zitouni raise an important question for the field of accelerometry in their letter: Which is the best cut point to estimate moderate-to-vigorous physical activity (MVPA) using the ActiGraph accelerometer in youth?
Physical activity is recognized as a key health component. Thus, it is currently recommended that youth engage in a minimum of 60 minutes/day of an activity that is of moderate intensity or, ideally, a moderate-to-vigorous physical activity (MVPA).1 In order to quantify accurately the physical activity level of youth, and for a fair examination of the relationships between physical activity and health outcomes, devices such as the MTI Actigraph accelerometer are used increasingly among this population.