MANUSCRIPT CITATION

Lowe J, Watkins WJ, Kotecha SJ, Kotecha S. Physical activity and sedentary behavior in preterm-born 7-year old children. PLoS ONE 2016; 11(5): e0155229. PMID 27168339.

REVIEWED BY

Benjamin F Mentiplay, PhD
Murdoch Children’s Research Institute
benjamin.mentiplay@mcri.edu.au

Tara L FitzGerald, BPhysio(Hons)
Murdoch Children’s Research Institute
tara.fitzgerald@mcri.edu.au

Alicia J Spittle, PhD
University of Melbourne
aspittle@unimelb.edu.au

TYPE OF INVESTIGATION

Prognosis

QUESTIONS

1. In seven-year-old preterm-born children, is lower gestational age at birth related to reduced physical activity and increased sedentary behaviour compared with children born at term?
2. Is this relationship mediated through an increase in respiratory symptoms?

METHODS

• Design: Cohort study
• Allocation: Children at seven years of age who were born preterm were separated into three groups based on their gestational age at birth: 25-32, 33-34, and 35-36 weeks’ gestation. Term born children at seven years of age were those born 37-43 weeks’ gestation.
• Blinding: The study does not report whether the study team were blinded to the gestational age of the children during assessments and data analysis.
• Follow-up period: Children wore the physical activity monitor (Actigraph) around their waist for a period of seven days.
• Setting: Physical activity monitoring was performed in the community.
• Patients: Children involved in this study were seven years of age and part of a larger prospective cohort study (the Millennium Cohort Study), a geographically representative cohort which disproportionally sampled areas of ethnic minorities and lower socioeconomic status in the United Kingdom. Children were recruited between the years 2000-2002 and were assessed at nine months, three, five and seven years of age. A total of 18,818 children were originally recruited, with 12,781 agreeing to wear the physical activity monitor at seven years of age. Of these, 6,422 children had valid physical activity monitor data and had their gestational age at birth recorded. From the included 6,422 children, 79 children were born at 24–32 weeks’ (very preterm), 119 were born at 33–34 weeks’ (moderate preterm), 275 were born at 35–36 weeks’ gestation (late preterm), and 5,949 children were born at 37–43 weeks’ gestation (term).
• Intervention: Not applicable.
• Outcomes:
• Primary outcomes: The three primary outcomes were total physical activity, time spent in moderate to vigorous physical activity (MVPA), and time spent sedentary. These outcomes were assessed with a uniaxial accelerometer (Actigraph), which was worn around the waist during waking hours for seven consecutive days. The data were considered valid where there was a minimum of 10 hours of recording on at least two days.
• Total physical activity was defined as the average number of counts per minute (cpm) over the period of valid recording. Time spent in MVPA was defined as the average time in minutes per day spent at >2241cpm over the period of valid recording, while time spent sedentary was defined as the average time in minutes per day spent at <100cpm over the period of valid recording. Measures of MVPA and sedentary behaviour were standardised to physical activity monitor wear time, as the length of wear time varied across participants.
• Secondary outcomes: Secondary outcomes were rates of wheeze and atopy (as measures of respiratory symptoms) in the last 12 months as taken from interviews during the home visit at the 7-year follow-up. Height, weight and social class (measured with the 5-category UK Office for National Statistics Socio-economic Classification) were also collected at the same home visit.
• Other data previously collected included birthweight, admission to the neonatal intensive care unit, length of hospital stay, gestational age at birth, gender, ethnicity, as well as the mother’s smoking habits during pregnancy, employment and education qualifications.
• Analysis and sample size: The analysis for this study involved multiple steps. Firstly, chi-squared tests were used to examine differences between the participants with valid physical activity data and those without valid physical activity data. Secondly, comparison of birth characteristics between the four groups were examined using an ANOVA (normally distributed data) or a Kruskal Wallis Test (non-normally distributed data). Thirdly, general linear models were used to examine the effects of gestation group on the physical activity measures. All analyses were stratified by gender, due to the author’s previous work showing differences between genders for physical activity. Four models were performed with various adjustments taken from the secondary outcome measures. Lastly, formal mediation analysis was conducted to examine if respiratory symptoms (current wheeze or atopy) impacted upon the association between gestational age and physical activity measures.
• The sample for this study was taken from a larger geographically representative cohort study and included participants who had valid physical activity measures. No specific sample size calculation was performed for this particular component of the larger cohort study.
• Patient follow-up: Of the total 18,818 children who were originally recruited, 14,043 (75%) participated in the home visit at seven years of age. A total of 12,781 (91%) agreed to wear the physical activity monitor, with 6,675 returning valid physical activity data (52%). Of these 6,675 children with valid data, 253 had missing gestational age at birth. Therefore, 6,422 of the total 12,781 who wore the physical activity monitor (50%) were included in the study.

MAIN RESULTS

A summary of participant characteristics at seven years of age is shown in Table 1. Similar rates of males and females were shown across gestational age groups. Children born very preterm (≤32 weeks’ gestational age) had significantly lower BMI (Body mass index) and increased levels of recent wheeze compared with those born at term. Rates of atopy were similar between each group. Participants with valid physical activity data were more likely to be female and born at term (p<0.05). Other characteristics such as maternal education, employment and smoking status during pregnancy, are also reported in the article.

PA: physical activity; BMI: body mass index; SD: standard deviation.

The physical activity measures for each gestational age group are shown in Table 2. Females had significantly decreased physical activity levels and increased sedentary behaviour compared with males.

Gestational age was associated with a reduction in time spent in MVPA for males born very preterm compared with term children, although this was not evident for males born moderate-late preterm. This significant association was found for all four models with varying levels of adjustments. Interestingly, this association revealed males born at term were involved in 9 minutes per day more MVPA compared with males born very preterm. Total physical activity and sedentary behaviour were also significantly different in males born very preterm compared with term males, however this association was only significant in the first linear model, which was minimally adjusted for covariates. In females, there was no significant associations between gestational age and any measure of physical activity across all models.

The formal mediation analysis confirmed the results, with no evidence that wheeze or atopy mediates the association between gestational age and physical activity.

CONCLUSION

The authors have shown a small but significant reduction in MVPA for seven-year-old males born very preterm compared with term born males, which was reported to equate to one hour less MVPA over the course of a week. There was also a trend to reduced total physical activity and increased time spent sedentary in males born very preterm compared with term born males, although this association was not significant in fully adjusted models. The association between gestational age and physical activity was not mediated by respiratory symptoms. The authors found no association between gestational age and physical activity in females.

COMMENTARY

Children born very preterm (<32 weeks’ gestation) are at a higher risk of various difficulties compared with children born at term, such as increased motor impairment (1). Physical activity is important for the general population, with physical activity often included in healthy living guidelines for children. Physical activity, when measured with questionnaires, has been shown to be reduced in adolescents and young adults who were born very preterm (2,3). Interestingly, when measured objectively with accelerometry, physical activity levels are similar in adolescents and adults born preterm compared with their term born peers (4-7). However, there appears to be little research in younger children born preterm using objective measures of physical activity. Therefore, Lowe and colleagues studied seven-year-old children who were born preterm to examine levels of physical activity and sedentary behaviour compared with term born children (8).

This large-scale cohort study of over 6,000 children across the United Kingdom born across the gestational age spectrum (8), found seven-year-old boys born very preterm spent significantly less time in moderate to vigorous physical activity (MVPA) compared with term born boys. Trends were also shown toward reduced total physical activity and increased sedentary time in boys born very preterm, although this association was not significant in fully adjusted models. Lowe and colleagues also found that this association was not mediated by respiratory symptoms, and they also found no association between gestational age and any physical activity measure in girls.

Participants in this study showed high levels of MVPA, with many children meeting or exceeding recommended physical activity guidelines. In contrast, previous studies have shown preterm and term born children fail to meet the recommended guidelines (4,5). Comparison of physical activity research is problematic due to the differing methods between studies (e.g. various types of accelerometers, data collection epochs and cut-points for intensity definitions). The monitor used by Lowe and colleagues was an Actigraph uniaxial accelerometer worn around the hip, with previous studies examining preterm physical activity using both the same (4-6) and different accelerometers (7). Further highlighting the disparities between physical activity research methodology, the cut-point used by Lowe and colleagues for MVPA differs with previous research, which may have led to a potential overestimation (or underestimation) of MVPA levels. Thus, caution is needed when considering the results of this study compared to previous physical activity research.

In conclusion, this cohort study provides informative and objective data on the physical activity levels of preterm children at seven-years of age, with boys born very preterm having lower physical activity levels compared with term born children. Recent research has examined the factors associated with physical activity in adolescents born preterm (9) however little is currently known in younger children. As this study found that respiratory symptoms did not mediate the association between gestational age and physical activity in seven-year-old children, further work in younger children is needed to examine what factors are associated with physical activity to inform the design of early intervention strategies aimed to improve physical activity in this potentially at-risk population.

REFERENCES

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8. Lowe J, Watkins WJ, Kotecha SJ, Kotecha S. Physical activity and sedentary behavior in preterm-born 7-year old children. PLoS ONE 2016; 11: e0155229.
9. Proulx K, Majnemer A, Dahan-Oliel N, Mazer B, Nadeau L, Vanier K, et al. Factors associated with moderate to vigorous physical activity in adolescents born preterm. Pediatr Exerc Sci 2017; 29: 260-7.