The regulation of physical activity in young children

Published on December 16, 2004 at 5:49 AM · No Comments

The evidence at present suggests that neither better facilities, nor an increase in the amount of time allocated to PE to four hours a week, will improve physical activity levels in children as a whole and thereby impact on the rising tide of obesity.

Nobody will deny the rising tide of obesity in childhood. It is as regularly the subject of reports in the media as it is in the medical journals. It is a recent phenomenon and is worsening, with an estimated three-fold increase in the prevalence of obesity over the last generation. Depending upon the criteria used to define obesity, nearly 30% of British girls are now overweight at five years, and some 10% of them frankly obese. While some European countries, such as France, show a lesser incidence of obesity, gradients of increase are much the same around the industrialised world. Whether measured by weight alone, body mass index (BMI: kg/m2) or waist circumference, children - and most particularly girls - are crossing upwards through centiles based onUK reference curves established little more than a decade ago.

High insulin levels

The issues surrounding weight excess in children are not merely cosmetic. Excess body fat, most particularly visceral fat, is associated with insulin resistance, which leads to a progressive rise in blood sugar. The islet cells, which produce insulin, are exquisitely sensitive to changes in blood glucose, and the rise in blood sugar resulting from insulin resistance leads to an increase circulating insulin. For decades, the relationship between insulin and glucose was looked upon as one which threatened only diabetes, but the last 15 years have brought new insights which reveal just how profound are the effects of high insulin levels on a wide variety of metabolic systems. Thus, cholesterol, triglycerides, ovulation, blood pressure, viscosity and coagulability are all disturbed adversely when the blood insulin level rises. In combination, these disturbances are termed the metabolic syndrome, currently the greatest threat to health in the industrialised world, out-pacing smoking-related disorders, trauma and cancers. Metabolic syndrome is nowadays the one condition most responsible for premature death in the industrialised world, and it can be attributed very largely to obesity.

Appetite Body weight is highly regulated. At its simplest, it reflects the cumulative balance between calories consumed and calories expended. Appetite is regulated by a neuro-humeral network, centred on the hypothalamus, sometimes referred to as the 'appestat'. If intake is not matched by a corresponding change in energy expenditure, a deviation of only 2% will lead to obesity over a short number of years.

Energy expenditure is made up of three components - resting energy expenditure, which is fixed, thermic response to feeding and free-living energy expenditure which is voluntary.

Given the sophisticated mechanisms that have evolved to regulate appetite, it would be surprising if corresponding mechanisms had not evolved to regulate physical activity.

This review of published data explores the evidence for regulation of physical activity in primary school children. The data derives largely from studies of healthy children carried out in the SW of England.

The EarlyBird Study

EarlyBird is a non-intervention prospective cohort study that asks the question 'Which children develop insulin resistance, and why?' It is unique in taking serial blood samples from a young age with which to monitor, serially, the behaviour of insulin resistance and its impact on health. It focuses on the natural evolution of insulin resistance in a cohort of healthy children, monitored from school entry to the age of 16.

Its aim is to identify the process that leads some, but not others, to develop diabetes and the metabolic syndrome. There are three key issues that surround the rising threat of metabolic disease - at what stage of life does insulin resistance emerge, what is the role of life-style factors and which component of insulin resistance is genetic?

There is a prevalent view that the answers are already known - that children eat too much of the wrong food, that they undertake too little physical activity and that a poor gestational e n v i r o n m e n t programmes the genetically susceptible for a lifetime of insulin resistance.

In reality, there is little information on the development of insulin resistance in children. Innumerable cross sectional studies have been conducted on the associations between birth weight, BMI and insulin resistance. Few, however, have studied the relationships prospectively in a large cohort, and none has obtained serial blood samples from such a young age with which to investigate the impact of body composition, fat distribution, dietary habits, energy expenditure and physical activity on insulin resistance and its metabolic correlates.

Random sample

The EarlyBird Study comprises of a random sample of the 1995/1996 Plymouth birth cohort and their parents. The 54 of 71 Plymouth Primary Schools that consented were stratified into quartiles according to their proportion of free school meal entitlement, as a socio-economic proxy, and a random selection made from each accordingly. With the parents' written consent and the children's assent, a total of 307 children (137 girls, 170 boys, mean age 4.9 years) who started school between January 2000 and January 2001 became the EarlyBird cohort. The protocol has been detailed elsewhere.

Physical activity is assessed annually in the EarlyBird cohort by the use of electronic accelerometers. We chose the MTI accelerometer (MTI, Fort Walton, Florida), which samples movement 600 times a minute in the vertical plane and integrates the data into one minute epochs, storing it on a chip which can be downloaded at the end of the sampling period. The accelerometer records clock time, intensity and duration of movement.

Such accelerometers are precise, and correlate well with activity-related energy expenditure measured by room respiration calorimetry (r=c.0.70). The accelerometers record continuously throughout a seven-day period, but it is possible to identify the waking time from the first recorded activity on the accelerometer in the morning to the last recorded at night.

This report explores the patterns of physical activity of the EarlyBird children at school, at weekends, from year to year and during the 'school run'.

The Three Schools Study

An independent study within the EarlyBird Programme was carried out to assess the impact of different provision for physical education within schools.11 Two hundred and fifteen healthy children (120 boys and 95 girls aged 7.0 - 10.5y, mean 9.0 years) from three schools with widely different curricular opportunity for physical education were tested. School 1, a private preparatory school with some boarding pupils, had extensive facilities and 9.0 hours a week of physical education in the curriculum. School 2, a village school awarded Activemark gold status for its focus on physical activity, offered 2.2 hours of timetabled physical education a week. School 3, an inner city school with no particular provision, offered 1.8 hours of physical education a week. Analysis of variance was used to compare means between schools and the least significant difference p-values are quoted. The study was conducted to establish whether the provision of physical education in the curriculum makes a significant difference to a child's overall weekly activity.

Results

Variation in Physical Activity over Time

Despite substantial differences in the structure of work days compared with weekend days, adults are remarkably consistent in the amount of daily physical activity they undertake when comparing the two (WK 1.8144 v WE 1.8135 PAL units by HR monitoring, p=0.98, r=0.69 p<0.01).

Although the variance in activity among the EarlyBird children was greater at weekends, the mean activity, whether weekday or weekend day, was the same (538 v 530 x103 counts p=0.53).

More importantly, the correlation between school day and weekend day activity, which reflects consistency, was not dissimilar in the children from that seen in adults (r=0.52 for girls, r=0.51 for boys, p< 0.001). The year-on-year correlation in seven-day physical activity measured at 4.9y and again at 5.9y was as strong (r = 0.49 for girls, r = 0.55 for boys, p< 0.001).

Comparison between Locations

EarlyBird children at 5.9 years were compared with a cohort of children from Glasgow, mean age 5.8, whose physical activity over a seven-day period was recorded in an independent study using similar MTI accelerometers.13 Overall, the mean daily activity recorded in these two very different locations was identical (Glasgow: 534 v Plymouth: 534 x 103). The Glasgow girls, like those in Plymouth, were less active than the boys, and to the same degree - the gender difference was systematic.

Impact of curricular provision on physical activity

As expected, the more privileged children in school 1 (School 1) undertook more than twice as much physical activity as those in schools 2 (School 2) or 3 (School 3), suggesting that good use was made of the extra provision. After school, however, children from School 2 and School 3 undertook more than twice the activity than those from School 1. As a result, the total physical activity in the three groups was no different (Figure 1).

Further analysis suggested that there was also no difference in the intensity of physical activity between children from the three schools (p>0.1, data not shown). Thus, while there was a wide variation in total physical activity within each of the three schools, there was little variation between them, despite a substantial difference in provision. Indeed, only 3% of the variance in physical activity could be accounted for by the school environment. What children lacked in school, they made up for out of school.

Driven to school

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