Associations of patterns of daily life, physical fitness and body composition of primary school age children

Abstract

The daily patterns of life, for example, food, physical activity and inactivity and sleep as well as physical fitness are associated with the accumulation of excess body fat in children. A positive energy balance between food (energy intake) and metabolism, particularly physical activity (energy output) is the accepted explanation. The reality of daily life for children is excessive calorie consumption, imbalances in macronutrient intake and missed opportunities for activity are being driven by the physical and social environment. Excess body fat tracks into adulthood and is associated with decreased insulin sensitivity, which may lead to increased risk of insulin resistance and chronic disease. Resting metabolic rate and substrate utilisation (measured by respiratory exchange ratio) are implicated in the prediction of weight gain in adults, but these relationships have been rarely explored in children. Both insulin and respiratory exchange provide insights into the pathways of accumulation of body fat. The purpose of this body of work presented here was to explore and explain how lifestyle patterns, substrate metabolism, physical fitness attributes and insulin resistance are related to excess body fat accumulation in children. “Project Energize” is a Waikato District Health Board-initiated through-school nutrition and physical activity intervention, operated in a growing number of primary schools throughout the Waikato. Data was collected from Project Energize control and programme schools between 2004 and 2006. The children that participated in this body of work were aged between 5 and 12 years of age (20% Māori). Fat mass (FM) change in Hamilton and Waikato primary school age children over a two year period is reported in study one (n=618). Study two (n=69) and three (n=169) are cross-sectional sub-studies of low decile schools (<3) that employ two methods of metabolic assessment; indirect calorimetry and glucose homeostasis; to investigate the relationships between food, activity, fitness with body composition and metabolic risk. More than 70% of the increase in body mass index (BMI) and percentage body fat (PBF) could be explained by the same measures two years earlier and more than 10% of the reported food, activity and sleep behaviours were able to be predicted from the responses two years earlier. There were no clear associations found with resting respiratory exchange ratio (RER), but resting metabolic rate (RMR) was best explained (45%) by fat free mass with a further 3% explained by cardiorespiratory fitness. Children with longer legs (as represented by height) travelled further, but FM attenuated final speed. Children with more fat had higher insulin resistance. Physical fitness was not associated with insulin resistance. Overall, a pattern of increased FM was linked to: 1. FM two years previously 2. a lesser speed attained in the 20m Shuttle Run Test and 3. higher insulin resistance. A focus on weight gain rather than change in FM and FFM, fitness and metabolic markers as the outcome of interventions is unlikely to show short or medium term changes. Therefore it is recommended that when instigating school-based nutrition and physical activity programmes, there must also be a focus on the daily patterns of life alongside community, family and culture-based partnerships to support sustainable behavioural change.