Development of body composition and its relationship with physical activity in healthy Swedish children A longitudinal study until 4.5 years of age including evaluation of methods to assess physical activity and energy intake

Sammanfattning: Childhood obesity according to the World Health Organization is one of the most serious public health challenges of the 21st century. The proportion of childhood obesity is high both globally and in Sweden. This is of great concern since obese children tend to stay obese in adulthood. In order to develop strategies to prevent early childhood obesity more knowledge is needed regarding factors explaining why children become overweight and obese. Preventive strategies require accurate and easy-to-use methods to assess physical activity in response to energy expenditure as well as energy intake in young children, but such methods are largely lacking or have shown limited accuracy. The aims of this thesis were: 1) to describe the longitudinal development of body composition from 1 week to 4.5 years of age; 2) to study relationships between measures of body composition and the physical activity level (PAL) at 1.5 and 3 years of age; 3) to evaluate if heart rate recording and movement registration using Actiheart can capture variations in total energy expenditure (TEE) and activity energy expenditure (AEE) at 1.5 and 3 years; 4) to evaluate the potential of a 7-day activity diary to assess PAL at 1.5 and 3 years of age; 5) to evaluate a new tool (TECH) using mobile phones for assessing energy intake at 3 years of age.Healthy children were investigated at 1 and 12 weeks (n=44), at 1.5 (n=44), 3 (n=33) and 4.5 (n=26) years of age. Body composition was measured using air-displacement plethysmography at 1 and 12 weeks and at 4.5 years of age. At 1.5 and 3 years, body composition, TEE, PAL and AEE were assessed using the doubly labelled water method and indirect calorimetry. Heart rate and movements were recorded using Actiheart (four days) and physical activities were registered using the 7-day diary. Energy intake was assessed using TECH during one complete 24-hour period.Average percentage of total body fat (TBF) and average fat mass index (FMI) were higher (+3 to +81 %), while fat-free mass index (FFMI) was slightly lower (-2 to -9 %), in children in the study from 12 weeks until 4.5 years of age when compared to corresponding reference values. A relationship between TBF% and PAL was found both at 1.5 and 3 years of age. At 3 years, but not at 1.5 years, this could be explained by a relationship between PAL and FFMI. Actiheart recordings explained a significant but small fraction (8%) of the variation in free-living TEE at 1.5 and 3 years, and in AEE (6 %) at 3 years, above that explained by body composition variables. At 1.5 and 3 years of age, PAL estimated by means of the activity diary using metabolic equivalent (MET) values by Ainsworth et al. was not significantly different from reference PAL, but the accuracy for individuals was low. Average energy intake assessed by TECH was not significantly different from TEE. However, the accuracy for individuals was poor.The results of this thesis suggest that 1) The higher body fatness of the children in the study compared to the corresponding reference values may indicate the presence of a secular trend in body composition development characterized by a high body fatness. 2) Body fatness might counteract physical activity at 1.5 years of age when the capacity to perform physical activity is limited, but not at 3 years of age when such a capacity has been developed. 3) Actiheart recordings explained a significant but small fraction of the variation in TEE at 1.5 and 3 years, and in AEE at 3 years of age, above that explained by body composition variables. 4) The activity diary and TECH produced mean values in agreement with reference PAL and TEE, respectively, but the accuracy for individual children was low.In conclusion, the results of this thesis suggest the presence of a secular trend in body composition development in healthy Swedish children, from infancy up to 4.5 years of age, which is characterized by a high body fatness. Methods to assess physical activity and energy intake at 1.5 and 3 years of age provided some promising results on a group level, although further research is needed to increase the accuracy of these methods in individual children.