Effect of manipulations in exercise and breakfast on metabolism in overweight and non-overweight children and adolescents

Obesity and insulin resistance are serious health concerns in children and adolescents (young people). Interventions to increase the potential for fat oxidation and improve insulin sensitivity could have widespread clinical relevance. Although exercise is often advocated for health, the factors implicated in the relationship between exercise, fat oxidation and insulin resistance are not well understood in young people. This thesis has investigated the effect of manipulations in exercise and breakfast on metabolism in young people, focusing on fat oxidation and postprandial blood glucose control. The first experimental study, Chapter 4, compared two different exercise protocols for estimating the intensity corresponding to maximal fat oxidation (Fatmax) in non-overweight prepubertal children. A 3 min incremental protocol was recommended to provide an estimation of Fatmax using a wide range of intensities in this population. Using this protocol, Chapter 5 demonstrated that Fatmax was higher for treadmill compared with cycling exercise in pre- to early pubertal children. Furthermore, treadmill exercise resulted in higher rates of fat oxidation over a range of absolute and relative intensities and fat oxidation remained high over a wider range of intensities. Therefore, treadmill exercise (walking or slow running) is clearly preferential for promoting fat oxidation in this population. Subsequently, Chapter 6 examined the effect of mixed breakfast meals containing high (HGI) and low (LGI) glycaemic index carbohydrates on blood glucose, plasma insulin and fat oxidation in overweight and non-overweight girls. Breakfast GI did not affect fat oxidation during the postprandial rest period or subsequent exercise. However, the main finding of this study related to blood glucose; the higher blood glucose response following the HGI compared with LGI breakfast was more pronounced in the overweight girls. This suggested a reduced ability to cope with the metabolic demands of HGI breakfast consumption in overweight girls and highlighted that strategies to reduce insulin resistance in this population are required. Consequently, Chapter 7 investigated the effect of treadmill exercise at Fatmax performed 16 h prior to HGI breakfast consumption on blood glucose, plasma insulin and fat oxidation in overweight and non-overweight girls. Fatmax exercise reduced the postprandial insulin response in the non-overweight, but not the overweight, girls while blood glucose was unchanged in both groups. More encouragingly, fat oxidation was increased after exercise in both the overweight and non-overweight girls. Collectively, the four experimental studies within this thesis have demonstrated that treadmill exercise at Fatmax is an effective means of elevating fat oxidation both during and up to 16 h after exercise. When considering postprandial glucose and insulin responses to HGI breakfast consumption, LGI breakfasts should be recommended for overweight girls, whilst acute treadmill exercise at Fatmax can reduce postprandial insulin concentrations in non-overweight girls. Walking or slow running (Fatmax treadmill exercise) and LGI breakfast consumption may be best advocated in combination for promoting fat oxidation and improving postprandial blood glucose control in young people. These two simple lifestyle-related strategies may provide an effective, safe and attractive means for preventing and treating obesity, insulin resistance and related disorders. Key words: exercise, metabolism, substrate oxidation, fat oxidation, glucose, insulin, glycaemic index, overweight, children, adolescents

Carbohydrate needs of the young athlete

Carbohydrate (CHO) typically provides the majority of energy in the athlete’s diet and is essential to fuel high intensity exercise. Ensuring adequate energy is available to meet the demands of high energy expenditures is important in the young athlete to ensure proper growth, development, and maturation. Physiological and metabolic changes that accompany the transition from childhood to adolescence and to adulthood, combined with the additional energy expenditure arising from exercise, mean that the dietary needs of young athletes require special consideration. However, in contrast to the well-documented literature in adults, little research attention has been given to child and adolescent populations. Thus, the development of specific recommendations for CHO intake in young athletes is difficult. Nevertheless, it is possible to make some general recommendations. Both the total daily CHO intake and the timing of CHO consumption in relation to exercise can determine whether adequate CHO substrate is available for muscles and the central nervous system or whether CHO fuel sources might limit exercise performance. In terms of the overall diet, CHO should contribute to the majority of energy intake, which must be high enough to support growth and maturation whilst fuelling the additional physical activity, and consequently elevated energy expenditure, in young athletes. In particular, CHO is an important fuel for high intensity exercise in young athletes. Decrements in exercise performance, fatigue and changes in body composition may serve as useful indicators that CHO intake may not be adequate, particularly in female adolescent athletes. During exercise, drinks containing CHO could be considered for young athletes engaged in endurance exercise due to the preferential use of exogenous CHO in younger athletes in the pre- or early- pubertal stages. However, evidence on CHO loading and CHO for post-exercise recovery does not appear to be available in children or adolescents. This chapter provides an overview of the available evidence that can be used to inform recommendations for CHO intake and timing in young athletes. Where no direct evidence in young athletes is available, we have relied on the relevant adult-based literature whilst emphasising that the direct translation and application of these findings to children and adolescents must be viewed cautiously

Metabolism and exercise during youth — The year that was 2017

Two publications were selected because they are excellent representations of studies examining different ends of the exercise-sedentary behavior continuum in young people. The first study is an acute response study with 13 mixed-sex, mid to late adolescents presenting complete data from 4 different randomized experimental crossover conditions for analyses. Continuous glucose monitoring showed that interrupting prolonged continuous sitting with body-weight resistance exercises reduced the postprandial glucose concentration compared with a time-matched uninterrupted period of sitting. Furthermore, the effects of the breaks in sitting time were independent of the energy content of the standardized meals, but variations in the area under the glucose time curves expression were important. The second study adopted a chronic 12-week exercise training intervention design with a large sample of obese children and adolescents who were allocated randomly to high-intensity interval training (HIIT), moderate-intensity continuous training, or nutritional advice groups. HIIT was the most efficacious for improving cardiorespiratory fitness compared with the other interventions; however, cardiometabolic biomarkers and visceral/subcutaneous adipose tissue did not change meaningfully in any group over the 12 weeks. Attrition rates from both HIIT and moderate-intensity continuous training groups reduce the validity of the exercise training comparison, yet this still provides a solid platform for future research comparisons using HIIT in young people