Abstract
Chronic diseases such as cancer, diabetes, and cardiovascular diseases (CVD) are the main health concerns of the 21st century, with CVD as the number one cause of mortality in New Zealand and worldwide. Although CVD hard endpoints such as stroke or heart attacks do not usually occur in children, there is evidence that the manifestation of CVD risk factors begins in childhood, preceding clinical complications of CVD in adulthood. Several factors including biological, environmental, and behavioural factors are associated with the development and advancement of CVD complications. Of these, dietary intake is a modifiable risk factor that has been shown to make a substantial contribution to the risk of death from CVD. Health professionals have long recognised the importance of diets high in fruits and vegetables, wholegrain/high fibre bread and cereals, and limited intakes of sugar and sugar-sweetened beverages in reducing the risk of CVD in adults. However, there is a lack of research in the paediatric population.
Thus, the aim of this thesis was to determine associations between dietary intake, particularly dietary patterns as a more global approach of assessing dietary intake and markers of cardiovascular health in 9-11 year-old children in Dunedin, New Zealand. The study was conducted in two phases. In the first phase, a short (28-item) non-quantitative food frequency questionnaire (FFQ) was developed and assessed for its reproducibility and relative validity. Fifty children (mean age±SD: 9.40±0.49 years old) from three Dunedin primary schools completed the FFQ twice, as well as a four-day estimated food diary (4DEFD) over a two-week period. Intraclass correlation coefficients (ICC) and Spearman’s correlation coefficients (SCC) were used to determine the reproducibility and relative validity of the FFQ, respectively. More than half of the food items/groups (52.2%) had an ICC ≥0.50. In relative validity analyses, 70% of food items/groups had a SCC ≥0.30. This FFQ has been used to rank children according to the frequency of consumption of specified food items/groups. The low respondent burden and relative simplicity of the FFQ make it suitable for use in large cohort studies in New Zealand children with similar characteristics.
The second phase of the thesis used data from the ‘Physical activity, Exercise, Diet, And Lifestyle Study’ (PEDALS), conducted in 17 primary schools in Dunedin. Of the children who took part in PEDALS, the mean age±SD was 9.72±0.68 years old, 76% were of normal weight, 80% met the guidelines of 60 minutes of daily moderate-vigorous physical activity, and 99% were categorised as fit based on the FITNESSGRAM standards.
The first objective of phase II was to identify dietary patterns using principal component analysis (PCA), using the FFQ validated in phase I. Two dietary patterns, namely ‘Snacks’ and ‘Fruit & Vegetables’ were identified. The mean ‘Snacks’ and ‘Fruit & Vegetables’ scores were -0.068±1.98 and -0.005±1.83, respectively. The two identified dietary patterns in PEDALS were similar to commonly identified dietary patterns in both international and national studies. The second objective of phase II was to determine associations between the two identified dietary patterns and components of physical fitness (i.e., cardiorespiratory fitness and handgrip strength). Cardiorespiratory fitness was measured as mean relative V ̇O2max obtained from a 20-metre shuttle run test (20msrt). A digital hand dynamometer was used to measure handgrip strength of both the dominant and non-dominant hands. Complete data was available for 398 participants. Mixed effects linear regression models with robust standard errors and school as a random effect were employed to assess relationships between dietary patterns and components of physical fitness.
Mean relative V ̇O2max was 48.7±4.75 ml/kg/min. Handgrip strength of the dominant and non-dominant hand was 15.2±3.29 and 14.4±3.17 kg, respectively. There were no significant associations between the dietary pattern scores and cardiorespiratory fitness. However, fat mass index (FMI) was independently associated with cardiorespiratory fitness. Excess body fat is associated with poorer performance and consequently lower estimated V ̇O2max (ml/kg/min). Furthermore, PEDALS did not find clinically meaningful associations between dietary patterns and handgrip strength of the dominant or non-dominant hand, while sex and fat-free mass index were independent determinants of handgrip strength. Considering the important impact of muscular strength on current and future health status, sex-specific exercise training to improve children’s fat-free mass and muscular strength from as young as 9 years old should be promoted.
The third objective of phase II was to investigate relationships between dietary patterns and indices of arterial stiffness (i.e., augmentation index (AIx) and pulse wave velocity (PWV)). Indices of arterial stiffness were assessed using the XCEL system. Data for AIx and PWV analyses were available for 337 and 389 participants, respectively. Mixed effects linear regression models were used to assess associations between dietary patterns and indices of arterial stiffness. Mean AIx and PWV were -2.14±14.1% and 5.78±0.79 m/s, respectively. There were no clinically significant relationships between the dietary pattern scores and AIx and PWV. Arterial stiffness is one of the earliest detectable measures of vascular damage and can be seen in the first decade of life. Although evidence has shown that obesity can accelerate the age-associated arterial stiffening process, the majority (76%) of PEDALS children were normal weight, which may explain the lack of an association.
Overall, there were no significant associations between dietary patterns and markers of cardiovascular health in children who took part in PEDALS. The majority of the PEDALS population had a healthy weight status, were of New Zealand European ethnicity, and from families of middle/high socio-economic status. Further research is suggested in a cohort of 9-11 year-old children from families of low socio-economic status and minority ethnic groups such as Māori and Pacific children, using the established methodology of PEDALS. Comparison of the results from PEDALS with a similar study on a group of children with different socio-demographic characteristics would be useful to inform policy and provide further insights on the importance of designing appropriate prevention strategies in both the general and high-risk paediatric populations.