Lifestyle and body composition in adolescents in the South Pacific

Abstract

Obesity prevalence among Pacific Island adolescents is high and is a major health concern. In view of this, obesity interventions targeted at this group are urgently required. However, evidence of suitable intervention strategies for this population is lacking. In addition, it is not clear how well measures of obesity that can be used in the field setting quantify fatness in South Pacific youth. To address these gaps of knowledge in the literature, this thesis aimed to examine the association between lifestyle risk factors of obesity and body composition variables in this population (Obesity Prevention In Communities (OPIC) study). To assist this objective, a second aim of this thesis was to examine the relationship between fatness estimated by field methods (anthropometric and bioimpedance analysis (BIA) variables) and measured by an accepted reference method in South Pacific youth (validation study). In the validation study, 432 adolescents (Pacific Island, Maori, Asian and European) were purposively selected from high schools in Auckland. Anthropometric variables were measured, impedance variables were measured on an 8-electrode BIA (BIA₈) device (Tanita BC-418; Tanita, Tokyo, Japan) and body composition variables were measured by dual-energy X-ray absorptiometry (DXA). In the OPIC study, data were collected from >17,000 students from 4 countries (New Zealand (NZ), Australia, Fiji and Tonga) and 8 ethnic groups (NZ Pacific Island, NZ Maori, NZ Asian, NZ European, Indigenous Fijian, Fijian Indian and Tongan), which comprised information on demographic, lifestyle and body composition variables. Results from the validation study showed that, compared to Europeans, for the same body mass index (BMI), Asian Indians had more percent body fat (%BF), while Maori and Pacific Islanders had less %BF. In boys, readily measured variables, waist circumference/height and conicity index, had notable effects on the %BF ethnic differences. Other factors that contributed to these differences in boys and girls were variation in muscularity, bone mass, fat distribution and relative leg length. BIA₈ estimated DXA-measured total fat mass (TFM), %BF and fat-free mass with significant bias. BIA-based prediction equations developed in the sample performed better than reliance on the manufacturer's equations and these equations depended upon ethnicity. For the same waist circumference (WC), compared to Europeans and Maori, Asians had more percent abdominal fat (%AbFM) and Pacific Islanders had less %AbFM. Adjustment for trunk impedance (ZTr) removed or reduced these %AbFM differences. In fact, at a given WC, ethnic differences in ZTr mirrored variation in %AbFM across ethnic groups. OPIC analyses revealed that Pacific Islanders had markedly higher fatness levels than other groups, including when %BF, TFM and %AbFM were used as fatness measures. Among all ethnic groups combined, TV watching was positively related to fatness in a dose-dependent manner. Overall effects showed strong, dose-dependent associations between fatness and soft drink consumption (positive relationship), breakfast consumption (inverse relationship) and after-school physical activity (inverse relationship). Differences in lifestyle obesity risk factors were associated with percentage differences in body composition variables that were greatest for TFM, followed by %BF and then BMI. This thesis supports the view that TV watching, soft drink consumption, breakfast consumption and physical inactivity contribute to increased obesity prevalence among Pacific Island youth. Body composition (DXA-measured fatness) notably varies at a given body size. Consistent with this, and an original finding, is that lifestyle factors are most strongly related to TFM and %BF, suggesting that obesity interventions and studies that use only BMI to quantify fatness may underestimate the "true" effect of lifestyle on adiposity.