CROSSROADS
The required sample size was calculated based on a cross-sectional study investigating the relationship between dietary GI/GL and metabolic health (O‟Sullivan et al, 2010). The calculation was based on logistic regression analyses that revealed a significantly (p =0.03) increased likelihood of having the metabolic syndrome per unit increase in dietary GI (odds ratio: 1.89 95% CI; 1.06-3.36) and GL (odds ratio: 1.62 95% CI; 1.05-2.49). The calculation estimated that a minimum of 137 (based on the smaller effect size observed for GL) participants are required in order to reach sufficient statistical power in logistic regression analysis when assessing the relationship between dietary GI/GL and metabolic health (alpha set at 0.05, power of 0.85).
SIRENS
Sample size was calculated, based on a previous paper (Kadoglou et al. 2007) in which a significant decrease (P =0.023) in HOMA-IR (primary outcome measure) was found following treatment (aerobic exercise training) compared to a control group. To produce a significant effect size on HOMA-IR between treatment and non-treatment groups, a minimum sample size of 3 in each group is necessary
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(alpha set at 0.05, power of 0.95). Moreover, a further power calculation was performed to determine the sample size required to detect a significant difference between two treatment types (e.g. aerobic exercise vs. diet), a sample size of 15 in each group was calculated (alpha set at 0.05, power of 0.80). Thus the required target sample size for the SIRENS study was 25 participants per group in order to achieve a minimum of 15 participants in each group (allowing for a 40% drop out rate).
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Chapter Four: Study One
Physical activity levels and Nutritional intake of postpubertal
Bedfordshire adolescents: associations with adiposity
4.0 Introduction
A healthy lifestyle can be characterised by one which consists of adequate PA and a „healthy‟ diet, both of which have been positively associated with a lower incidence of overweight and obesity (Wareham et al., 2005). Obesity and particularly centrally located adiposity has been associated with a cluster of metabolic disorders associated with type II diabetes, atherogenic dyslipidemia and subsequent CVD (Despres and Lemieux, 2006).
Diet and Adiposity
Energy dense foods are considered a primary determinant of obesity and related diseases such as CVD (Phillips et al., 2010) and metabolic complications (Druet et al., 2007). Energy restriction has fundamentally been used to reduce obesity (Abete et al., 2011) but manipulation of macronutrient distribution has also been associated with weight reduction (Muzio et al, 2007). Traditionally, high fat diets were associated with increased adiposity and thus treatment of overweight and obesity was facilitated through lowering fat intake (Abete et al., 2010) which was subsequently compensated for by increasing CHO in place of fat. Increased dietary fat has been positively associated with body weight in large cross-sectional studies of males and females (Satia-About, 2002; Park et al., 2005) as well as in youth populations (Tucker et al., 1997; Ortega et al., 1995). This „low fat focused‟ approach proved successful at reducing energy density and weight loss for a short period; however, these diets appeared to have poor adherence over a longer period due to a lack of satiety which is now associated with low fat diets (Atrup, 2008) and some studies have shown weight re-gains after 18 months of dieting (Summerbell et al., 2008). Since 1976, as dietary fat consumption has declined, rates of obesity and its co-morbidities have continued to rise (Weinburg, 2004). It thus appears that low fat, high CHO diets may not be the most appropriate technique for targeting adiposity. Over more recent years evidence has emerged that the composition of CHOs can alter the speed at which they are metabolised and this can have implications for satiety and fat metabolism leading to weight gain (Ludwig, 2000, Du
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et al., 2006). To this end, research has assessed the associations of CHO consumption with adiposity, in terms of the speed at which CHOs are absorbed into the blood stream; ranked by the GI and GL. The GI classifies CHO containing foods according to their impact on the body‟s postprandial glycaemic response (Du et al., 2006), furthermore, as the quantity and not just the quality (GI) of CHO will impact on postprandial glucose levels the GL of the diet has also been considered. It has been evidenced that GI is positively associated with adiposity (Brand-Miller et al., 2002), in adults; prospective cohort studies of adults have shown BF% and WC significantly increase in relation to dietary GI (Du et al., 2009; Hare-Bruun et al., 2006). Furthermore, GI has been positively associated with BMI in 1354 Japanese females (20-78 yrs) (Murakami et al., 2006).
There is limited evidence supporting positive associations of GI and GL in youngsters; in normal weight children (Scaglioni et al., 2005; Buyken et a.l, 2008) and overweight youths (Hui and Nelson 2006) higher glycaemic CHO was not associated with fatness. Furthermore, Joslowski et al (2011) observed that postprandial insulinemia was associated with adiposity but that GI and GL shared no association in 262 9-15 year olds. However, in 486 children and adolescents from Denmark, both GI and GL were associated with body fatness, in 16 year old males but not children or females; the lack of association in girls was purported to be related to the increased reporting bias observed in the young females of this study Nielsen et al (2005). These findings (Neilsen et al., 2005) appear to be the only to report association of GI and GL with adiposity in a potentially entirely postpubertal group (16 year old males; n = 181), however, pubertal status was not assessed. One study has assessed associations of glycaemic CHO and adiposity in (818) British children (4-10 yr olds) and adolescents (11-18 yr olds) based on data from The National Diet and Nutrition Survey (NDNS) (Murakami et al., 2013). The authors observed that increasing GL was independently associated with increased risk of overweight (BMI) in children (P= 0.04) and central obesity (assessed by waist to height ratio) in adolescents (P= 0.02) but GI was not associated with adiposity (Murakami et al., 2013). Although this recent publication is a relatively large representative sample of UK youths, the NDNS data analysed is not current; data were collected in 1997 and therefore are unlikely to represent current glycaemic CHO consumption. A further limitation is that this investigation utilised an age range of adolescents (11-18 yrs) that is likely to encompass individuals of varying pubertal status and thus findings may be confounded by puberty (Moran et al., 1999).
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During puberty hormonal changes are responsible for altered substrate metabolism and a transient increase in insulin resistance which return to pre-pubertal levels in the last stage of maturational growth (Staiano and Katzmarzyk, 2012, Moran et al., 1999); it is understood that these factors can influence metabolic health parameters (Hannon et al., 2006). Furthermore, during puberty, fat mass, including visceral fat has been shown to increase in males and females, although, females are more susceptible to increased adiposity (Staiano and Katzmarzyk, 2012). Therefore, puberty may confound associations of GI and GL with adiposity.
In summary, there is limited evidence of an association between glycaemic CHO and adiposity in youths. However, studies of children and adolescents tend to assess participants across a broad age range which encompasses pre-pubertal, pubertal and post-pubertal individuals. Therefore, it will be of benefit to assess these associations in a solely postpubertal adolescent population in an attempt to negate these confounding factors. The few studies investigating associations of adiposity with GI and GL in adolescents have only assessed adiposity according to BF% and BMI and not assessed associations with WC. Adolescents are at a stage in their lives where they are making more autonomous lifestyle choices regarding their eating behaviours (Ebbeling et al., 2003) and thus an understanding of their dietary GI and GL and the associations they share with adiposity important. However, little is known of the dietary GI and GL of adolescents from the UK. One reason for this could be that the lack of published GI data on foods commonly consumed in the UK and many of the food brands published in GI tables are not accessible in the UK (Aston et al., 2008). Currently the most comprehensive data are based upon analysis conducted in Australia and the USA and thus the application of dietary GI and GL in a health context is little understood in UK adolescents (Aston et al., 2008).
Physical Activity, CRF and Adiposity
It is well established that time spent being physically active shares an important association with obesity and associated co-morbidities (Wareham et al., 2005). Physical inactivity, or being sedentary may also play an equally important role in the development of adiposity (Healy et al., 2011). A cross-sectional study of European adults showed that being physically inactive (time spent sitting) shared a dose response relationship with obesity as assessed by BMI (Martínez-González et al.,
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1999). Two studies, one of healthy American and one of diabetic Australian adults, provide evidence in adults that objectively assessed SED time may be a more important determinant of obesity than MVPA. All physical activity categories (negatively) including sedentary (positively) were associated with an increased WC (Healy et al., 2008b, Healy et al., 2011). SED time was shown to be independently associated with WC regardless of time in MVPA, however after adjustment for SED time MVPA was no longer associated with WC. Additionally, (Healy et al., 2008a) evidenced that breaks in SED time were associated with a lower WC suggesting that a transition from SED to LPA may be a useful strategy for weight reduction. In children and adolescents PA engagement is negatively associated with adiposity (Deforche et al., 2003, Ekelund et al., 2012); however, compared to adults there is contrasting evidence in respect to which activity category is most important for adiposity. A study of school children identified that obese pupils, compared to non- obese, engaged in a similar amount of PA in leisure time (LTPA) but took part in significantly less higher intensity sporting activities (Deforche et al., 2003), although PA was assessed by questionnaire. In a study of 9-10 year old British males, SED time was associated with an increased WC and fat mass (as in adults), however, associations were attenuated after adjustment for time in MVPA (Steele et al., 2009). Furthermore, total PA and MVPA, were inversely associated with adiposity following adjustment for SED time (Steele et al., 2009). Similarly, MVPA was associated with WC independent of time spent SED in a large study of pooled data (n = 20871) in 4-18 year olds (Ekelund et al., 2012). As part of living a more independent lifestyle, adolescents are also making important decisions about their PA and exercise engagement. As children enter their adolescent years PA engagement begins to decline (Kimm et al., 2002) and a concomitant increase in weight status has been observed (Kimm et al., 2005). This may have important health implications as physical inactivity (measured by self assessed questionnaire) during adolescence (16-18 years old) has been shown to independently predict total and abdominal obesity levels in adulthood (25 years old) (Pietiläinen et al., 2008). In light of the evidence that MVPA is an important determinant of fatness and related health outcomes, the UK government recommend that children and adolescents should engage in >60 minutes of MVPA per day (DOH, 2011).
In addition to PA, CRF is an important factor associated with obesity and metabolic aberrations (Ekelund et al., 2007b). In a study of 366 SED males, those with moderate CRF had a lower average total fat mass, WC and visceral fat than those
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categorised in to the low CRF group (Janssen et al., 2004). Similarly, in a sample of 3719 male and 3854 female adults (20-60 yrs), those with high CRF levels, demonstrated a lower WC and total fat mass compared to those with low CRF (Ross and Katzmarzyk, 2003)
In 1045 children aged 6-13 total and abdominal body fat (assessed by the skinfold method) were lower in those with a high, compared to low CRF (estimated via 20 metre shuttle run) (Nassis et al., 2005). In a large study of Spanish adolescents (13- 19 year olds), BMI and WC were inversely associated with CRF (20 metre shuttle run) and positively with SED activities (assessed by self report questionnaire); however, no relationship was seen between LTPA and adiposity variables (Ortega et al., 2007). CRF (20 metre shuttle run) and PA were objectively assessed (RT3 accelerometer) in 7-10 year olds from Ireland; in girls fitness was also the only factor associated with body composition (inversely). In boys however body composition was negatively associated with both fitness and VPA (Hussey et al., 2007). Very few studies have assessed the association of adiposity with PA and CRF in children and adolescents and there is a distinct lack of evidence for objectively assessed PA and directly measured CRF (VO2 uptake). Furthermore
very few studies have assessed these relationships whilst controlling for dietary variables.
Therefore the following study investigated the current: 1) dietary GI and GL intakes; 2) objectively assessed physical activity levels; 3) directly measured CRF levels, of postpubertal adolescents from Bedfordshire. Due to the implications of overweight and obesity on cardio-metabolic health, the associations of dietary GI and GL, CRF and PA with adiposity were assessed in this group.