Chapter Summary

This study strengthens the findings of the original study by Stannard et al. (Stannard, Holdaway, Sachinwalla, & Cunningham, 2007), and shows that even over time, a relationship exists between adiposity, aerobic fitness and insulin sensitivity in Māori men. My results have also shown that the detrimental effects of age and reduced aerobic fitness upon ‘metabolic health’ are reduced with increased muscle mass and reduced body fat. Clearly however, further studies of this relationship in both Māori and non-Māori would make the observations of this study stronger if a larger sample size can be employed.

6

Study 2: Aerobic capacity and adiposity as

predictors of fasting insulin concentration: A

comparison between glucose tolerant Mā_{ori and}

non-Māori men.

6.1 Introduction

When compared with New Zealanders of European ancestry, Māori in New Zealand are disproportionately afflicted by diabetes and it’s precursor condition insulin resistance (David Simmons & Thompson, 2004). Physiological characteristics of an individual, such as skeletal muscle fibre type proportion (Lillioja et al., 1987a), aerobic capacity (C. R. Bruce et al., 2003), and body composition (Bogardus, Lillioja, Mott, Hollenbeck, & Reaven, 1985), are all associated with risk of insulin resistance. It is thought that these physiological variables all impact on whether there is accumulation of lipid, and subsequent impairment of insulin- mediated glucose disposal, within liver, and particularly, skeletal muscle (Seppälä-Lindroos et al., 2002; Stannard & Johnson, 2004).

On the other hand, population-based studies have identified lower standards of employment, education, and living standards as contributors to an increase in diabetes prevalence (Ministry of Health, 2004b). When taken in isolation, the latter approach suggests that Māori, who are overrepresented in the lower deciles of socioeconomic status, have a lower degree of ‘metabolic’ health because they are exposed to a lifestyle that is conducive to obesity and insulin resistance. Alternatively, it is possible that a physiological disposition for lipid

accumulation and/or reduced lipid turnover within Māori predisposes this population to greater risk of obesity and metabolic disorder.

Separating socioeconomic from physiological factors is difficult however, because any phenotype, including that which results in diabetes, develops from the interaction between inherent traits and environmental factors (Neil Pearce, Foliaki, Sporle, & Cunningham, 2004). Furthermore, comparing Māori and European participants in a physiological study can be difficult as ethnicity is a self identified trait and not necessarily a genetic or biological characteristic. On the other hand, because epidemiological data is also based on self- identification of ethnic identity, it would be expected that a comparative study of physiological factors between two different self-identified ethnic groups would produce similar trends to epidemiological data. Nevertheless, an understanding of which physiological characteristics may be associated with increased risk of developing insulin resistance and diabetes may prove useful in preventing insulin resistance and prescribing treatment.

Previous research has identified both aerobic capacity (VO2max) of the lean body mass, and

body adiposity independently, as predictors of insulin resistance in young Māori men (Stannard, Holdaway, Sachinwalla, & Cunningham, 2007). Others have also shown the significance of adiposity and associated increases in blood lipids in predicting insulin sensitivity in Māori (K. McAuley et al., 2001). Aerobic capacity is, in part, a function of the oxidative capacity of the muscle and thus also its ability to utilize (oxidise) lipids. Accordingly, oxidative capacity of skeletal muscle and resting whole body lipid oxidation rates have been shown to be significant predictors of whole body insulin sensitivity (Goodpaster, He, Watkins, & Kelley, 2001).

Regardless of the epidemiological trends previously discussed, Māori actually appear to be more physically active than New Zealanders of European Origin (NZEO) (Utter, Scragg, Schaaf, & Fitzgerald, 2006), and sporting participation rates in Māori are high (Chadwick & Palmer, 2006). Furthermore, Māori have less adiposity when compared to NZEO of a similar BMI (Swinburn, Ley, Carmichael, & Palnk, 1999), and even at a similar %BF, Māori are still more insulin resistant than NZEO (K. McAuley, Williams, Mann, Goulding, & Murphy, 2002). These data beg the question as to whether the observed relationship between physical fitness, adiposity and insulin sensitivity exist in Māori and non-Māori. Understanding these relationships better, may assist in shedding light upon the reasons why Māori are more predisposed to developing type-2 diabetes in New Zealand. To date, only one published study has shown that the relationship between aerobic capacity, body composition and insulin sensitivity exists in Māori (Stannard, Holdaway, Sachinwalla, & Cunningham, 2007). Additionally, I have shown that this relationship is consistent over time (see Chapter 5). Nevertheless, neither of these studies tried to compare these relationships between two ethnic groups.

Thus, the purpose of this study was to investigate whether the relationships observed between %BF, aerobic capacity and blood markers of insulin sensitivity, differed between Māori and non-Māori men.

6.2 Methods

6.2.1 Study Design

The methods used in this study, including the measurement of blood biochemistry, aerobic capacity and body composition, were identical to those in Study 1. However, this was a cross- sectional study rather than a repeated measures study, including a non-Māori cohort for comparative purposes.

6.2.2 Participants

Forty eight healthy men (31 Māori, 17 non-Māori) aged 28 ± 5 (mean ± SD) years old. All participants answered a health questionnaire (see chapter 5) and had no cardiovascular, respiratory or metabolic disorders. All participants were randomly selected from employment, university or church networks.

Participants were asked to self-identify their ethnicity. All those who were included as Māori identified as such, while in the non-Māori group all but one participant (Asian) identified as European. Pacific Islanders were excluded from all studies in this body of research.

Table 6-1 – Participant physical characteristics Entire Group n = 48 Mā_ori n = 31 Non-Mā_ori n = 17

Mean SD Mean SD Mean SD

Age (yrs) 27.9 5.40 28.5 5.5 26.7 5.1

Height (m) 1.80 0.07 1.80 0.07 1.79 0.07

Weight (kg) 87.8 17.2 93.7 17.4 77.0 10.4

BMI (kg/m2) 27.1 4.7 28.8 4.8 24.1 2.7

6.2.3 Statistical Analyses

Independent samples t-tests were used to identify any significant differences in the variables measured, between the Māori and non-Māori cohort. Simultaneous multiple regression was performed to ascertain whether the dependant variables relating to insulin sensitivity (HOMA- IR, Fast-Ins, 2hr-Ins, Fast-Gluc, 2hr-Gluc), could be significantly predicted by the independent variables (%BF, VO2maxLBM). Ethnicity was also included as an independent

variable, to determine the effect of ethnicity on these relationships. SPSS version 10 was used for all statistical analyses.

6.3 Results

Although 61 participants took part in this study (39 Māori, 22 non-Māori), I was unable to collect accurate VO2max measures for 13 of these subjects (8 Māori and 5 non-Māori) so data

6.3.1 Body Composition

The Māori group had a significantly greater mean weight (93.7kg Māori, 77.0kg non-Māori), BMI, %BF and LBM than the non-Māori group (p<0.01). Neither age nor height were significantly different between groups (p>0.05).

Table 6-2 – Body composition (percent body fat and total lean body mass)

Entire Group Māori Non-Māori

Mean SD Mean SD Mean SD

%BF 19.7 7.0 21.5 7.4 16.5 5.0

LBM (kg) 69.3 9.3 72.4 9.2 63.8 6.5

6.3.2 Aerobic capacity

The non-Māori group had a significantly greater VO2maxLBM than the Māori group (60.8

L/min/kg l.b.m and 53.0 L/min/kg l.b.m: p<0.05). The non-Māori group also had a higher absolute VO2max than the Māori (3853 ml/min and 3805 ml/min), though this was not

significant (p>0.05).

Table 6-3 – Aerobic capacity (absolute and relative to lean body mass)

Entire Group Māori Non-Māori

Mean SD Mean SD Mean SD

VO2max (ml) 3822 507 3805 505 3853 525

VO2maxLBM

(ml.kgLBM -1.min-1)

6.3.3 Blood Biochemistry

Eight of the 48 participants did not receive 2hr insulin blood samples, and 5 did not receive 2hr glucose samples. None of the blood markers measured were significantly different between the Māori and non-Māori group (p>0.05).

Table 6-4 – Blood Biochemistry

Entire Group Māori Non-Māori

Mean SD n Mean SD n Mean SD n

HOMA-IR 1.67 1.28 48 1.88 1.32 31 1.46 1.23 17

Fast Ins (pmol.L-1) 55 38.7 48 60 39.3 31 47 37.2 17

Fast Gluc (mmol.L-1) 4.8 0.4 48 4.8 0.5 31 4.8 0.2 17

*2hr Ins (pmol.L-1) 143 141.3 40* 161 165.0 26* 110 75.5 14*

*2hr Gluc (mmol.L-1) 4.3 1.1 43* 4.4 1.1 27* 4.0 1.0 16*

* 8 participants did not have 2hr insulin samples, 5 did not have 2hr glucose