The measurement and influence of body composition phenotype in ageing: an analysis of health -related changes in Irish adults

Ageing is a process associated with adverse effects on nearly every facet of human body composition and is generally accompanied by progressive loss in lean mass (sarcopenia) and bone mineral density (BMD) (osteoporosis) and concomitant increase in total and visceral adiposity (fat obesity). However, body mass index (BMI), the metric commonly used to quantify changes in body composition, does not adequately represent these health-related changes. The technological advance in bone and soft-tissue imaging offered by dual energy X-ray absorptiometry (DXA) provides an accepted criterion measure of body composition into three components (adiposity, lean mass, BMD). Using DXA as the reference method of measurement, this thesis reports a cross-sectional characterisation of body composition in a convenience sample of 1,606 Irish adult men and women age 18-81 years. Interrogation of these data afforded the opportunity to challenge the validity and sensitivity of metrics such as BMI, redefine the criterion reference of adiposity and monitor age-related change and interrelationships between components of body composition. The first study recommends measurement of a body fat mass index (BFMI) for accurate classification of adiposity that is independent of change in fat-free mass. An age-, BMI- and gender-specific reference equation is generated that allows accurate estimation (R2=0.9, SEE=1.1%) of BFMI for use clinically. Additionally, reference ranges of BFMI based on a young adult Z-score and centiles are presented, offering classification of the individual based on adiposity. The second study tracks the age-related change in body composition from the median young adult (18-29y), to middle age (30-49y) and older age (≥50y), observing an increase in BFMI, coinciding with a re-distribution of adiposity from subcutaneous to visceral compartments. A decline in lean mass and BMD was observed to begin at age 30 years in men and women in this cohort. Examining the interrelationships between these parameters, the third study investigates the association between adiposity (BFMI and visceral) on BMD. Adiposity was shown to negatively influence BMD (p<0.05) to a greater extent at the whole body vs. site-specific regions and in younger vs. older adults. Taking a combination of approaches, this thesis defines the criteria that describe ageing in an Irish cohort, and tracks the body compositional changes that may be detrimental to health. These results should form a basis to devise methods that offset the decline in composition that leads to frailty, disability, disease and loss of independence in the elderly, and promote health and functional status for longer.