Over the past four decades, there has been an unprecedented increase in the prevalence of overweight and obesity, contributing to the burden of chronic disease. The overarching aim of this thesis was to determine plausible dietary mechanisms for nutrient and nutrient combinations and weight gain, and to examine trends in the populations nutrient intakes between 1983 to 2012. A secondary aim was to focus on the socio-demographic determinants of poor dietary choices; first in the whole population and then in young adults. Secondly, trends in adult’s nutrient intakes were determined using the cross-sectional 1983, 1995 and 2011/12 dietary surveys representative of the Australian population. From a systematic review of reviews, twenty studies provided strong evidence that reducing total fat and dietary energy-density is inversely associated with body weight; moderate evidence that lowering total sugars and increasing dietary fibre is inversely associated with body weight; inconsistent evidence for alcohol and protein while total carbohydrate appears to play no direct role in body weight regulation. For trends in population macronutrients, the percentage energy from protein (%E) was 17.7%, 16.8% and 18.3% energy in 1983, 1995 and 2011/12 respectively (P<0.001). Carbohydrate (%E) increased between 1983 and both 1995 and 2012 and was 40.0%, 45.4% and 43.3% respectively (P<.0001). Fat (%E) declined between each survey and was 35.3%, 31.8%, and 30.9% respectively (P<0.001). A decline in alcohol intake was seen between 1983 and 2012 for all subpopulations. The mean dietary energy-density was 6.6 kJ/g and 6.9 kJ/g (P<0.0001) in 1995 and 2011/12. There were increases in dietary energy-density and reductions in protein and further investigation is warranted. Populations at greatest risk of poor diet quality included males, young-adults, obese, lower socio-economic status and Australian country-of-birth.