On a population basis, young adult women have a high rate of weight gain. While adult weight gain has many well-known health implications, there are additional consequences for young women in terms of their reproductive health, pregnancy and offspring health outcomes. Typically, weight gain in this population has been associated with pregnancy and the postpartum period, however, young mothers are not the only women who gain weight at this life stage. A literature review was conducted at the beginning of this thesis work to improve understanding of the non-pregnancy related determinants of weight gain in young women. This review found that quitting smoking, dietary behaviour, physical activity (PA) and transition to university were associated with weight gain. The finding that both dietary behaviour and PA were associated with weight gain was not surprising, given their contribution to energy balance. Available data on energy intake in young women suggest that energy intake has increased and nutritional quality has decreased over time. In contrast, little is known about changes in energy expenditure over time. Energy expenditure is determined by daily activity patterns, including time spent in light, moderate-vigorous (MV) intensity physical activities, as well as in sedentary behaviours. Very few data are available on the patterns of PA and sedentary behaviour in young women. Therefore, the aims of this thesis were to determine the patterns of PA and sedentary behaviour in young women, and the energy costs of these patterns. The thesis also examines whether the patterns change over time and, if so, whether they influence health markers at this life stage.
A 12-month prospective study of 65 full-time employed, non-pregnant women aged 18-36 years was conducted. The women were selected to reflect two distinct occupational groups, characterised by (a) women who spend a large proportion of their time at work sitting (high sitters) and (b) women who spend a large proportion of their time at work on their feet (low sitters). Participants wore an accelerometer for seven days and completed a detailed activity diary for the same time period, at baseline, 6 and 12 months. Anthropometric, physiological and metabolic data were collected at baseline and 12 months. Diary and accelerometer data were merged to allow accelerometer data to be separated into specific domains: domestic, leisure whilst at home, screen time, work, travel and leisure outside the home, and average MET values for each domain and total daily energy expenditure (TDEE) were calculated.
Baseline analysis to determine the patterns of PA and sedentary behaviour and the energy cost of these, in these two groups of women, revealed that young women participated in sufficient levels of MVPA (as indicated by accelerometer data and results from the Active Australia survey) but spent much of the rest of their time in very low-energy activities. On workdays, more than half of all non-work time in both groups (52%) was sedentary (221 [SD 80.8] mins/d), with a further 40% in light or very light activities (208 [SD 108.6] mins/d). The only difference in activity patterns between the low (average MET value 1.9) and high sitters (average MET value 1.5) was whilst at work, p<0.001. This difference translated to a difference in TDEE of 200 MET-mins between the low and high sitters. On non-workdays, overall average energy expenditure was also higher in the low sitters than the high sitters (p=0.01). This finding reflected slightly (but not significantly) higher energy expenditure in every domain in the low sitting group.
Over 12 months, participants gained an average of 410g, however, weight change ranged from -8kg to +5.4kg. There was no change in time spent in MVPA, but there were small decreases in time spent in active domains (travel and domestic) and concomitant increases in sedentary domains (screen time). This resulted in an overall decrease in TDEE. At 12 months, more than half the women dropped out of the study, because of pregnancy, overseas travel or quitting work. Due to the lack of change in any of the health outcomes except weight, and the small sample size, none of the activity measures were associated with changes in health outcomes over time. Comparison of weight gainers (gained >500g/year) with non-gainers (maintained or lost weight) found no significant differences in any of the activity variables, or in changes in these variables over time. The odds of being a weight gainer were however more than 5 times higher in women with baseline BMI >23kg/m2 or waist circumference >72 cm.
Overall, the findings support the possibility that patterns of physical activity and sedentary behaviour, in the domains of occupation, travel, and leisure both at and outside the home, may explain small differences in TDEE, which may account for the energy imbalance that underlies weight gain in this population. Further work with a larger sample over a longer period will be needed to confirm this hypothesis.