Secular trends and distributional changes in the BMI and physical activity of New Zealand adults

Health is known to be inextricably related to physical activity. The world-wide trend of increasing obesity is a major problem for health professionals, however, the extent to which physical activity contributes to these obesity patterns is not fully understood. The aim of this study was to track secular trends in body mass and physical activity patterns among New Zealand adults aged 18 years and over between 1997 and 2007 to better understand changes over time and the relationship between body mass and physical activity. Data from the 1997 National Nutrition Survey, the 2002/03 New Zealand Health Survey, and the 2006/07 New Zealand Health Survey was analysed for changes in body mass and BMI over the ten year period, and weekly physical activity patterns over a 4 year period. Over the ten year period, mean weight increased by 4.8 kg (95% confidence limits CL 3.9-5.8 kg) in males and 3.7 kg (CL 2.8- 4.5 kg) in females. No substantial change in height was observed. Mean BMI increased over the same period by 1.6 kg/m² and 1.3 kg/m² for males and females respectively with an 11.7% increase in the proportion of males being classified as overweight or obese and a 7.6% increase in the proportion of females classified as overweight or obese. Between 2003 and 2007 the total weekly moderate to vigorous-intensity physical activity decreased by 39 and 25 minutes for males and females respectively. Brisk walking increased from 196 minutes in 2003 to 260 minutes in 2007 for males and 142 minutes in 2003 to 179 minutes in 2007 for females, which probably contributed to the lack of change in physical activity levels. In conclusion the increase in body mass of New Zealand adults continues unabated. At least some of this increased weight is likely to be due to a reduction in moderate and vigorous intensity physical activity witnessed.