Objective: To compare estimates of population levels of 'adequate activity' for health benefit in different age and sex groups using two different measures kilocalories (kcals) and Mets.mins. Methods: 10,464 mid-age women (47-52 years) from the second survey of the Australian Longitudinal Study on Women's Health (ALSWH, 1998) and 2,500 men and women (18-75 years) from the 1997 Active Australia national survey, answered questions about physical activity. Kcals and Mets.mins were calculated from self reported time spent in walking, moderate and vigorous activity, and self-reported body weight. 'Adequate activity' was defined as a minimum of 800 kcals or 600 Mets.mins. Results: There were differences in the estimates of 'adequate activity' using the two methods among women participants in both surveys, but not among the male participants in the Active Australia survey. A significant proportion of the women in both surveys (6.4% of the ALSWH women and 8.5% of the Active Australia women, mean weight 60 kg) were classified as 'inactive' when the kcals method was used despite reporting levels of activity commensurate with good health. Fewer than 1% (mean weight 105 kg) were classified as 'active' using kcals when reporting lower than recommended levels of activity. Agreement between the two methods was better among men; only 3% were misclassified because of low or very high weight. Conclusions: The Mets.mins method of estimating 'adequate' activity assesses physical activity independently of body weight and is recommended for use in future population surveys, as it is less likely to under-estimate the prevalence of physical activity in women. Implications: Women and men aged 45-59 and women aged >60 should be the target of specific health promotion strategies to increase population levels of physical activity.
