OBJECTIVE-Diminished daily physical activity explains, in part, why obesity and diabetes have become worldwide epidemics. In particular, chair use has replaced ambulation, so that obese individuals tend to sit for ϳ2.5 h/day more than lean counterparts. Here, we address the hypotheses that free-living daily walking distance is decreased in obesity compared with lean subjects and that experimental weight gain precipitates decreased daily walking.RESEARCH DESIGN AND METHODS-During weight-maintenance feeding, we measured free-living walking using a validated system that captures locomotion and body movement for 10 days in 22 healthy lean and obese sedentary individuals. These measurements were then repeated after the lean and obese subjects were overfed by 1,000 kcal/day for 8 weeks.RESULTS-We found that free-living walking comprises many (ϳ47) short-duration (Ͻ15 min), low-velocity (ϳ1 mph) walking bouts. Lean subjects walked 3.5 miles/day more than obese subjects (n ϭ 10, 10.3 Ϯ 2.5 vs. n ϭ 12, 6.7 Ϯ 1.8 miles/day; P ϭ 0.0009). With overfeeding, walking distance decreased by 1.5 miles/day compared with baseline values (Ϫ1.5 Ϯ 1.7 miles/day; P ϭ 0.0005). The decrease in walking that accompanied overfeeding occurred to a similar degree in the lean (Ϫ1.4 Ϯ 1.9 miles/day; P ϭ 0.04) and obese (Ϫ1.6 Ϯ 1.7 miles/day; P ϭ 0.008) subjects.CONCLUSIONS-Walking is decreased in obesity and declines with weight gain. This may represent a continuum whereby progressive increases in weight are associated with progressive decreases in walking distance. By identifying walking as pivotal in weight gain and obesity, we hope to add credence to an argument for an ambulatory future. Diabetes 57:548-554, 2008 O besity is epidemic in developed countries and is emerging in middle-and even low-income countries; this in part explains the unprecedented increase in type 2 diabetes worldwide (1). It is widely agreed that this partially reflects mounting sedentariness (2,3). At the beginning of the 20th century 90% of the population of the world was rural. However, over the last century, more than two billion agriculturalists have become city dwellers (4). In the latter transition, physical activity has declined (5). In particular, chair use has replaced ambulation (6) such that obese individuals tend to sit for ϳ2.5 h/day more than lean counterparts (7). Walking is the principal component of nonexercise activity thermogenesis (NEAT), which is the energy expenditure association with free-living daily activities (8,9). This is because people walk for several hours each day and, even at slow velocity, walking doubles energy expenditure (10); hence, the product of the time engaged in walking and its energetic equivalent is numerically substantial. We therefore wondered whether walking might be mechanistically important in weight gain and obesity. To address the hypothesis that free-living daily walking distance is decreased in obesity, we examined the characteristics of free-living walking in lean and obese people. Then, to address the hypothesis t...