Metabolic disruptions induced by reduced ambulatory activity in free-living humans

Physical inactivity likely plays a role in the development of insulin resistance and obesity; however, direct evidence is minimal and mechanisms of action remain unknown. Studying metabolic outcomes that occur after transitioning from higher to lower levels of physical activity is the best tool to answer these questions. Previous studies have successfully used more extreme models of inactivity, including bed rest, or the cessation of exercise in highly trained endurance athletes, to provide novel findings. However, these models do not accurately reflect the type of inactivity experienced by a large majority of the population. Recent studies have used a more applicable model in which active (∼10,000 steps/day), healthy young controls are asked to transition to an inactive lifestyle (∼1,500 steps/day) for a 14-day period. The transition to inactivity resulted in reduced insulin sensitivity and increased central adiposity. This review will discuss the outcomes of these studies, their implications for the cause/effect relationship between central adiposity and insulin resistance, and provide rationale for why inactivity induces these factors. In addition, the experimental challenges of directly linking acute responses to inactivity to chronic disease will also be discussed.