No abstract

Despite major investment in both research and policy, many pressing contemporary public health challenges remain. To date, the evidence underpinning responses to these challenges has largely been generated by tools and methods that were developed to answer questions about the effectiveness of clinical interventions, and as such are grounded in linear models of cause and effect. Identification, implementation, and evaluation of effective responses to major public health challenges require a wider set of approaches 1,2 and a focus on complex systems. 3,4 A complex systems model of public health conceptualises poor health and health inequalities as outcomes of a multitude of interdependent elements within a connected whole. These elements affect each other in sometimes subtle ways, with changes potentially reverberating throughout the system. 5 A complex systems approach uses a broad spectrum of methods to design, implement, and evaluate interventions for changing these systems to improve public health.Complex systems are defined by several properties, including emergence, feedback, and adaptation. 3 Emergence describes the properties of a complex system that cannot be directly predicted from the elements within it and are more than just the sum of its parts. For example, the changing distribution of obesity across the population can be conceptualised as an emergent property of the food, employment, transport, economic, and other systems that shape the energy intake and expenditure of individuals. Feedback describes the situation in which a change reinforces or balances further change. For example, if a smoking ban in public places reduces the visibility and convenience of smoking, and this makes it less appealing, fewer young people might then start smoking, further reducing its visibility, and so on in a reinforcing loop. Adaptation refers to adjustments in behaviour in response to interventions, such as a tobacco company lowering the price of cigarettes in response to a public smoking ban.Rhetoric urging complex systems approaches to public health is only rarely operationalised in ways that generate relevant evidence or effective policies. 1,6 Public health problems that emerge as a property of a complex system cannot necessarily be solved with a simple, single intervention, but the interacting factors within the system can potentially be reshaped to generate a more desirable set of outcomes. 7,8 Achievement of meaningful impacts on complex multicausal problems, like obesity, requires more than single interventions, such as traffic light food labelling or exercise on prescription, many of which require high levels of individual agency, have low reach and impact, and tend to widen health inequalities. 9-11 Shifts within multiple elements across the many systems that influence obesity are required, some of which might only have small effects on individuals but can drive large changes when aggregated at population level. 12 Although randomised controlled trials of individual-level interventions are relatively strai...

The global obesity epidemic has been on the rise for four decades, yet sustained prevention efforts have barely begun. An emerging science using quantitative models has provided key insights into the dynamics of this epidemic, and made it possible to combine different pieces of evidence and calculate the impact of behaviors, interventions and policies at multiple levels – from person to population. Forecasts indicate large effects of high levels of obesity on future population health and economic outcomes. Energy gap models have quantified the relationships of changes in energy intake and expenditure to weight change, and documented the dominant role of increasing intake on obesity prevalence. The empirical evidence base for effective interventions is limited but growing. Several cost-effective policies are identified that governments should prioritize for implementation. Systems science provides a framework for organizing the complexity of forces driving the obesity epidemic and has important implications for policy-makers. Multiple players (including governments, international organizations, the private sector, and civil society) need to contribute complementary actions in a coordinated approach. Priority actions include policies to improve the food and built environments, cross-cutting actions (such as leadership, health-in-all policies, and monitoring), and much greater funding for prevention programs. Increased investment in population obesity monitoring would improve the accuracy of forecasts and evaluations. Embedding actions within existing systems in both health and non-health sectors (trade, agriculture, transport, urban planning, development) can greatly increase impact and sustainability. We call for a sustained worldwide effort to monitor, prevent and control obesity.

No abstract