Structuring the obesity literature into a system dynamics model PJ Giabbanelli, T Torsney-Weir, DT Finegood

Outline Background Objective Model Conclusion

Background Recent mathematical models of obesity focus on influences by peers. ▫ Bahr et al., Obesity 2009 ▫ Hill, Rand, Nowak, Christakis 2010 Weight was directly ‘spreading’

Background Eating Exercising We suggested a model in which behavioural norms spread. However weight is the result of the energy balance. Giabbanelli, Alimadad, Dabbaghian, Finegood; ICO 2010

Background We have models to understand influences between individuals.But we do not have models to understand the logic of one individual.

Objective We want to establish a new mathematical model of obesity that explicitly models determinants of weight at the individual level. The model must rely on findings from the literature structured using system dynamics.

Model At the center of the model is energy balance.One thing an individual may do with respect to weight is… weighing. Weight monitoring is indeed efficient for weight control (O’Neil et al).

Model You won’t do anything particular for small fluctuations (Resnicow et al). You consider acting when you think there may be a weight issue. Your intention to change is a stock: it builds up over time.

Model Gaining five pounds may not sound as strong as reaching 200 pounds. Particular values correspond to life events, with a strong impact (Gorin). You can think of similar pathways for estimations of body size (Gardner et al.).

Model Several psychological models explain how the intention to change is translated into the actual action (Schwarzer et al.) The change feeds back into the system.

Model Perceived self-efficacy may be hampered by depression. There are social and physiological mechanisms.

Model

Model

Model We have 38 variables, and 52 interdependencies. Energy balance Weight monitoring Body size estimate Change Intake monitoring Exercise Stress Depression Medical trigger

Model Obesity is a complex phenomenon: a more realistic model is also complex. If we stopped there, we would have only a conceptual model. But we want a mathematical model.. So we need numbers, and a good calibration of that beast!

Model We actually have pretty good numbers for several of the relationships. Healthiness issue: you’ve developed diabetes. 56% of people seek counseling (Briscoe et al) ▪ lack of time ▪ lack of training ▪ poor reimbursement ▪ vague guidelines Broad medical triggers are reported by 22.9% of people in weight loss programs.

Model We actually have pretty good numbers for several of the relationships. How much weight gain do antidepressants produce? ▪ Tricyclics: 3.5 kg / 6 weeks; old ▪ SSRIs: 1 kg / 6 weeks; >50% market share ▪ Olanzapine: 8.5 kg / 11 weeks (severe case) ▪ Aripiprazole: 4.4 kg / 11 weeks (Correll 2009; Ruetsch 2005; Olfson 2006)

Model We actually have pretty good numbers for several of the relationships. How much more do individuals eat when with friends? (social facilitation) From 11% to 96%, and most studies suggest between 40-50% (Herman)

Model We actually have pretty good numbers for several of the relationships. How does it all turn into a change in weight? Energy expenditure (MJ/d) = Physical activity * (0.083*Weight ) * Energy intake

Conclusion Mathematical models in obesity focus on the spread of weight or behavioural norms between individuals. Using system dynamics, we propose the first model of obesity that is individual-centered and based on the literature.

Conclusion ▪ In order to be broad, we performed a non-systematic review. There are limitations to this study. → some relationships may have been missed. ▪ Numbers are available for some relationships but not for all. → the mathematical model may have several variables → meaningful values may have to be estimated through simulation

Conclusion The next stage is to go from the conceptual model onto the mathematical.

Acknowledgement This works was supported by the Canadian Institutes of Health Research (MT-10574) and the Modeling of Complex Social Systems Program (MoCSSy) at Simon Fraser University.

Conclusion