Structuring the obesity literature into a Structuring the obesity literature into a system dynamics modelsystem dynamics model

PJ Giabbanelli, T Torsney-Weir, DT Finegood

OutlineOutline

Background

Objective

Model

Conclusion

BackgroundBackground

Recent mathematical models of obesity focus on influences by peers.

▫ Bahr et al., Obesity 2009

▫ Hill, Rand, Nowak, Christakis 2010

Weight was directly ‘spreading’

BackgroundBackground

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

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

ObjectiveObjective

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.

ModelModel

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).

ModelModelYou 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.

ModelModelGaining 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.).

ModelModelSeveral psychological models explain how the intention to change is

translated into the actual action (Schwarzer et al.)

The change feeds back into the system.

ModelModelPerceived self-efficacy may be

hampered by depression.There are social and physiological

mechanisms.

ModelModel

ModelModel

ModelModelWe have 38 variables, and 52 interdependencies.

Energy balance

Weight monitoring

Body size estimate

Change

Intake monitoring Exercise

Stress Depression

Medical trigger

ModelModelObesity 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!

ModelModelWe 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.

ModelModelWe 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)

ModelModelWe 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)

ModelModelWe 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 + 0.85) + 0.1 * Energy intake

ConclusionConclusion

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.

ConclusionConclusion

▪ 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

ConclusionConclusion

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

AcknowledgementAcknowledgement

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.

ConclusionConclusion